C Chemistry in industry and technology - pedagogics.capedagogics.ca/cambridge/source_data/options/Options_C.pdf · 2012-02-20 · Properties and uses of steel and iron The properties

C CHEMISTRY IN INDUSTRY AND TECHNOLOGY 1CHEMISTRY FOR THE IB DIPLOMA © CAMBRIDGE UNIVERSITY PRESS 2011

C Chemistry in industry and technology

C1 Iron, steel and aluminium

IronIron is a high-density, magnetic transition metal and the most abundant element on Earth owing to its presence in the Earth’s core (it is usually regarded as the fourth most abundant element in the Earth’s crust). Iron is vital for life as part of haemoglobin in red blood cells, but its primary industrial uses are as a building material and for conversion to steel. Iron is a fairly reactive metal and is rarely found native (in an elemental from); it reacts with oxygen to form a mixture of oxides and can combine with other elements to form a number of ores. The main mineral sources of iron include:

Haematite: Fe2O3 – iron(III) oxideMagnetite: Fe3O4 – a mixed ore of iron(II) oxide and iron(III) oxideSiderite: FeCO3 – iron(II) carbonateGoethite: FeO(OH) – hydrated iron(III) oxideLimonite: FeO(OH).nH2O – a mixture of hydrated iron(III) oxides

The largest producers of mined iron ore are China, Australia and Brazil, producing between 250 and 500 million tonnes of iron ore annually.

The blast furnaceUsable iron ores can be chemically reduced from the oxide forms to yield metallic iron. Although iron is reasonably reactive, it is below carbon in the reactivity series.

Iron can be reduced from its +2 or +3 oxidation states to the element by reacting it with carbon or carbon monoxide.

A specialised brick-lined furnace, called the blast furnace (Figure C1), is used to reduce iron ores to metallic iron, as the process requires extremely high temperatures.

In addition to iron ore, a number of other raw materials are needed in the reduction process. These include:

• coke – as a source of carbon, produced from heating coal in the absence of oxygen

• hot air – as a source of heat and oxygen

• limestone (mostly CaCO3) – to react with impurities in the ore.The solid raw materials are added from the top of the furnace. Once the

furnace has been charged with the raw materials, hot, oxygen-enriched air at between 900 and 1300 °C is blown in to the furnace from the bottom through pressurised inlet pipes called tuyères. The heat from the injected air causes a series of reactions to take place, which result in the reduction of the iron ore. The temperature inside the furnace may reach in excess of 2300 °C.

Learning objectives

• Describe the manufacturing processes for iron, steel and aluminium

• Understand how the modi� cation of iron, steel and aluminium a� ects their properties and uses

• Describe the environmental impact of ore and metal production

Wüstite, FeO and iron pyrite, FeS2, are two additional mineral sources of iron. These are, however, rarely used in the manufacture of the metal because of: (1) scarcity in the case of wüstite; and (2) being used in other processes, such as the production of sulfur dioxide and iron(II) sulfate from iron pyrite.

If the temperature in the furnace drops too much, fuels such as hydrocarbons are injected into the hot air blast to provide extra heat energy and carbon monoxide when they combust:

2CH4(g) + 3O2(g) → 2CO(g) + 4H2O(l)

Examiner’s tipOther than mineral sources, an important source of iron is through the recycling of scrap iron and steel.

2 CHEMISTRY FOR THE IB DIPLOMA © CAMBRIDGE UNIVERSITY PRESS 2011C CHEMISTRY IN INDUSTRY AND TECHNOLOGY

Reactions in the blast furnace

Inside the furnace, carbon monoxide (CO) acts as the primary reducing agent.

Carbon monoxide may be produced in two ways in the blast furnace. The combustion of coke can produce carbon monoxide:

2C(s) + O2(g) →2CO(g)

This reaction is exothermic, and the energy released helps to maintain the extremely high temperatures required in the blast furnace.

Coke can also react with carbon dioxide (produced either when limestone decomposes or when some coke burns in oxygen):

CO2(g) + C(s) → 2CO(g)

The hot carbon monoxide rises up the furnace and reacts with the iron ore, reducing it to molten iron and becoming oxidised to CO2 in the process. The overall reaction is:

Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g)

Iron is reduced, as its oxidation number decreases from +3 to 0.The molten iron formed falls to bottom of the furnace, where it pools,

waiting to be tapped o� .

input of rawmaterials

waste gasoutflow

refractorylining

hot-airtuyère

tap hole

tap hole

450 °C

HOT

VERY HOT

HOTTEST

1750 °C

slag

iron

Figure C1 The blast furnace for the reduction of iron ore to molten iron.

Carbon in the coke may also act as a reducing agent:

Fe2O3(s) + 3C(s) → 2Fe(l) + 3CO(g)


Hot, unreacted gases exit at the top of the furnace and are circulated back to the tuyères at the base of the furnace, providing a ‘jump start’ to the air-heating process. This recycling helps to reduce the energy requirements of the industrial process.

SlagThe iron ores are not pure and contain a signi� cant amount of impurities, usually in the form of silicon and aluminium oxides, which have very high melting points. These contaminant compounds are removed by reaction with calcium oxide (CaO) produced by the thermal decomposition of the limestone:

CaCO3(s) → CaO(s) + CO2(g)

CaO(s) + SiO2(s) → CaSiO3(l)

Alumina (Al2O3) dissolves in the molten calcium silicate (CaSiO3) to form a complex aluminosilicate slag. This slag is less dense than the molten iron and � oats on top of the pool of iron. It can be tapped o� separately.

SteelmakingThe molten iron formed in the blast furnace is still impure, despite the removal of the high-melting-point impurities. This iron contains small amounts of sulfur, phosphorus and silicon, as well as up to 5% carbon. Iron with a carbon content this high is called pig iron and is extremely brittle as a solid and therefore of limited usefulness. In order to remove the impurities and to reduce the carbon content of the iron, a process called basic oxygen conversion is employed (Figure C2).

Molten iron is poured into a vessel lined with calcium oxide and magnesium oxide. Hot, 99% pure oxygen is injected at high pressure into the molten iron and causes many of the impurities to be oxidised.

C + O2 → CO2

4P + 5O2 → P4O10

Si + O2 → SiO2

Molten slag is processed into pellets by spray cooling; these pellets are used as an aggregate in roadmaking or used to produce cement.

Extension

The iron ore added to the furnace is a mixture of iron oxides. In the blast furnace, a number of di� erent reactions occur between the reducing gases and the ores, depending on the temperature and position in the furnace. These reactions serve to purify the iron by gradually reducing the oxygen content of the ore:

3Fe2O3(s) + CO(g) → 2Fe3O4(s) + CO2(g) 450 °C Top of furnace

2Fe3O4(s) + 2CO(g) → 6FeO(s) + 2CO2(g) 600 °C

6FeO(s) + 6CO(g) → 6Fe(l) + 6CO2(g) 700 °C

Combining the equations we get:

3Fe2O3(s) + 2Fe3O4(s) + 6FeO(s) + 9CO(g) → 2Fe3O4(s) + 6FeO(s) + 6Fe(l) + 9CO2(g)

which simpli� es to:

Fe2O3(s) + 3CO(g) → 2Fe(l) + 3CO2(g)


The CO2 escapes as a gas; the phosphorus(V) oxide and silicon(IV) oxide react with added calcium oxide (lime) to form a layer of calcium phosphate (Ca3(PO4)2) and calcium silicate (CaSiO3) slag. Sulfur is removed by adding a large quantity of powdered magnesium, which reduces the sulfur to magnesium sul� de in an extremely violent exothermic reaction. This reduction reaction and the basic oxidation reactions are all exothermic and produce the heat necessary to keep the iron molten and the reaction rate high. However, the temperature of the process must be carefully controlled in order to produce steel of the desired quality. To prevent the temperature from rising too high, scrap steel is added to the vessel, which melts when the reaction temperature is high enough, preventing it from increasing further.

Steel alloysAlloying metals changes the mechanical and chemical properties of the metal product: for example, resistance to corrosion, hardness and strength. The properties of steel can be modi� ed by adding di� erent transition metals to the molten steel towards the end of the steelmaking process. The conferred properties of the steel alloy will depend on the identity of the metal added. The alloying process allows the production of metal alloys for speci� c purposes based on their properties (Table C1).

tap-holewhen tilted

CaO/MgOrefractorylining slag

gas outlet

hood

input of CaO, Mg,Mn, etc

oxygen lance

steelO2O2

Figure C2 The basic oxygen converter, which removes impurities from the molten iron and converts it to steel.

During the basic oxygen conversion process, the carbon content of the iron is reduced to 0.1–2.0%, depending on the grade of steel desired.

An alloy is a homogeneous mixture of two or more metals, or a mixture of a metal and non-metal. Brass is an example of a two-metal alloy, containing copper and zinc; mild steel is an alloy of iron and the non-metal carbon.


Steel heat treatmentDi� erent heat treatments on simple carbon steel (i.e. steel with no added metals) can a� ect the mechanical properties of the steel. Typically, properties such as hardness, strength and impact resistance are enhanced.

TemperingTempering increases the toughness and malleability of the steel. In the tempering process, the steel is heated to between approximately 200 and 250 °C (although the exact temperature depends on the type of steel) and then cooled slowly in a controlled manner. This modi� es the crystal structure within the alloy; that is, it causes the rearrangement of the carbon atoms within the steel structure, resulting in a new form that is less brittle.

AnnealingAnnealing results in softer, more ductile steel. Steel is heated to a much higher temperature than required for tempering – between 700 and 1000 °C, depending on the carbon content of the steel – and held at that temperature for a long period. The steel is then control cooled and, as with tempering, a structural rearrangement of the atoms in the alloy occurs, yielding highly ductile steel.

QuenchingQuenching is the process of rapid cooling. Quenching can be carried out in cool gas, oil, water or salt solutions. The rapid cooling of steel heated to over 900 °C causes the metal to increase in hardness and strength, owing to the formation of a new internal crystal structure. Tempering and quenching often go hand-in-hand to reduce the brittleness of hardened steel.

Properties and uses of steel and ironThe properties and uses of some di� erent types of iron and steel are shown in Tables C2 and Table C3.

Type of iron Properties Uses

pig iron high carbon content (4–5%), hard, brittle

steelmaking

cast iron 2–4% carbon, brittle, wear resistance, easily machined

piping, machine and car parts

wrought iron <0.01% carbon, malleable, ductile, easily welded

nails, chains, railings, gates, piping; rarely produced now

Table C2 The properties and uses of different forms of iron.

Metal added Amount added / % Properties Uses

chromium 10–15 corrosion resistance stainless steel in kitchenware and cutlery

vanadium 0.25–5 increased strength tools

nickel 10 increased strength, corrosion resistance stainless steel applications

molybdenum 1–8 ultra-high strength even at high temperature, easily welded

tools, aircraft and missile parts

Table C1 Various modifi ed steel alloys with their associated characteristics and uses.


Type of steel Properties Uses

mild steel 0.15–0.3% carbon, cheap, malleable, not ductile, will corrode

structural steel used in construction

medium carbon steel 0.3–0.5% carbon, wear resistance, balanced strength and ductility

car parts (body and engine)

high and ultra-high carbon steel 0.5–2% carbon, very strong and hard springs, high-strength wires, specialist uses, e.g. punches, axles

stainless steel 15% chromium, 10% nickel, corrosion resistance, strong, hard

cutlery, kitchenware, surgical equipment, major appliances

Table C3 The properties and uses of different forms of steel.

AluminiumAluminium is a low density, reactive metal not found native in nature. It is the most abundant metal in the Earth’s crust. Aluminium is more reactive than iron and carbon and cannot be extracted by heating with carbon or carbon monoxide.

In the � rst step of aluminium production, alumina (Al2O3) must be extracted from the bauxite and puri� ed.

Refi ning bauxiteThe bauxite is crushed and then dissolved in hot sodium hydroxide at 175 °C. The aluminium compounds dissolve to form sodium tetrahydroxoaluminate (NaAl(OH)4):

Al(OH)3(s) + NaOH(aq) → NaAl(OH)4(aq)

AlO(OH)(s) + NaOH(aq) + H2O(l) → NaAl(OH)4(aq)

but the impurities are insoluble and can be � ltered out.The resulting solution is cooled, and this causes solid aluminium

hydroxide to precipitate from solution as � u� y, white crystals:

NaAl(OH)4(aq) → Al(OH)3(s) + NaOH(aq)

The aluminium hydroxide crystals are removed from the solution and heated to over 1000 °C in a process called calcination. This causes the hydroxide to decompose into alumina and water:

2Al(OH)3(s) → Al2O3(s) + 3H2O(l)

Electrolysis of aluminaOwing to the high reactivity of aluminium, electrolysis must be employed in order to extract it from alumina.

Alumina is an ionic solid comprising Al3+ and O2− ions. In order to conduct electricity, the ions must be free to move. This requires melting the alumina (so that the strong electrostatic forces between the oppositely charged ions are broken); however, alumina has a very high melting point (2072 °C) owing to the high charges on the individual ions. Heating the alumina to this temperature and then maintaining it for the electrolytic process would cost more in energy and electricity than the aluminium produced at the end would be worth. Therefore, a clever method is employed to produce molten alumina at lower temperatures.

The principal ore of aluminium, bauxite, is a mixture of aluminium hydroxide (Al(OH)3) and hydrated aluminium oxides (AlO(OH)) with iron oxide impurities.

This process, called the Bayer process, relies on the amphoteric nature of aluminium oxide and hydroxide and the basic nature of other metal oxides. Amphoteric oxides and hydroxides are able to act as a base or an acid. In the Bayer process, aluminium hydroxide is acting as an acid, reacting with sodium hydroxide.

Smelting (extraction of the aluminium) is an expensive and energy-intensive process that uses huge amounts of electricity. Many aluminium smelters are therefore located near to hydroelectric or other power stations to ensure a good (and preferably cheap) supply of electricity.


Alumina will dissolve in molten cryolite (sodium aluminium � uoride, Na3AlF6) which melts at only 1012 °C (a full 1060 °C lower than alumina itself). A solution of aluminium oxide in cryolite is formed and costs much less to keep molten.

Now that the aluminium and oxide ions are no longer held in a lattice and are free to move, they can be separated in an electrolytic cell. Electrolysis of dissolved alumina is called the Hall–Héroult process (Figure C3).

graphiteanode

electrolytecrust

molten cryoliteand alumina

moltenaluminium

graphite cathode

steelcasing

refractorylining

tap-hole

+ +

The electrolytic cell is made of steel, with a refractory ceramic lining to withstand the high temperatures. The base of the cell is further lined with graphite, which acts as the cathode. The molten mixture of alumina and cryolite sits on top of this graphite cathode, and several large graphite blocks suspended from a support are immersed in the molten � uid; these act as the anode.

Figure C3 The Hall–Héroult cell for the electrolysis of molten alumina/cryolite.

The internal resistance of the passage of electricity through the molten cryolite generates enough heat to keep the compounds molten without the need for an external heat source such as a furnace.

Examiner’s tipThe usefulness of state symbols on the ion in these half equations is dubious, but if one is required in the examination then ‘(l)’ is probably most appropriate.

Hundreds of cells are lined up in series, and a huge current of several hundred thousand amps is applied through the circuit. The application of a current causes the free ions to move towards the oppositely charged electrode.

Aluminium ions are attracted to the cathode at the base of the cell, where they gain electrons and are reduced to liquid aluminium:

Al3+(l) + 3e− → Al(l)

Elemental aluminium is denser than molten cryolite, and so remains pooled at the bottom of the cell until it is tapped o� .

The free oxide ions are attracted to the suspended graphite anodes and are oxidised to oxygen.

2O2−(l) → O2(g) + 4e−

Graphite is used for a number of reasons:

• it conducts electricity well

• it is cheap and easily replaced

• it is relatively inert

• it has a high melting point, well above that of the molten alumina.

Extension

Cryolite is an uncommon mineral, and there is not nearly enough of it easily extractable from the crust to use in aluminium production. Most cryolite used in aluminium smelting nowadays is produced synthetically by reacting some of the sodium tetrahydroxoaluminate from the Bayer process with hydro� uoric acid obtained from � uorspar, CaF2 (a far more common mineral), and soda ash (sodium carbonate).


The overall reaction for the process is:

2Al2O3(l) → 4Al(l) + 3O2(g)

The high temperatures in the cell cause the oxygen gas produced at the anode to react slowly with the graphite, oxidising the carbon to carbon dioxide:

C(s) + O2(g) → CO2(g)

The anodes gradually erode away and have to be replaced periodically. This is done by recycling the unreacted graphite and re-pressing it into blocks with new graphite. Much of the � oor space of aluminium smelters is taken up with anode recycling.

Properties and uses of aluminiumSome properties and uses of aluminium are shown in Table C4.

Use Related properties

aircraft, bicycle, motorcycle frames, etc.

low density, strength, malleability, corrosion resistance

packaging (foil, cans), cookware corrosion resistance, good heat conductance, malleability

overhead power cables good electrical conductance, low density, ductility

mirrors, refl ectors (heat and light) high refl ection

Table C4 The properties and uses of aluminium.

Aluminium alloysAluminium can be alloyed with numerous metals to alter its mechanical properties. Most aluminium alloys are harder, less malleable and more durable than aluminium alone. This is because of the incorporation of metals of di� erent sizes – for example, copper disrupts the lattice structure and prevents the layers sliding past one another as easily.

Environmental impact of iron and aluminium productionProduction of aluminium and iron has a signi� cant e� ect on the environment, and wherever possible these metals should be recycled.

Mining

• Opencast mining causes irreversible damage to the landscape.

• Mining machinery and transportation of ore creates signi� cant pollution.

Blast furnace

• The blast furnace produces signi� cant amounts of greenhouse and toxic gases (CO2, CO, SO2, NOx).

• The blast furnace requires lots of energy (ultimately from fossil fuels) to keep the reaction temperature high.

• The blast furnace is unsightly and noisy.

• Unused waste products such as slag are pollutants.

• Waste from coke production is polluting.

Despite being a reactive metal that oxidises readily, aluminium is a corrosion-resistant and inert material in many of the forms in which it is used. This is because of the formation of a thin, unreactive and impermeable layer of aluminium oxide on the surface of the metal. This protects the more reactive metal underneath.

Examples of aluminium alloys include:

• Duralumin (copper): strong, but susceptible to corrosion; used in aircraft

• Silumin (silicon): strong and highly corrosion resistant; used in humid environments

• Birmabright (magnesium): strong, easily welded; sheet metal in vehicle production

Examiner’s tipFor a discussion of the advantages of recycling metals see Option E on the CD-ROM.


Aluminium smelter

• The aluminium smelter requires huge quantities of electricity, which may come from fossil-fuel-� red power stations.

• ‘Red mud’ waste from bauxite re� ning is unsightly and polluting when disposed of.

• The aluminium smelter is unsightly and noisy.

• CO2 and SO2 byproducts are polluting.

• Hazardous � uorine compounds are produced from cryolite.

Test yourself1 Both iron and aluminium can be extracted from

their ores using electrolysis. Suggest why electrolysis is not used for the production of iron.

2 Write an ionic equation, including state symbols, to show the reaction that occurs when aluminium hydroxide dissolves in sodium hydroxide solution.

C2 The oil industry

Oil as an energy source or chemical feedstock?Crude oil is one of humankind’s most useful and used resources. For centuries it has been used as a source of heat and energy, and in more modern times as a chemical feedstock for producing petroleum-based products such as plastics. However, it is only a limited resource; produced millions of years ago from the compressed, heated remains of prehistoric sea life. We cannot ‘make’ oil, only � nd and tap new pockets yet to be discovered.

Current estimates put the world’s proven oil reserves at 1.25 trillion barrels (170 billion tonnes), with more in unproven or probable reserves. This may seem a lot, but global daily usage is about 85 million barrels, meaning these proven reserves will provide oil for a mere 40 years before they are depleted. With a burgeoning world population, oil usage is likely to increase over that time period.

Crude oil as an energy source

Advantages

• Good source of combustible hydrocarbons from re� nery gases (propane, butane), through gasoline (mainly octane) to aviation fuel (kerosene), diesel and fuel oil.

• Complete combustion of these hydrocarbons liberates large quantities of energy: C4H10 + 6.5O2 → 4CO2 + 5H2O ∆H = −2876.5 kJ mol−1

C8H18 + 12.5O2 → 8CO2 + 9H2O ∆H = −5470.2 kJ mol−1

• More e� cient release of heat energy compared with other traditional sources of energy, such as coal, wood and peat.

• Easy to store and transport owing to the majority of fuel compounds being easily compressible gases or liquid at room temperature.

• Relatively cheap compared with renewable fuels.

Disadvantages

• Production of greenhouse gas (CO2) and poisonous carbon monoxide (complete combustion is less likely, as carbon chain increases in length).

• Diverts oil supplies away from use as a chemical feedstock.

Learning objectives

• describe the use of crude oil as a fuel and a chemical feedstock

• describe the main di� erences between the various types of cracking of petroleum products


Crude oil as a chemical feedstock

Advantages

• Excellent source of compounds for making plastics and synthetic � bres.

• Excellent source of precursors for production of petrochemicals including pharmaceuticals, waxes and lubricants.

• Source of compounds for other uses, e.g. road building.

Disadvantages

• Polymers and petrochemicals are di� cult to dispose of (they are non-biodegradable).

• When incinerated, these produce CO2, CO and toxic halogenated gases.

• Alternative renewable and environmentally ‘friendly’ sources of polymers are becoming increasingly available.

• It diverts oil away from production of fuels.Ultimately, use of crude oil as both a fuel and as a chemical feedstock needs to be reduced if global reserves are to remain during our lifetimes. A balance needs to be struck as to which is the most prudent use of this dwindling resource. Many would argue that using crude oil as a fuel is less appropriate, given the number of alternatives such as coal, natural gas, nuclear power and renewable sources, which cannot be used as chemical feedstock to the same degree. The global demand for plastics and petrochemicals is huge, and the most chemically straightforward manufacturing process is from crude oil. From a chemical point of view, � nding alternative fuels is an easier proposition than � nding alternative sources of chemicals for polymerisation, etc.

Cracking – formation of alkenesCrude oil is a good source of highly combustible short-chain alkanes; however, a signi� cant proportion of the bulk of crude oil is less-useful, longer-chain alkanes that combust less e� ciently and are, generally, less pro� table for oil companies. These longer-chain alkanes can be used as a chemical feedstock to produce both shorter-chain alkanes (for gasoline) and alkenes for making polymers and plastics.

There are three di� erent methods for cracking alkanes: catalytic cracking, thermal cracking and steam cracking.

Catalytic crackingThis process is used industrially to break alkanes with lengths in excess of 14 carbon atoms and tends to produce gasoline-length products. The alkanes are passed over a hot powdered catalyst at over 700 °C. When the alkanes come into contact with the catalyst, they vaporise and decompose to form a mixture of smaller hydrocarbons, including alkenes:

C16H34 → C8H18 + C2H4 + 2C3H6 octane ethene propene

The C16 compound could be used a fuel oil, but the products octane, propane, ethene and propene are far more valuable. The original alkane may split into any number of alkanes and alkenes of varying length; the exact reaction is di� erent every time.

Possible catalysts that can be used include alumina (Al2O3) and silica (SiO2), although most modern facilities use a zeolite, which is

The process by which long-chain alkanes are split into shorter alkanes and alkenes is called cracking.

Examiner’s tipAlthough the following methods produce a number of products, often a simple equation showing the production of one alkane and one alkene will su� ce, e.g.:

C10H22 → C8H18 + C2H4

Do not forget that the equation must balance!


a specialised material comprising both aluminium and silicon in an aluminosilicate. Zeolites have a porous three-dimensional structure that permits maximum e� ciency and some control over the sizes of the products.

Thermal crackingThis method of cracking alkanes involves a free radical mechanism. The alkanes are heated to between 500 and 750 °C at a pressure of approximately 70 atmospheres (7000 kPa). This causes homolytic � ssion of C–C bonds within the alkanes to form alkyl free radicals, e.g.:

CH3CH2CH2CH3 → 2CH3CH2• Initiation

These free radicals can then undergo internal rearrangement to yield alkenes or react further (propagation and termination) to produce a variety of di� erent products.

Steam crackingSteam cracking is primarily used to produce low molecular weight alkenes such as ethene and propene from small alkanes such as naphtha, propane and ethane. The alkanes are mixed with superheated steam (>1000 °C) in a furnace in the absence of oxygen. The reaction occurs in a fraction of a second. The mechanism by which steam cracking works also relies on free radicals, as in thermal cracking.

Environmental impactThe re� ning of crude oil into more useful products has a signi� cant impact on the environment. Primary areas of concern are: air pollution caused by volatile hydrocarbons at the re� nery and the combustion of the � nal products; water pollution if a leak or spillage of crude oil occurs; the toxic nature of some products of cracking and re� ning, e.g. benzene and other aromatic hydrocarbons; global warming due to the combustion of hydrocarbon fuels; disposal problems with non-biodegradable plastics generated from the polymerisation of alkenes.

CH3CH2• + CH3CH2CH2CH3 → CH3CH2CH3 + CH3CH2CH2• Propane produced

CH3CH2CH2• → CH3CHCH2 + H• Propene produced

CH3CH2• → CH2CH2 + H• Ethene produced

CH3CH2• + H• → CH3CH3 Ethane produced

Test yourself3 Write an equation to show the cracking of an

alkane with 20 C atoms.4 Write an equation to show the cracking of an

alkane with 14 C atoms to form ethene, octane and one other product. Is the other product an alkane or alkene?


C3 Addition polymersThe alkenes produced in the cracking methods described above are extremely valuable commodities. The presence of a C=C double bond makes them far more reactive than alkanes, and they are therefore useful as precursors for making more complex molecules. One particular reaction that makes alkenes valuable is addition polymerisation, in which thousands of individual alkene molecules (monomers) combine to form straight or branched-chain alkanes called polymers. We call the majority of these polymers plastics. They have many useful properties, including the ability to be moulded into shape, thermostability, corrosion and water resistance, strength and durability.

Structure and propertiesThere are several factors that a� ect the structure and properties of polymers, including branching and the position of side groups.

BranchingEthene monomers do not necessarily add to one another in a linear fashion to create a long, straight-chain alkane polymer, and the polymerisation process normally results in the formation of polymer chains with branches. The amount of branching present in polyethene has signi� cant e� ects on its physical properties and uses.

Low-density polyetheneLow-density polyethene (LDPE) contains a high proportion of branching (Figure C4a). Highly branched polymer chains are less able to pack closely together, and therefore contact points between chains are reduced. This results in weaker van der Waals’ forces. As the intermolecular forces between the chains are weaker, the polymer is more � exible, has a lower melting point and is more easily stretched. LDPE can be used to make rigid containers, but is more commonly used to make plastic bags and wrapping � lms because of its � exibility.

High-density polyetheneHigh-density polyethene (HDPE) has a very low incidence of branching (Figure C4b), and this permits the polymer chains to pack together more tightly, increasing the density of the plastic. The more e� cient packing of the chains increases the strength of van der Waals’ interactions, and so the chains are held together more tightly. This makes the polymer more rigid and increases its melting point and tensile strength. HDPE can be used for di� erent purposes from those of LDPE because of its di� erent mechanical properties. HDPE is commonly used for making plastic bins, water pipes and food containers such as milk cartons and margarine tubs.

Position of side groupsWhen propene is polymerised to polypropene, the repeating unit contains a methyl group side chain (Figure C5).

Propene molecules can be added on to one another in a di� erent orientation, with the methyl group pointing up or down in relation to the methyl group at the growing end of the polymer. The orientation of the methyl groups in relation to each other can a� ect the properties of the � nal material.

Learning objectives

• describe how structure a� ects the properties of a polymer

• describe how the properties of polymers are modi� ed

• understand the advantages and disadvantages of polymer use

The exact properties of any given polymer are dependent on its chemical structure and the manner in which the polymer chains interact with one another.

The frequency of branch formation can be reduced by changing the reaction conditions: lowering the temperature and pressure and including a Ziegler–Natta catalyst (see Higher Level Section C9 on pages 31–32).

a LDPE

b HDPE

Figure C4 The proportion of branching in LDPE (a) is signifi cantly higher than in HDPE (b) and contributes to its different properties and uses.

CH

CH3

CH2

n( (

Figure C5 The repeat unit of the polymer polypropene.


Isotactic polypropene is the polymer of polypropene in which all the methyl groups are pointing in the same direction. Atactic polypropene contains methyl groups orientated in a random manner.

The regular arrangement of methyl groups in isotactic polypropene allows the chains to pack together more easily and therefore maximises the van der Waals’ force strength. Isotactic polypropene is therefore crystalline, rigid and strong, with a higher melting point than atactic polypropene.

Isotactic polypropene is used for a wide range of purposes, including � ip-top lids, plastic kettles, crates, chairs and ropes.

The irregular positioning of the methyl groups in atactic polypropene means that the polymer chains do not align themselves very well, and as a result the intermolecular forces between the chains are weaker. Atactic polypropene has a low melting point and is soft and rubbery rather than rigid. It has a limited number of applications, which include use as a roo� ng material, as a waterproof membrane and in paper lamination.

Modifi cation of addition polymers

Adding plasticisersPlasticisers are small molecules added to the polymer to increase its � exibility and durability.

Unmodi� ed PVC is a very rigid material used for guttering and piping; however, incorporation of phthalate plasticisers makes it � exible (Figure C6). This � exibility, coupled with its impressive durability, makes plasticised PVC suitable for making garden hoses, � ooring, in� atable structures and some clothing. The plasticiser molecules insert between the polymer chains, forcing them apart and so reducing the strength of the intermolecular forces, allowing the chains to move more freely.

ExpansionPolystyrene is a rigid, high-melting-point (240 °C), glassy plastic polymer in its unexpanded form. It is used to make plastic models, CD cases and disposable cutlery. However, when asked to describe polystyrene, most people would invariably describe a white, low-density, gas-� lled packaging material. This well-known form of polystyrene is the expanded form of the polymer. It is created by � rst dissolving the volatile hydrocarbon pentane (C5H12) in the polymer during initial manufacture. The beads of polystyrene that are formed during the polymerisation process are then heated in steam. The steam causes the pentane to vaporise and form gas bubbles within the polystyrene beads. This causes the beads to expand to

H3CHH3C

HH3C

HH3C

HH3C

H

isotactic

H3CHH3C

HH H3C

CH3 CH3HH

atactic

Most commercially produced polypropene is isotactic, due to the carefully controlled choice of catalyst during the polymerisation.

Atactic polypropene is not manufactured commercially, and almost all atactic polypropene is produced as a byproduct of isotactic polypropene manufacture.

In plastic polymers, the most common plasticisers are phthalates.

a

b

plasticiser moleculesforce chains apart

Figure C6 Non-plasticised PVC (a) and PVC containing a plasticiser (b).


about 60–70 times their original volume. Expanded polystyrene beads can then be pressed together in moulds to form sheets or appropriate packaging shapes.

Advantages and disadvantage of addition polymers

Advantages

• Strength and durability: HDPE, PVC and Perspex™ are particularly strong, rigid and hardwearing.

• Flexibility: LDPE and plasticised PVC are easily rolled into sheets or � lms.

• Density: plastics have variable densities, but expanded polystyrene has an extremely low density (0.015–0.03 g/cm3), making it an ideal packaging material; Perspex™ has a density of 1.2 g/cm3, which makes it a good lightweight substitute for glass (density ~2.5 g/cm3).

• Impact absorption: expanded polystyrene absorbs impact very well because of the gas pockets within the macrostructure.

• Lack of reactivity: addition polymers contain unreactive, saturated C–C bonds and generally inert side groups. This makes them excellent for storage of corrosive substances and resistant to oxidation in air and hydrolysis in water.

• Insulation: polypropene, polyethene and polystyrene have high speci� c heat capacities and extremely low thermal conductivity, making them excellent thermal insulators.

• Recycling: many polymers, especially polyethene and polypropene can be easily recycled by melting and reforming.

Disadvantages

• Depletion of crude oil: addition polymers, plasticisers and foaming agents (pentane) all come from petroleum-based feedstock.

• Disposal: the lack of reactivity of plastic polymers makes disposal tricky. They are non-biodegradable, meaning they will not rot. Current methods for disposal include land� ll, which takes up vast spaces, and incineration, which contributes to the greenhouse e� ect by producing CO2 and releases toxic gases from halogenated polymers such as PVC.

• Not all polymers can be easily recycled, and recycling can be expensive.

Examiner’s tipSee Option E on the CD-ROM for a more detailed discussion of the options for disposing of polymers and some of the problems of recycling them.

Petroleum-derived polymers can be combined with additives that will increase the polymer’s susceptibility to hydrolysis or oxidative decomposition. In addition, a number of species of bacteria have been found to decompose plastic polymers. These can be added to land� ll sites to reduce the volume of waste.

Test yourself5 Draw three repeat units of the addition

polymer formed from but-2-ene and suggest whether it is possible to produce di� erent forms.

6 The basic structure of the polymer chains in plastics A and B are shown below. Explain which will have the higher melting point.

plastic A

plastic B


C4 CatalystsMany spontaneous chemical and biological reactions occur incredibly slowly at room and body temperatures. Catalysts are of vital importance to manufacturing and life processes, as they increase the rate of these reactions without the need for dramatic changes to the reaction conditions.

Catalysts provide an alternative reaction pathway that has a lower activation energy than the uncatalysed pathway.

Catalysts can be broadly categorised as homogeneous or heterogeneous, depending on the state of matter (phase) in which they and the reactants exist.

Homogeneous catalysts

Homogeneous catalysts are in the same phase as the reactants.

The most common form of homogeneous catalysis involves catalyst and reactants in solution (aqueous or organic), as in most enzyme-catalysed biological processes.

An example of homogeneous catalysis is the iron(II)-catalysed reaction between persulfate and iodide ions. In the catalytic cycle, a two-stage reaction occurs involving � rst the transition metal in a low oxidation state and then a second reaction involving the high oxidation state ion, or vice versa.

S2O82−(aq) + 2I−(aq) → 2SO4

2−(aq) + I2(aq) Overall reaction

activation energy = Ea1

With catalyst:

S2O82−(aq) + 2Fe2+(aq) → 2SO4

2−(aq) + 2Fe3+(aq) First catalysed stage


2Fe3+(aq) + 2I−(aq) → 2Fe2+(aq) + I2(aq) Second catalysed stage


The Fe2+ ion is regenerated in the second reaction, so its � nal concentration remains una� ected at completion.

The enthalpy level diagram for a catalysed process such as this is shown in Figure C7.

Besides transition metal ions, another common homogeneous catalyst is the proton (H+). Reactions in which H+ is the catalyst are called acid-catalysed reactions. Carboxylic acids react with alcohols to form esters in an acid-catalysed reaction.

Many reactions involving a homogeneous catalyst involve the catalyst forming an intermediate with a reactant. This intermediate reacts more easily than the original reactant. Acid-catalysed esteri� cation involves protonation of the carbonyl group on the carboxylic acid.

The ability to act as a catalyst relies on a transition metal atom/ion being able to exhibit varying oxidation states and to coordinate other molecules/ions.

Note: all the reactants, including catalyst, are in the same phase (solution).

The proton is regenerated in the � nal stage of the esteri� cation reaction mechanism.

Learning objectives

• Understand the di� erences between heterogeneous and homogeneous catalysis

• Discuss advantages and disadvantages of heterogeneous and homogeneous catalysis

• Discuss the factors that in� uence whether heterogeneous or homogeneous catalysis is used


In enzyme-catalysed reactions, the reactant or reactants must dock into a speci� cally shaped active site within the enzyme protein. This forms an enzyme–substrate complex, which forms with a lower activation energy. The active site of the enzyme only recognises a subset of molecules with an adequate three-dimensional shape for binding in the active site, like a key � tting into a lock.

Heterogeneous catalysts

Heterogeneous catalysts are in a di� erent phase from the reactants.

Heterogeneous catalysts are usually solids, and reactions occur on their surface. Transition metals and their compounds are particularly good at adsorbing gases, and so they are commonly used as heterogeneous catalysts in industry, e.g. Fe in the Haber process, Ni in the hydrogenation of unsaturated hydrocarbons, V2O5 in the Contact process, and Pt, Rh and Pd in vehicle catalytic converters. In the iron-catalysed production of ammonia, the reactants are gases but the catalyst is a solid:

Fe(s)N2(g) + 3H2(g) 2NH3(g)

Many heterogeneous catalysts rely on their ability to adsorb reactant molecules on to active sites on their surfaces (Figure C8). Adsorption (note ‘adsorption’ and not ‘absorption’) performs three tasks: it increases the localised concentration of the reactants on the surface of the catalyst and thereby increases collision rate; it brings the reactants together in the correct orientation for reaction; and it weakens the covalent bonds in the reactants, reducing the energy barrier for reaction.

Many heterogeneous catalysts are added to reactions as powders, as a � ne mesh or attached to high-surface-area structures, e.g. catalytic converters. This is because heterogeneous catalysis occurs only at the surface of the catalyst. This means that there is a reduced ratio of active sites to catalyst molecules compared with homogeneous catalysts.

Reaction progress

uncatalysed

catalysed byFe2+(aq)

S2O82–+2I–

+2Fe2+

2SO42–+2I–

+2Fe3+

2SO42–+I2

+2Fe2+

Ea1

Ea3

Ea2

E

Figure C7 The change to the reaction profi le associated with homogeneous catalysis of an exothermic reaction.

Examiner’s tipAdsorption means that the reactants bind to the surface of the catalyst – it is not the same as absorption.


Some advantages and disadvantages of homogeneous and heterogeneous catalysts are shown in Table C5.

Advantages Disadvantages

Homogeneous • all reactant molecules exposed to catalyst• extreme reaction conditions, e.g. high pressure

and temperature, not required

• diffi cult to separate the catalyst from unused reactants and products

• prone to permanent deactivation, in which catalyst becomes useless

Heterogeneous • easily removed from unused reactants and products by fi ltration

• one metal may catalyse more than one reaction

• reactant molecules exposed only to the surface of the catalyst

• prone to poisoning (blockage of active sites) by contaminants, e.g. soot in catalytic converters

Table C5 The advantages and disadvantages of employing heterogeneous and homogeneous catalysis.

Ni

Ni

HH

C C

reaction

dissociation

active sitefree

adsorption

H H

H HNi

Ni

Ni

Ni

Ni

Ni

HHC CH H

H H

C CH H

HHH H

C CH H

HHH H

Figure C8 The adsorption of ethene and hydrogen gases onto the surface of a heterogeneous solid nickel catalyst during hydrogenation.

Choice of catalytic methodOver 90% of industrial processes use heterogeneous catalysis, despite the apparent advantages of homogeneous catalysis. This is almost entirely down to the ease of separating a heterogeneous catalyst from the reaction products. But what factors should an industrial chemist take into account when deciding whether to use a heterogeneous or homogeneous catalyst for a given reaction?

• Homogeneous catalysis is far more speci� c to a particular reaction, so if selectivity of the product is desired, homogeneous catalysis is more useful.

• Is e� ciency essential or can unreacted material be recycled?

• Homogeneous catalysis is more e� cient, owing to greater availability of active sites.

• Does the reaction require severe conditions? Homogeneous catalysts tend not to work well in extreme conditions such as high temperature, whereas heterogeneous transition metal catalysts withstand high temperature and pressure well.

• Can the catalyst be easily replaced or repaired if poisoned? Heterogeneous catalysts become poisoned by the build-up of substances such as sulfur or carbon; however, they can be cleaned fairly


easily and need fully replacing less often than homogeneous catalysts. Once a homogeneous catalyst has been deactivated, it generally needs to be replaced completely.

• Are there any environmental considerations to the use and disposal of the catalyst: for example, the disposal of toxic metals such as mercury or cadmium?

C5 Fuel cells and rechargeable batteriesSafe, reliable and portable sources of electrical energy were once a pipedream. However, technological advances and some inspired chemistry now mean that such devices are commonplace; the humble battery really is a masterpiece of chemical engineering. In recent years, battery technology has advanced to the extent that batteries can be easily recharged and produce impressive amounts of electricity, and now fuel cell technology permits the generation of a current from unlikely sources of electricity such as hydrogen and ethanol.

Fuel cellsA fuel cell is a specialised type of electrochemical cell in which a fuel and an oxidising agent, e.g. oxygen, react in the presence of an electrolyte to produce electrical energy. In the hydrogen–oxygen fuel cell, hydrogen reacts with oxygen to produce water and electricity.

Figure C9 shows an alkali hydrogen–oxygen fuel cell with a potassium hydroxide electrolyte. Electrodes are typically made of carbon and incorporate a metal catalyst such as platinum.

Learning objectives

• Understand the functioning of a hydrogen–oxygen fuel cell

• Describe the workings of lead–acid, NiCad and lithium-ion rechargeable batteries

• Discuss the di� erences and similarities between fuel cells and rechargeable batteries

Examiner’s tipThe following examples are electrochemical cells, in which the anode is the negative electrode and the cathode is the positive electrode – the opposite to electrolytic cells.

+

e–

external circuit

OH–

H2O

H2OH2O

H2

H2

H2

H2 O2

O2

O2

unusedO2

KOH(aq)electrolyte

O2OH–

OH–

OH–

anod

e

cath

ode

The two electrodes are separated by a porous matrix saturated with an electrolyte, which may be either an alkaline or acidic solution. In the case of an alkaline electrolyte, typically a solution of potassium hydroxide (KOH) is used.

At the anode, hydrogen reacts with free hydroxide ions (OH−) to form water. In the process, the hydrogen is oxidised and releases electrons:

H2 + 2OH− → 2H2O + 2e− Anode

Figure C9 An alkali hydrogen–oxygen fuel cell with a potassium hydroxide electrolyte.


At the cathode, oxygen reacts with water, gaining electrons from the electrode in a reduction reaction to form hydroxide ions.

O2 + 2H2O + 4e− → 4OH− Cathode

Overall redox reaction:

2H2 + O2 → 2H2O

The separation of the oxidation and reduction reactions permits the � ow of electrons and the generation of an electrical current.

The alkaline electrolyte can be substituted for an acidic solution, commonly phosphoric acid (Figure C10). The reactions occurring at anode and cathode in an acidic (proton-exchange) fuel cell are di� erent:

H2 → 2H+ + 2e− Anode: oxidation

4H+ + O2 + 4e− → 2H2O Cathode: reduction

+

e–

external circuit

H2O

H2O

H2

H2

H2

H2

H+

H+

H+

H+

O2

O2

H2OH2OO2

O2

unusedH2

H3PO4(aq)electrolyte

anod

e

cath

ode

Figure C10 A proton-exchange fuel cell, using a phosphoric acid electrolyte.

Both types of fuel cell rely on external sources of fuel and oxidising agent, which are depleted during use and require constant replenishing. They are therefore very di� erent from batteries that contain stored chemical energy within them. In a hydrogen–oxygen fuel cell in a vehicle, the hydrogen is stored on board the vehicle in a pressurised liquid form; oxygen may come from puri� ed air or an oxygen tank.

Hydrogen–oxygen fuel cells do not produce any pollution, as the only products are water, electricity and a small amount of heat. However, the production of hydrogen from the cracking of ethane, steam cracking of methane and hydrolysis of water must be considered when thinking about the overall environmental impact of fuel cell use.

Rechargeable batteriesRechargeable batteries, or secondary cells, are a portable type of electrochemical cell that generate a current via electrically reversible reactions. That is, once the forward reactions have taken place and yielded the desired electrical energy (electrical output), electrical input can be used to reset, or recharge, the battery by causing the reactions to occur in the reverse direction.

Owing to the use of platinum catalysts and the need for hydrogen storage and precision engineering, hydrogen–oxygen fuel cells are currently very expensive to produce.


Lead–acid batteryOne of the oldest forms of rechargeable cell is the lead–acid battery, still used in the majority of car engines. The lead–acid battery works on the simultaneous oxidation and reduction of lead in a concentrated sulfuric acid electrolyte.

When the battery is charged, i.e. is capable of releasing electrical energy, lead within the battery is in the form of elemental lead at the anode and lead(IV) oxide, PbO2, at the cathode. A typical car battery comprises six electrical cells in series and can therefore generate a signi� cant potential di� erence of about 12 V. When the battery discharges, lead at the anode is oxidised, releasing electrons, while the lead(IV) at the cathode is reduced. The product is solid lead(II) sulfate, which accumulates on the electrodes and within the battery.

During discharge:

Pb(s) + SO42−(aq) PbSO4(s) + 2e− Anode: oxidation

PbO2(s) + 4H+(aq) + SO42−(aq) + 2e− PbSO4(s) + 2H2O(l)

Cathode: reduction

Pb(s) + PbO2(s) + 4H+(aq) + 2SO42−(aq) 2PbSO4(s) + 2H2O(l)

Overall reaction

When the battery loses charge, the lead(II) sulfate can be converted back to lead and lead(IV) oxide by applying an external electrical source; the chemical reactions above are reversed.

Nickel–cadmium batteryNickel–cadmium (NiCad) batteries are often seen as the classic small rechargeable batteries (sizes AAA to D) used for powering small electronic devices such as clocks, calculators, remote controls and toys. They typically generate smaller voltages than the lead–acid battery, of around 1.2 V. NiCad batteries use a nickel oxide hydroxide (NiO(OH)) cathode and a metallic cadmium anode separated by a potassium hydroxide electrolyte.

During the discharge, the following reactions occur at the electrodes:

Cd(s) + 2OH−(aq) Cd(OH)2(s) + 2e− Anode: oxidation

NiO(OH)(s) + H2O(l) + e− Ni(OH)2(s) + OH−(aq) Cathode: reduction

Cd(s) + 2NiO(OH)(s) + 2H2O(l) Cd(OH)2(s) + Ni(OH)2(s) Overall reaction

Lithium-ion batteryLithium-ion (Li-ion) batteries are commonly used rechargeable cells in mobile telephones, laptop computers and high-energy usage portable electronic devices. They are able to generate higher voltages (around 3–4 V) than NiCad batteries. Typically, the anode is made of carbon with lithium atoms inserted in the lattice, and the cathode is a metal oxide such as manganese(IV) oxide or cobalt(IV) oxide, into the lattice structure of which Li+ ions can be inserted. The electrolyte is a complex lithium salt like lithium hexa� uorophosphate (LiPF6) dissolved in an organic solvent (Figure C11).

When a car is in motion the battery is charged, as it is linked to the alternator.


During discharge, lithium atoms are oxidised at the negative electrode and electrons are released to the external circuit:

LixC6 → xLi+ + xe− + 6C Oxidation

The Li+ ions move through the electrolyte to the cathode, where they become inserted into the lattice of the transition metal oxide:

CoO2 + xLi+ + xe− → LixCoO2 Reduction

The overall redox reaction during discharge is:

LixC6 + CoO2 → 6C + LixCoO2

The net e� ect of these reactions is that lithium atoms are oxidised at the negative electrode and cobalt ions are reduced at the positive electrode. For every lithium ion present in LixCoO2, the oxidation number of a cobalt ion must be reduced by 1 from +4 to +3 (an Li+ and Co3+ ion together give a charge of 4+ to cancel out the charge on two O2− ions).

The reverse reactions occur during charging, and lithium ions move in the reverse direction.

Similarities and differences between fuel cells and rechargeable batteries

Similarities

• They both generate electrical energy from chemical energy.

• They both require anode, cathode and electrolyte.

• They both generate a current based on the separation of reduction and oxidation reactions and the � ow of electrons through an external circuit.

Differences

• Fuel cells require an external source of chemical energy (fuel), but rechargeable batteries carry their chemical energy source with them.

• Fuel cell reactants need replenishing by replacement, but rechargeable battery reactants are replenished by reversing reactions.

• Rechargeable batteries are generally much smaller than fuel cells.

• Rechargeable batteries are far cheaper than fuel cells.

• Fuel cells are capable of generating a far greater quantity of electricity than rechargeable batteries.

e–

external circuit

Li

discharge

CoO2 electrodecontaining Li+ ions,forms LixCoO2

grap

hite

ele

ctro

deco

ntai

ning

Li a

tom

s

+

LixC6

LiPF6 electrolyte

Li+ ions Li+Li

Li Li

Li Li

Li+

Li+

Figure C11 A lithium-ion battery. Lithium ions are exchanged between complexed lithium and graphite electrodes.


• Fuel cells can produce drinkable water as a byproduct; there is no byproduct of rechargeable batteries.

• Fuel cells are non-polluting; rechargeable batteries may contain toxic metals (Cd, Pb), so are di� cult to dispose of.

• Rechargeable batteries have a � nite life and must be replaced eventually; fuel cells have a much better longevity.

C6 Liquid crystalsWhen asked to describe the � rst thing that comes to mind when we hear the words ‘liquid crystal’, the majority of us would no doubt think of the ubiquitous liquid crystal displays (LCDs) present in numerous electronic devices. These displays contain specially developed chemical compounds with liquid crystal properties, but many common and ‘ordinary’ compounds can also be classed as liquid crystal.

What is a liquid crystal?Liquid crystal is a phase of matter in which the properties of a compound may exhibit characteristics of both a liquid and a solid. They are � uids with electrical, optical, elastic and some other physical properties that depend on the orientation of the molecules relative to some � xed axis in the material. Generally, in the liquid crystal phase, the molecules are orientated uniformly, as in a solid crystal, but retain the ability to � ow and move, as in a liquid.

Examples of substances that possess liquid crystal properties under certain conditions include graphite, cellulose, DNA and the solution secreted by spiders to make their silk. These substances do not necessarily display liquid crystal properties in their standard states.

Thermotropic liquid crystalsMaterials that show thermotropic properties are pure substances that exist in the liquid crystal phase only over a certain temperature range between the true solid and liquid phases. If the temperature rises too far, the molecular orientation is disrupted as the molecules gain kinetic energy, and a liquid forms; too low a temperature causes the substances to form a normal solid crystal with no � uidic properties. The biphenyl nitriles used in some liquid crystal displays are examples of thermotropic liquid crystals.

4-cyano-4'-pentylbiphenyl (Figure C12) is a liquid crystal between 18 and 36 °C, giving it liquid crystal properties at room temperature. Other

Test yourself7 Write ionic half equations for the following

reactions: a the reaction that occurs at the electrode

attached to the negative side of a power supply when a lead–acid battery is charged.

b the reaction that occurs at the electrode attached to the positive side of a power supply when a NiCad battery is charged.

8 Write an overall equation for the reaction that occurs when a lithium-ion cell is charged.

Learning objectives

• Describe the behaviour of thermotropic and lyotropic liquid crystals

• Explain the functioning of a liquid crystal in terms of the arrangement of the molecules

• Understand how a liquid crystal display works


compounds such as para-azoxyanisole (PAA) were instrumental in the development of LCDs, although they are liquid crystal over less useful temperature ranges; PAA exists in the liquid crystal phase between 118 and 136 °C.

The two molecules above could be roughly described as rod-shaped; they are signi� cantly longer in one direction than the other two. In the liquid crystal phase, these rods show some degree of alignment, with the rod-shaped molecules, on average, pointing in the same direction (Figure C13). However, there is no speci� c positional order, i.e. the molecules are positioned randomly relative to each other, so they can � ow past each other. This phase is called the nematic phase.

biphenyl group

nitrile group

C N

Figure C12 The liquid crystal molecule 4-cyano-4'-pentylbiphenyl, a member of the biphenyl nitriles.

a b c

Lyotropic liquid crystalsThese are formed by materials depending on the concentration of the compound in solution. The detergent molecules in soap show lyotropic properties; molecules such as sodium stearate (C17H35COO−Na+) contain a long hydrophobic, non-polar hydrocarbon chain and a polar, hydrophilic carboxylate group. In a dilute aqueous solution, the distance between the molecules is great, and they show no semblance of ordered orientation; however, when the concentration increases, the molecules begin to line up in a speci� c manner owing to the formation of intermolecular forces and the minimisation of the interaction between the hydrophobic chains and the water. Soap molecules and related compounds, such as the phospholipids found in cell membranes, can form micelles (spheres) or bilayers to maximise favourable intermolecular forces (Figure C14).

Figure C13 The orientation and distribution of molecules in a thermotropic liquid crystal: (a) solid – regular arrangement and orientation; (b) nematic liquid crystal phase – random arrangement but fairly regular orientation; and (c) above the liquid crystal temperature range – random orientation and arrangement.

Molecules that exhibit liquid properties are often rod-shaped and polar.

Examiner’s tipIn IB examinations, orientational order is alternatively referred to as directional order.

Micelles can position themselves in an ordered arrangement that shows liquid crystal properties. At higher concentrations, micelles join to form bilayers that can stack to form a lamellar (layered) phase liquid crystal.


Liquid crystal displaysThe ability of the molecules in a liquid crystal to transmit light is dependent on their orientation. As the molecules are polar, their orientation can be controlled by applying a low voltage. In the LCD, a small electrical � eld is applied to a thin � lm of the liquid crystal material held between two layers. By altering the orientation of the crystals using an electrical � eld, the areas of the display that can and cannot transmit light, and thus appear light or dark in colour, are controlled.

Current applications for LCDs include pocket calculators, digital watches and laptop computer screens. Liquid crystal displays are ideal for these purposes because of the requirement for only a tiny electrical current – making them more energy e� cient than other types of display. The main problems with liquid crystal displays are that they can be damaged fairly easily and they only operate over the temperature range in which the molecules exist in the liquid crystal phase; extreme hot and cold temperatures will temporarily disable an LCD.

micelle

bilayer sheet

Figure C14 Two structural forms that can be adopted by lyotropic liquid crystal molecules.

Test yourself9 Explain which of the following molecules would be more likely to

show liquid crystal properties and be useful for a liquid crystal display.

I II

C N

The ideal properties for a compound that can be used successfully in an LCD are:• chemically stable• existing in the liquid crystal

phase over a suitable and wide range of temperatures

• polar, so that they change orientation when an electrical � eld is applied

• rapid switching speed between orientations

C7 Nanotechnology

What is nanotechnology?When the term ‘nanotechnology’ is mentioned, most of our thoughts would wander to some sort of science-� ction future with microscopic robots and super-fast computers. The truth is rather di� erent, but nanotechnology is certainly out there and with us to stay. Nanotechnology deals with the research and development of technology at the ultramicroscopic level between 1 and 100 nm (1 nm is one billionth of a metre). Nanotechnology creates and uses structures possessing novel

Learning objectives

• Describe what nanotechnology is and how physical and chemical methods can be used

• Describe the structure and properties of carbon nanotubes

• Discuss the implications of nanotechnology


properties because of their small size. Nanotechnology builds on the ability to control or manipulate at the atomic scale.

Atoms can be manipulated or moved into position by both chemical and physical techniques. Although the outcome may be similar, the routes to success are quite di� erent. Chemical manipulation relies on the use of speci� c chemical reactions or interactions to position the atoms in a molecule. For example, DNA nanotechnology uses the intermolecular hydrogen bonding rules of base pairing to yield self-assembling branched DNA complexes with potentially useful applications in computing and nanorobotics. Physical manipulation positions atoms to speci� c requirements. A powerful piece of technology called atomic force microscopy (AFM), which has multiple applications, including three-dimensional visualisation of the surface of individual molecules, has been used to deposit individual atoms in speci� c positions.

Carbon nanotubesCarbon nanotubes are allotropes of carbon and members of the fullerene family. Rather than having carbon atoms arranged in a spherical shape, as in C60 fullerene (see Chapter 3, page 135 of the Coursebook, they form a cylinder made only of hexagons of carbon atoms, much like in the layers of graphite (Figure C15). Some carbon nanotubes are closed at the end and some are open. In order to allow the ends of the tubes to be sealed, pentagons must also be present in the structure.

It is possible to create single-walled nanotubes (SWNTs) and multi-walled nanotubes (MWNTs). In MWNTs, there is a concentric arrangement of two or more nanotubes. Physical properties, including electrical conductivity, are dependent on the arrangements of the tubes. MWNTs are signi� cantly more resistant to chemicals than are SWNTs.

A single nanotube is an extremely strong structure, because only covalent bonds are present between the atoms. Nanotubes are among the strongest and sti� est materials ever created. In tests, SWNTs exhibit a tensile strength up to 50 gigapascals (compare with stainless steel, up to 3 GPa).

It may be possible to make bundles of these tubes that also show exceptional mechanical properties, but the challenge is to produce su� ciently long and well-aligned � bres so that the strength of the bundle as a whole relies more on the strength of the covalent bonds between atoms in the individual nanotubes rather than the van der Waals’ forces between tubes. This can be compared with graphite, in which, although each individual layer (called graphene) has exceptional mechanical properties (it is very strong), when the layers come together to form graphite, a soft substance used as a dry lubricant is produced because of the weak van der Waals’ forces between the layers.

As in graphite, carbon nanotubes contain delocalised electrons as a result of the presence of only sp2 hybridised carbon atoms and overlapping, electron-containing p-orbitals across the entire interconnected network of atoms.

The degree to which a carbon nanotube conducts electricity is dependent on its length, as a result of changes in the behaviour of the delocalised electrons. This is because quantum-level e� ects predominate at the nanoscale. Some nanotubes are full conductors, whereas some exhibit semiconducting properties; the use of nanotubes in electronic circuitry has been long proposed.

Physical techniques: atoms are manipulated and positioned to speci� c requirements.Chemical techniques: atoms are positioned in molecules using chemical reactions.

A network of strong covalent bonds extends throughout the entire length of the tube.

Figure C15 A capped single-walled carbon nanotube. Note the hexagons that make up the main body of the tube and the inclusion of pentagons at the capped ends.

HL


Implications of nanotechnologyOther uses of carbon nanotubes include as a synthetic � bre in conjunction with existing polymers to improve many of the desirable physical properties of these materials. Carbon nanotubes can also be used as highly e� ective heterogeneous catalysts, as they have a large surface area because both the outside and inside surfaces are available for binding to reactants. Nanotubes could be developed into specialist � lters, in which the diameter of the tubes is set to permit the passage of particles up to a certain maximum size, for example in the desalination of water by allowing small water molecules through but excluding larger chloride ions.

There are some serious concerns about nanotechnology in the health arena. Determining the toxicity of nanotubes and nanoscale particles is di� cult, as their properties depend on their size. There is speculation that they could cross cell membranes and thereby induce harmful e� ects. Concerns have surfaced that the human immune system would not be able to recognise particles on the nanoscale and would be defenceless against them. The similarity between carbon nanotubes and asbestos threads has been noted and has led to worries that nanotubes might be able to cause respiratory and other health issues in the manner that asbestos does.

Currently, the technology is so new that not enough is known about the potential implications for human health. New materials being created may have new and unforeseen health risks. Thorough testing and regulation will be essential. Government regulatory bodies and the industry itself will need to take responsibility for the safe introduction of nanotechnology into the world at-large. Public education would go some way to allaying fears and reducing risk factors, if and when nanotechnology enters the public domain. Informed debate and public involvement in policymaking will be crucial.

C8 Condensation polymers

Addition polymerisation vs condensation polymerisationAddition polymerisation involves adding together many alkene monomers to form a long polymer chain. There is no loss of small molecules when monomers join, and the resulting polymer contains repeating units joined by carbon–carbon single bonds, e.g. poly(ethene).

Condensation polymerisation involves the joining of molecules that contain two reactive functional groups, e.g. dicarboxylic acids, dihydric alcohols and amino acids. In the process, a molecule of water, or another small molecule, is eliminated at each junction (Figure C16).

Other interesting proposed applications of nanotechnology include: paper batteries, synthesis of chemical nanowires, visual displays, solar cells, stealth technology, and many, many more.

HLLearning objectives

• Describe the structure and formation of condensation polymers using speci� c named examples

• Understand how structure a� ects the properties and uses of a polymer

• Understand how modi� cation of condensation polymers a� ects their properties and uses

• Discuss the advantages and disadvantages of the use of condensation polymers

See also Chapter 10, page 473 of the Coursebook.

Polyesters are formed when monomers with carboxylic acid and alcohol groups react; polyamides are formed when monomers with carboxylic acid and amine groups react together.


CO

OH

H H

CO

OO

ester group

H H

CO

OO

dicarboxylic acid dicarboxylic aciddihydric alcohol dihydric alcohol

H H

CO

OO

chain cancontinue

condensationreaction

chain cancontinue O H

CO

OHC

O

OC

O

OC

O

OH OH O H H OH O HH

Phenol–methanal plasticsPhenol dissolved in methanal is able to undergo condensation polymerisation, catalysed by either acid or base, to yield a pink-coloured, tacky thermosetting plastic. The polymerisation occurs in two stages.1 Phenol reacts with methanal to form a mixture of 2-(hydroxymethyl)

phenol and 4-(hydroxymethyl)phenol with a small number of polyhydroxymethylated derivatives (Figure C17).

2 The hydroxymethyl group reacts with either the 2- or 4- position of a phenyl group (benzene ring) in another molecule, eliminating a molecule of water in the process (Figure C18). Multiple reactions at the 2- positions build up long linear chains of repeating units; reaction at the 4- position creates cross-links between chains and creates a complex three-dimensional structure (Figure C19).

Figure C16 Formation of a polyester by a condensation polymerisation reaction.

The structure of phenol:

In the presence of a base catalyst, ether (C–O–C) bridges can also be formed between the monomers, as well as methylene (–CH2–) bridges.

OH

OH

CH2OH

CH2OH

+

CH2OH

C OH

H

OH

CH2OHHOH2C

OH

CH2OHHOH2C

OH

OH

CH2OH

OH

CH2OH

4-position

2-position

++

C

OH OH

C

OHOH

OH

H2O

H

H

H

H

Figure C17 The reaction of methanal with phenol, forming a mixture of products that can undergo polymerisation.

Figure C18 The polymerisation reaction in the formation of a phenol–methanal polymer.

HL

One of the earliest created phenol–methanal plastics was Bakelite, used for many purposes, including casings for electrical items such as radios and telephones.


polymer chain

cross-link

polymer chain

OH OH

OH OH

CH2

CH2 CH2

CH2 CH2CH2

OH OH

CH2

OH OH

CH2 CH2CH2

PolyurethanePolyurethane polymers are created from the reaction between a diisocyanate and a dihydric alcohol in the presence of a catalyst.

The resulting polymer contains monomers joined together by carbamate linkages (Figure C20).

The isocyanate functional group is R–NCO.

See also Chapter 10, page 480 of the Coursebook.

diisocyanate

carbamate linkage

+

O C C ON N

N

H

CC N

H

HO OH

O

On

O

O

Figure C20 The polymerisation of a diisocyanate and a dihydric alcohol to yield a polyurethane polymer.

The formation of polyurethane is classi� ed under condensation polymerisation on the IB syllabus; however, polyurethane production does not yield water (or any other small molecules) during formation. So, should polyurethane be classi� ed as an addition polymer? The problem arises because the classi� cation of polymerisation reactions as addition and condensation is not the only way of classifying reactions. Reactions can also be, perhaps more usefully, classi� ed according to the mechanism of polymerisation. Polymers formed in chain reactions include the addition polymers that we have met, whereas polymers formed in stepwise reactions include the condensation polymerisation we have met and polyurethane.

Polyurethanes have a wide spectrum of uses, including foam seating and insulation, elastane � bres, seals and carpet underlay.

Polyethylene terephthalatePolyethylene terephthalate (PET) is a polyester formed by the reaction of benzene-1,4-dioic acid (terephthalic acid) with ethane-1,2-diol (Figure C21).

PET is highly resistant to a number of solvents and is easily moulded. It is an ideal material for making drinks bottles, wrapping � lms and similar products.

Figure C19 Part of the structure of a phenol–methanal polymer.

HL


Properties of condensation polymersAs explored in Section C3 on page 12, the structure of a polymer has a signi� cant impact on its physical properties – e.g. atactic vs isotactic polypropene. The same is true for many polymers formed by condensation reactions.

KevlarKevlar is a para-aramid polymer (‘aramid’ is short for ‘aromatic amide’) created by the reaction between 1,4-diaminobenzene and terephthaloyl chloride (the diacyl chloride of benzene-1,4-dioic acid) (Figure C22).

The polymer chains align themselves in such a way that allows for the formation of comparatively strong intermolecular hydrogen bonds between amide groups (C=O to H–N) along the whole chain (Figure C23). Additional intermolecular interactions arise in the form of extensive van der Waals’ forces. These hold the polymer chains together very tightly and contribute to Kevlar’s high tensile strength.

CI

ONH2

NH2

CI

OC

C

a b

Figure C22 The two monomer units for the production of Kevlar: (a) 1,4-diaminobenzene and (b) teraphthaloyl chloride.

An acyl chloride has a similar structure to a carboxylic acid, except that the OH group has been replaced with a Cl. This means that in the formation of Kevlar, HCl, rather than water, is eliminated.

Kevlar is used in protective clothing, including armour, synthetic ropes and sporting equipment.

hydrogenbond

N N

H

O

H

C

O

C

C C

O

H

O

N N

H

O

H

C

O

CN

H

N

Phenol–methanal plasticsIn Kevlar, intermolecular forces hold the polymer chains together. As we saw in Figure C19, in phenol–methanal plastics there are covalent bonds, called cross-links, between chains. As covalent bonds are much stronger than intermolecular forces, cross-linked phenol–methanal plastics are harder and more rigid than Kevlar; hence their use in making items such as pool balls.

Figure C23 A short double-fi bre section of the condensation polymer Kevlar.

benzene-1,4-dicarboxylic acid

PET

ethane-1,2-diol

heat

+

C C H H

H

H

O+ Cn n

O

O

O

OH H

O

C C

O

O

O

OH

H

H

C

H

n

(2n–1) H2O

H

H

C O

H

H

C

Figure C21 The formation of polyethylene terephthalate (PET).

To create a polymer with di� ering levels of cross-linking, the methanal : phenol ratio is varied. If the molar ratio of methanal : phenol is less than one, few cross-links form. However, when the molar ratio rises to about 1.5, the incidence of cross-linking is much higher and the plastic produced is far more rigid, thermostable and resistant to chemicals.

HL


Modifi cation of polymersThe properties of many polymers can be modi� ed during manufacture to produce materials with more speci� c applications. On pages 13–14, we looked at expanded polystyrene as a protective packing material due to its high shock-absorption capacity, compared with non-expanded polystyrene used to make model kits and CD cases.

Polyurethane foamPolyurethanes possess the desirable quality of being able to be converted into a foam using a blowing agent. Water is added to the mixture of diisocyanate and dihydric alcohol. Water reacts with the diisocyanate, producing carbon dioxide gas. As the polyurethane forms, the CO2 bubbles cause it to expand before it solidi� es. Other blowing agents that are also used include highly volatile liquids that boil during the polymerisation process, giving o� large quantities of gas. The size and number of bubbles can be varied to yield foams of di� ering strengths and densities, depending on the intended application. The foams produced can be used in seating or for insulation.

PolyethyneWhen ethyne (HC=−CH) is polymerised, one of the π bonds in the carbon=−carbon triple bond is broken, and the resulting polymer contains alternating double and single bonds. This forms an extensive conjugated system along the entire length of the polymer chain (Figure C24).

In this unadulterated form, polyethyne is a semiconductor; the electrons are not truly free to move, as there are no ‘gaps’ for them to move into. However, doping the polymer with iodine leads to oxidation of the polymer, and one or more electrons are lost from the system. This creates a space for other electrons to � ll, and so the π electrons can move freely from one end of the chain to the other. Doping polyethyne with iodine increases its conductivity by almost one billion times, making it as good a conductor as the best conducting metals, silver and copper. Doping of polyethyne with a group 1 metal has a similar e� ect by adding extra electrons, which can move along the chain. The process is similar to doping silicon semiconductors with group 3 or group 5 atoms (see Section C10).

PolyestersWhen woven alone, polyester � bres can often feel uncomfortable to wear and do not retain coloured dyes well (they are not dye-fast). By blending polyester � bres with other synthetic or natural � bres such as cotton, comfort is signi� cantly increased and the ability of blended � bre to retain dyes is increased.

Examiner’s tipAir is stated as the blowing agent for the manufacture of polyurethane foams on the IB syllabus.

alternatingcreates a conjugated system

CC

H

H

CC

H

H

CC

C/CC C

H

H

Figure C24 A section of the polymer, polyethyne, comprising three repeat units.

A conjugated system is a series of alternating single and double bonds. p orbitals can overlap side-on to produce a π delocalised system – see Option A on the CD-ROM for more details.

HL


Test yourself

C9 Mechanisms in the organic chemicals industry

Manufacture of low-density polyetheneLow-density polyethene (LDPE) is manufactured via a free-radical mechanism initiated by the introduction of oxygen or an organic peroxide to ethene at a high temperature (>300 °C) and extreme pressure (~2000 atm).

As with other free radical mechanisms, polymerisation of ethene to LDPE involves initiation, propagation and termination steps.

InitiationThe weak O–O bond in the organic peroxide initiator (R–O–O–R) undergoes homolytic � ssion under high temperature to yield two free-radical species:

R–O–O–R → 2RO•

Learning objectives

• Describe the free radical mechanism for the production of low-density polyethene

• Outline the use of Ziegler–Natta catalysts in the production of high-density polyethene

Advantages and disadvantages of polymer use HL

Advantages Disadvantages

strength – Kevlar and phenol–methanal plastics are extremely strong and durable

use of natural resources – many aromatic compounds are derived from crude oil

density – strength : density ratio is greater than that of metals

disposal – PET is extensively recycled disposal – incineration creates signifi cant quantities of polluting gases such as CO and CO2; phenol–methanal plastics are thermosets and cannot be re-melted to make new articles because of the presence of the cross-links

insulation – incorporation of air pockets in polyurethane foam gives it excellent insulating properties

chemical resistance – ester and amide linkages are highly resistant to most chemicals except concentrated acids and alkalis

biodegradability – owing to their chemical resistance, polymers are not broken down by moisture and/or bacteria; landfi ll requirements are large, polluting and unsightly

Table C6 Advantages and disadvantages of condensation polymers.

10 Draw the repeat unit of the polymer formed when the two molecules shown below react. Name the functional groups in the monomers, the linkage formed between the groups and the type of polymer formed.

11 Write an equation for the formation of Kevlar using the monomers given in Figure C22. Name the functional group that links the monomers together and the type of polymer formed.

H

C N

HH

N

H

C

H

C

H

H

C

H

H

C

HH

O OH H

H

CC O +CO


Extension

PropagationThe free radical formed then reacts with an ethene molecule, breaking the π bond; and in the process, a single covalent bond is created between the � rst carbon and the radical’s oxygen atom. This produces an unpaired electron at the C at the end of the chain:

H

C

HHn

RO+

H

CRO

H

C

H

H

C

H

H

C

HHn

H

CRO OR

H

C

H

H

C

H

H

C

HHn

+

H

CRO

H

C

H

H

C

H

H

C

HHm

H

CRO

H

C

H

H

C

H m

H

C

H

H

C

H

H

C

HHn

H

CRO OR

H

C

H

H

C

H H

H

C

H

H

C

or

R O CH2 CH2 R O CH2 CH2

H2C CH2

CH2 CH2chain flexes H-atom transferred

CH2

R O CH CH2+ H2C CH2

R O CH CH2

CH3 CH2

CH2

CH2

CH3R O CH CH2 CH2 CH3chain straightens

H

C

HH

RO +

H

C

H

C

HH

RO

H

C

H

C

HH

RO +

H

C

H

C

H

H

C

H

H

C

HH

RO

H

C

H

C

H

H

C

H

The unpaired electron interacts with the π bond of another ethene molecule, creating a C-C single bond and an unpaired electron at the end of the chain:

This cycle of breaking the π bond and forming a C–C single bond to extend the chain continues until termination occurs.

TerminationTermination can occur in a number of ways when two free radicals combine. For example:

HL

Branching in LDPE occurs via a mechanism called backbiting, in which the free radical centre is relocated to a position within the growing chain, not at the end. This happens when the chain twists back on itself and an intramolecular reaction occurs, transferring

a hydrogen atom from a methylene (CH2) group to the end of the chain. When the unpaired electron then reacts with an ethene molecule, it starts to create a side chain rather than extend the main chain:


Manufacture of high-density polyetheneHigh-density polyethene (HDPE) contains very little branching and is therefore more crystalline with a higher density than LDPE and increased rigidity. The polymer chains are better able to align with each other and maximise contact points, increasing the strength of the van der Waals’ interactions. Manufacture of HDPE is by a mechanism completely di� erent from the free radical polymerisaton that yields LDPE.

HDPE is polymerised under low pressure (1–30 atm) and reduced temperature in the presence of a specialised catalyst called a Ziegler–Natta catalyst.

The mechanism is complex and not fully understood, but polymerisation occurs via ionic intermediates in which the incoming ethene monomers coordinate with the titanium in the catalyst, which is already attached to the nascent chain; no free radicals are involved. In this manner, the addition of monomers is highly controlled and the incidence of branching is reduced.

A proposed mechanism is as follows. An ethyl group from Al(C2H5)3 coordinates to the Ti ion. The ligand is C2H5

−, hence the ionic nature of the mechanism. Ethene coordinates to the Ti using the π electrons from the double bond:

Owing to the control that Ziegler–Natta catalysts impart to polymerisation of alk-1-enes, they are used to control the tacticity of polymers such as polypropene, increasing the yield of isotactic structures rather than the less useful atactic variety.

ClCl

ClCH2 HH

HH

CH3

TiC

C+

ClCl

ClCH2 HH

HH

CH3

TiC

C

The C2H5− group then migrates to the ethane, lengthening the

polymer chain:

ClCl

ClCH2

CH2

CH2

CH3

Ti

ClCl

ClCH2 HH

HH

CH3

TiC

C

There is now a free space on the Ti for coordination of another ethene.This type of polymerisation results in less chain branching and a more

regular, crystalline product, as the ligands around the Ti prevent branching.

Test yourself 12 Draw the mechanism for the polymerisation of propene under

high temperature and pressure using an organic peroxide initiator.

HL Ziegler–Natta catalysts are generally a mixture of titanium(III) chloride (TiCl3) or titanium(IV) chloride (TiCl4), with an organoaluminium compound such as triethylaluminium (CH3CH2)3Al.


C10 Silicon and photovoltaic cells

Silicon is a semiconductorSilicon is a silvery, non-metal with a giant covalent structure similar to that of diamond (see Chapter 3, page 133 of the Coursebook. Electrons within the structure of silicon are � xed in position within the covalent bonds; this makes silicon a poor electrical conductor under standard conditions. However, under certain conditions, silicon acts as a semiconductor, with the ability to conduct electricity to a degree that can be used in electrical circuits.

The electrical conductivity of solids can be described in terms of band theory. The outer orbitals of the atoms making up a solid overlap to form valence and conduction bands of orbitals:

Learning objectives

• Describe how the doping of silicon is used to produce p- and n-type semiconductors

• Describe how sunlight enhances electrical conductivity in a semiconductor

conduction band

band gap

insulator semiconductor conductor

valence band

conduction band

valence band

conduction band

valence band

conduction band

empty conductionband orbitals

closely spacedorbitals containingpairs of electrons

valence band

These bands consist of a very large number of orbitals that are closely spaced in energy.

At absolute zero in insulators and semiconductors, the valence band containing the outer shell electrons of the solid is full, whereas the conduction band is empty. Because the valence band is full, there is nowhere for the electrons to move, and neither insulators nor semiconductors conduct electricity at absolute zero. However, the band gap in silicon is su� ciently small that as the temperature is raised some electrons are promoted to the conduction band and become free to move. The electrical conductivity of silicon thus increases as the temperature increases. The promotion of electrons from the valence band also creates ‘holes’ in this band that other electrons can move into; this also contributes to the electrical conductivity (Figure C25).

electronmovement

hole

Si Si Si SiSi Si Si Si

holemovement

hole

Si Si Si SiSi Si Si Si

Figure C25 The movement of a negatively charged electron in one direction is equivalent to the movement of a positively charged hole in the opposite direction.

A hole is regarded as a positive charge carrier – if an electron has been removed, what is left has a positive charge.

In an insulator, the energy gap between valence and conduction bands is large, and at normal temperatures electrons cannot jump to the higher level. In a metal, the conduction and valence bands overlap, and the metal is a good conductor of electricity, as a large number of electrons are free to move.

Doping of siliconThe electrical conductivity of silicon can be increased by incorporating atoms of group 3 or 5 elements as impurities into the lattice. This process is called doping.

Inclusion of a small proportion of a group 3 element such as boron, aluminium or gallium into the silicon structure creates a p-type semiconductor (where ‘p’ stands for positive). These group 3 atoms

HL


supply only three electrons to the valence band instead of four (as supplied by Si), and therefore there is a hole in the valence band. The introduction of extra gaps in the valence band increases the electrical conductivity of the silicon. Because the main charge carriers are positively charged holes, this is now called a p-type semiconductor (Figure C26).

Incorporation of a group 5 element (e.g. phosphorus, arsenic or antimony) into the silicon lattice also increases the electrical conductivity of the semiconductor. These atoms have � ve valence electrons, of which four are used in forming covalent bonds and go into the valence band, but the � fth is promoted easily to the conduction band. As the current is carried by negatively charged electrons, it is termed an n-type semiconductor.

The interaction of sunlight with siliconWe have seen above that electrons can be promoted from the valence band to the conduction band of a semiconductor by thermal energy. Photons of light with wavelength shorter than about 1100 nm have su� cient energy to cause electrons to be promoted from the valence band to the conduction band and thus increase the electrical conductivity of silicon. This e� ect is used in photovoltaic cells (solar cells).

Figure C26 The movement of holes in p-type semiconductors.

Visible light has wavelengths between 400 and 750 nm.

Test yourself 13 When silicon is doped with the following atoms, would n- or

p-type semiconductors be produced? a Al b In c As d Sb

HL

Extension

In photovoltaic cells, such as the solar cells found in calculators and other small electronic devices, a p-type semiconductor and an n-type semiconductor are joined together. When these two types of semiconductor (both neutral) are joined, electrons move from the n-type to the p-type semiconductor, where they combine with some holes; they leave behind positively charged ions. Holes move from the p-type to the n-type semiconductor and combine with the additional electrons, leaving behind negative charge. There is thus a build up of charge at the p–n junction that prevents

any further movement of charge across the junction (Figure C27).

When light hits the n-type semiconductor, electrons are promoted to the conduction band. These electrons are not able to � ow from the n-type to the p-type directly, because the voltage at the junction prevents this. They must therefore travel from the n-type to the p-type through the external circuit, and this can be used to power an electrical device (Figure C28).

p-typesemiconductor

depletionlayer

+–

+–

e–

hole

e– e– e–

p-typesemiconductor

junction voltageprevents movementof electrons from n pand of holes from p n

n-typesemiconductor

n-typesemiconductor

depletionlayer

+–

+–

p-typesemiconductor

externalcircuit

n-typesemiconductor

light

e–

e–

Figure C27 The build up of charge at a p–n junction produces a layer that prevents any more movement of charge across the junction.

Figure C28 How a photovoltaic cell can produce a current.

electronmovement

hole

Si Si Si SiSi B Si Si

holemovement

hole

Si Si Si SiSi B Si Si


C11 Liquid crystals

Identifying liquid crystalsMolecules that display liquid crystal properties generally possess a number of structural features in common. The biphenyl nitriles can be used to illustrate this. Molecules such as 4-cyano-4'-pentylbiphenyl (Figure C29a), have a nitrile group at one end which makes the molecules polar. This provides a strong enough intermolecular force to align the molecules in the same direction, but not too strong that they only form solid crystals. The two phenyl groups provide signi� cant rigidity (Figure C29b) and make the molecule rod-shaped; the long alkyl chain ensures that the molecules cannot pack too closely together, and this helps maintain the liquid crystal phase (rather than forming the solid).

Examples of other molecules that possess similar features and display liquid crystal behaviour are shown in Figure C30.

Learning objectives

• Identify molecules with liquid crystal properties

• Describe how a twisted nematic liquid crystal works and is used in liquid crystal displays

• Understand why Kevlar exhibits liquid crystal properties in acid

biphenyl group

alkyl chain

a

b

C Nδ

C CC

CC

C

+ δ –

C CC

CC

C

HO

H H

H

ba

N

O

Figure C29 (a) 4-cyano-4’-pentylbiphenyl; (b) overlap of p orbitals between the benzene rings means that the rings are less likely to rotate relative to each other.

Figure C30 Molecules that display liquid crystal properties: (a) cholesterol; (b) N-(4-methoxybenzylidene)-4-butylaniline (MBBA).

Twisted nematic liquid crystalThe twisted nematic liquid crystal cell is the basic unit behind the functioning of a simple liquid crystal display (Figure C31). The cell comprises a thin � lm of the liquid crystal sandwiched between two glass plates, each layered with a polariser on the external surface and a transparent electrode (a coating of indium tin oxide) on the inner surface. The inner surfaces of the glass plates are covered in thousands of tiny

HL


parallel scratches. The liquid crystal molecules positioned close to the plates line up with these scratches. In the twisted nematic cell, the top glass plate is rotated exactly 90° relative to the bottom plate. This causes the liquid crystal molecules to adopt a twisted arrangment as they try to align with both sets of scratches.

light cannot pass throughsecond polariser - displayappears black

liquid crystalmolecules alignedwith electric field

switch closed - voltage appliedacross the liquid crystal

display

polariser at 90°

light passes through– display appears clear

liquid crystal

glass plate with scratches

polariser - polarises the light

light in

glass plate with scratchesat 90° to bottom one

switch open - no voltage

display

Figure C31 A twisted nematic liquid crystal cell commonly found in simple LCD devices.

Light entering the cell is � rst polarised and then passes through the liquid crystal layer. When no electrical � eld is applied and the molecules are in a twisted arrangment, the plane-polarised light is rotated by exactly 90°. This means that when the light hits the top glass plate and polariser it has been rotated the precise amount to pass through the polariser. The image formed appears transparent as the light passes through the cell uninterrupted. However, when an external electrical � eld is applied, the liquid crystal molecules align with the � eld rather than the scratches; this completely obliterates the twisted arrangement. Now, when plane-polarised light passes through the liquid crystal it is no longer rotated. The second polariser angled at 90° to the � rst completely blocks out the light, and the image formed appears dark. This technology is used in the classical digital displays in pocket calculators and wristwatches.

KevlarThe structure of Kevlar has already been discussed on page 29. It consists of long polymer chains held together by strong intermolecular hydrogen bonding formed between N–H and C=O groups in the amide linkages. Under normal conditions, Kevlar shows no liquid crystal properties.

Kevlar contains numerous polar groups, is rigid due to the aromatic rings and is rod-shaped; all prerequisites for liquid crystal molecules. Indeed, in concentrated sulfuric acid, Kevlar acts as a lyotropic liquid crystal, in which the concentration of Kevlar determines the extent of liquid crystal behaviour. At a speci� c concentration, the Kevlar strands will align in the same direction but not form a solid crystal because the intermolecular bonding is not strong enough.

When Kevlar is dissolved in concentrated sulfuric acid, the oxygen and nitrogen atoms in the amide linkage become protonated and the hydrogen bonding is lost. This separates the Kevlar strands.

HL


Test yourself 14 Look at the molecules in Figure C30. Explain why they should

form a liquid crystal phase.

15 Explain why the molecule shown below would be less suitable than the biphenyl nitrile shown in Figure C29 for use in a liquid crystal display.

H3C CH3

C12 The chlor-alkali industryBoth chlorine and sodium hydroxide are extremely important and lucrative chemicals. In the USA, approximately 12 million tonnes each of chlorine and sodium hydroxide are manufactured by the chlor-alkali industry annually. In the USA, this industry is directly worth several billion dollars every year, but end-use sales coming from chlorine chemistry are worth hundreds of billions of dollars. Both chemicals can be manufactured from brine.

Chlorine is a powerful oxidising agent, and so it is di� cult to oxidise chloride ions to chlorine gas chemically. Rather than using chemicals to oxidise chloride ions, electrical energy is used to remove electrons from the ions to form the element. Electrolysis of both molten sodium chloride and brine yields chlorine gas. It is cheaper to electrolyse brine to obtain chlorine, as high temperatures are not required to permit free movement of the ions.

Electrolysis of brine produces three important chemicals: chlorine gas, hydrogen gas and sodium hydroxide. A number of di� erent electrolytic methods have been used to separate the chlorine from the sodium.

Mercury cellIn the mercury cell, a thin layer of � owing mercury is used as the cathode and graphite or titanium as the anodes (Figure C32).

Application of a current through the solution causes negatively charged chloride ions to migrate to the anodes, where they are oxidised to gaseous chlorine, which escapes and is collected:

2Cl−(aq) → Cl2(g) + 2e− Anode

At the cathode, rather than hydrogen ions being reduced, as would be expected given the relative reactivity of hydrogen and sodium, sodium ions are reduced to sodium metal, which then forms an amalgam with the mercury cathode:

Na+(aq) + e−(+ Hg(l)) → Na(l) (+ Hg(l)) Cathode

Owing to di� erences in density, the mercury and amalgam remain at the bottom of the cell, with the brine � oating on top. The metals can be easily tapped o� from the bottom of the vessel, from where they � ow into a second vessel part-� lled with water. As the amalgam mixes with the water, the sodium metal reacts to form hydrogen gas, which escapes from the top of the vessel, and sodium hydroxide solution, which � oats on the surface

Learning objectives

• Describe the production of chlorine gas, sodium hydroxide and hydrogen gas using the mercury, diaphragm and membrane cells

• Understand the environmental impact of these electrolytic cells

• Outline the major uses of the products of the electrolysis of brine

Brine is a concentrated aqueous solution of sodium chloride.

Electrolysis of molten sodium chloride is principally used to produce sodium metal, which cannot be obtained from electrolysis of an aqueous solution.

HL


of the remaining mercury and can be siphoned o� . With the sodium now removed, the mercury can be recycled into the electrolytic cell.

2Na(l) (+ Hg(l)) + 2H2O → 2NaOH(aq) + H2(g) (+ Hg(l))

Although this is an e� ective method for the manufacture of chlorine and sodium hydroxide, it has met with � erce opposition from environmentalists because of the use of toxic mercury. Despite best e� orts to recycle the mercury, several hundred kilos will be lost by each plant every year and end up in the water system. Mercury is highly toxic and can cause neurological disorders in humans and animals, as well as pollute farmland and waterways.

There are two other types of electrolytic cell employed in the chlor-alkali industry, which do not require a mercury cathode: the diaphragm cell and the membrane cell.

Diaphragm cellIn the diaphragm cell (Figure C33), the vessel is separated into two sections by a permeable asbestos diaphragm. Brine is introduced into the anode-containing section of the cell, where oxidation of chloride ions to chlorine gas occurs. The chlorine escapes as a gas and is collected and puri� ed. The remaining brine � ows through the diaphragm and into the cathode-containing section of the cell. Here, at the cathode, water is reduced, forming hydrogen gas and hydroxide ions:

2H2O(l) + 2e− → H2(g) + 2OH−(aq) Cathode

The hydrogen gas escapes and the asbestos diaphragm prevents the hydroxide ions from returning back to the anode-containing side of the vessel.

The level of brine in the anode compartment is higher than that in the cathode compartment – brine � ows through the diaphragm – which helps to keep the hydroxide ions away from the chlorine gas, as the net � ow of liquid is from the anode to the cathode compartment.

Once chlorine and hydrogen gases have been removed, the remaining solution is a mixture of about 12% sodium hydroxide and a slightly higher percentage of unused brine. The sodium hydroxide and sodium chloride

HL

brinein

brineout

waterin

CI2

H2

H2

H2ONaOH/

mercurycathode

graphite/Ti anode

NaOHout

Hg

Hg

Hg

+ + + +

Figure C32 The mercury cell.

Water is easier to reduce than sodium ions (see Chapter 9, page 407 of the Coursebook.

The use of a diaphragm means the chlorine and alkali are kept separate, to avoid reaction between the two and contamination of the solution with hypochlorite (chlorate(I)).

Cl2 + 2OH− → OCl−+ Cl− + H2O


are separated by evaporating away the water. The solubility of NaCl in water is three times lower than that of NaOH (~360 g dm−3 for NaCl and 1110 g dm−3 for NaOH), and so as the water is removed, NaCl crystallises out of solution and can be removed.

Again, there are environmental and health considerations that need to be taken into account with use of the diaphragm cell. Asbestos is the common material used in construction of the diaphragm, and this mineral is associated with some serious respiratory diseases, including asbestosis and mesothelioma, which can prove fatal. In many countries, the use of asbestos diaphragms has been banned by law.

Membrane cellMembrane cell technology has now superseded both mercury and diaphragm cells, as a result of its impressive e� ciency, and because it is far less environmentally damaging. Membrane cells work on a similar principle to the diaphragm cell but use a specialised ion exchange membrane to separate oxidation and reduction reactions (Figure C34).

The membrane is made of � uorinated compounds and allows positive ions to pass through but not negative ions. It thus allows sodium (Na+) ions to pass through but not chloride (Cl−) ions or hydroxide (OH−) ions. As hydroxide ions are produced only in the cathode compartment, chlorine and hydroxide ions are thus kept separate from each other.

brine +sodiumhydroxide

steel cathode

titanium anode

chlorine gas hydrogen gas

asbestos diaphragm

brine

+

flow of brine

Figure C33 The diaphragm cell.

sodiumhydroxidesolution

steel cathode

titanium anode

chlorine gas hydrogen gas

ion permeable membrane

brine

+

Na+ ions

CI– OH–

Figure C34 The membrane cell.

HL


HLAs in the diaphragm cell, the brine is injected into the anode-containing side of the cell, where Cl− ions are oxidised to Cl2. Any unoxidised chloride ions remain on the anode side of the cell, as they cannot move through the membrane. Sodium ions pass across the membrane into the cathode-containing side of the cell; here they combine with hydroxide ions formed by the reduction of water (the same equation as for the diaphragm cell) to yield sodium hydroxide. The main di� erence between the diaphragm and membrane cells is that, in the latter, the unused brine and sodium hydroxide remain separate and so there is no requirement for separating NaCl and NaOH. The concentration of NaOH formed in the cell is almost three times greater in the membrane cell compared with the diaphragm cell (32–34% compared with 12%). The ability to recycle unused brine more easily, and generate a more concentrated NaOH solution, makes the membrane cell a more e� cient and economical technology.

Comparison of cellsThe best available technology is the membrane cell – this produces high purity (although also less than the required concentration) sodium hydroxide without the environmental problems of the other two cells. It is also the most energy-e� cient process. The cells are compared in Table C7.

Mercury cell Diaphragm cell Membrane cell

Construction costs most expensive to build relatively cheap more expensive than diaphragm cell but cheaper than mercury cell

Environmental issues toxic mercury in effl uent

asbestos diaphragms must be replaced regularly – problem with disposal of asbestos

some issues but much less than the other two

Energy consumption most energy effi cient

Purity of NaOH(aq) high purity and required concentration (50%)

lowest purity and lowest concentration high purity but lower than required concentration

Table C7 A comparison of the three cell types.

Uses of chlorine, sodium hydroxide and hydrogenChlorine chemistry is worth hundreds of billions of dollars each year in the USA alone. Much of this is generated from the pharmaceutical industry. Other important industrial areas in which chlorine is important include:

• manufacture of bleaches

• manufacture of acids, e.g. HCl

• agrochemical industry, e.g. insecticides

• organic solvents, including those used in dry cleaning

• plastics industry, e.g. PVC manufacture

• water puri� cation, in swimming pools and main water suppliesThe use of chlorine, especially in the production of organic solvents, is deeply opposed owing to the damaging e� ect these toxic and volatile compounds have on the depletion of the ozone layer. Once in the upper atmosphere, ultraviolet light from the Sun is able bring about homolytic fi ssion of the C–Cl bond. This generates Cl• free radicals, which catalyse the destruction of ozone.

See Option E on the CD-ROM.


Sodium hydroxide is arguably the most important strong base used in the chemicals industry. It is used in a wide range of applications, including:

• re� ning of alumina in aluminium production

• cleaning products and bleaches

• soap manufacture

• making paper.Hydrogen gas is used in the manufacture of ammonia (for fertiliser

production), hydrogenation reactions in syntheses or production of margarine, production of methanol and as a fuel in fuel cells.

Exam-style questions

1 Iron is a common, but highly important, metal used in construction and the manufacture of numerous household items. The metal is obtained by reduction from one of its ores.

a Name and give the chemical formula of one ore of iron. [2]

b The reduction of iron ore takes place in the blast furnace. i Write an overall equation, including state symbols, for the production of molten iron from iron(III)

oxide in the blast furnace. [2] ii Identify the reducing agent in your equation. [1]

c Steel is made by blowing oxygen through molten iron at high pressure in a process called basic oxygen conversion.

i What is the primary purpose of the basic oxygen converter? [1] ii Explain, using balanced equations, how phosphorus and silicon impurities are removed during this

process. [5]

d Expert swordmakers treat steel to make it harder and stronger and less likely to bend under stress. Name a treatment suitable for realising this outcome and explain how it is carried out. [3]

e Another metal made in large quantities is aluminium, produced by electrolysis of alumina. Alumina is produced by re� ning bauxite, which contains, among other things, aluminium hydroxide.

i What property of aluminium hydroxide is crucial in allowing it to be converted to pure alumina? [1] ii Give one advantage and one disadvantage of using graphite anodes in the production of aluminium,

rather than an inert metal. [2]

2 Crude oil is a non-renewable resource worth trillions of dollars annually. The products of crude oil re� ning � nd uses in almost every aspect of everyday life.

a i State two uses of crude oil. [2] ii Give one advantage and one disadvantage for each use given. [4]

b i Name three methods for obtaining alkene molecules from the products of fractional distillation. [3] ii State and explain which of these methods could be used safely in a classroom environment. [2]

HL


3 Addition polymers and plastics are highly versatile products used to make items from garden hoses to prosthetic limbs. Their versatility and wide spectrum of usages derives from di� erent chemical structures and modi� cation processes.

a i What is the major structural di� erence between LDPE and HDPE? [2] ii How does ‘backbiting’ contribute to the formation of LDPE? [1]

b Explain why Te� on (polytetra� uoroethene) exists in only one structural form, whereas PVC exists in a number of di� erent structural forms. [3]

c i Name two methods used in the modi� cation of addition polymers. [2] ii For one of the methods in part i, describe how it is performed and state how the properties of the

product di� er from the starting material. [2]

4 The reaction between aqueous persulfate ions (S2O82−(aq)) and aqueous iodide ions is catalysed by iron(II) ions

in an example of homogeneous catalysis. The catalysed reaction � rst involves the reduction of the persulfate ions to sulfate ions followed by oxidation of the iodide ions to molecular iodine.

a Draw an enthalpy level diagram showing the progress of the catalysed reaction, indicate the species, including state symbols, present at each stage, and also activation energy where needed (assume the reaction to be exothermic). [6]

b Why is heterogeneous catalysis favoured in industrial processes over the more speci� c and e� cient homogeneous catalysis? [1]

c i Carbon nanotubes have a potential use as which type of catalyst? [1] ii Suggest two reasons for this. [2]

5 Electricity can be generated by an electrochemical cell by converting chemical energy to electrical energy. This is generally achieved by the � ow of electrons from a reactive metal electrode to a less reactive metal electrode. Rechargeable batteries work on a slightly di� erent principle, relying on reactions that can be easily reversed by applying a voltage.

a i Other than their ability to covert chemical energy to electrical energy, suggest three similarities between the lead–acid, NiCad and Li-ion rechargeable batteries. [3]

ii In the context of the lead–acid battery, why is it important for a car to be driven regularly rather than left stationary for a long period of time? [2]

iii State the changes in the oxidation states of cadmium and nickel that occur during discharge in a NiCad battery according to the equation:

Cd(s) + 2NiO(OH)(s) + 2H2O(l) Cd(OH)2(s) + 2Ni(OH)2(s) [2]

b Electrical energy can also be produced via the conversion of chemical energy from non-metallic reactants in a fuel cell. Suggest one reason why liquid hydrogen and ethanol can be used as the fuel in a fuel cell, but a solid fuel source cannot. [1]

6 Liquid crystals were discovered towards the end of the 19th century. Since then, advances in liquid crystal technology have been enormous. Colour liquid crystal displays are now commonplace.

a i Explain how the shape and physical properties of biphenyl nitriles allow them to be used in LCDs. [3] ii Describe the e� ect of increasing temperature on the relative orientation of biphenyl nitrile molecules. [1]

b The stearate ion is an example of a lyotropic liquid crystal with behaviour that depends on its concentration in solution. Describe the e� ects on the positional and directional order of molecules of sodium stearate during evaporation of water from a dilute aqueous solution until a solid is formed. [6]

c Explain why liquid crystal displays fail to work properly in overly cold and hot environments. [2]


7 Nanotechnology has the potential to become a world-changing scienti� c discipline, with potential applications ranging from the fairly ordinary to those that would seem more at home in a science-� ction novel. Most nanotechnology revolves around carbon atoms that can be constructed into various shapes, including balls, tubes and pipes.

a How and why does the length of a carbon nanotube a� ect its electrical conductivity? [3]

b Explain in terms of structure and bonding why carbon nanotubes have one of the highest tensile strengths of all known materials, but graphite is soft, with a low tensile strength. [3]

c Suggest two bene� ts and two possible problems associated with the use of nanotechnology. [4]

8 Condensation polymers include well-known compounds such as nylon, polyester and Bakelite, to name but a few. Condensation polymers come in a large variety of structures, with a similarly large range of properties and applications. The polyaramid molecule, Kevlar, is made by reacting a diamine with a diacyl chloride in a condensation reaction.

a i With reference to structure and bonding, explain why Kevlar is useful in protective armour. [3] ii Why must protective clothing made from Kevlar must be replaced if it is ever exposed to acids? [2]

b Phenol–methanal plastics, of which Bakelite is the most famous, can adopt complex three-dimensional structures due to cross-link formation between polymer chains. Draw the structure of a phenol–methanal plastic comprising two cross-linked chains, each containing three aromatic rings. [3]

c The alkyne molecule ethyne, C2H2, contains a carbon=−carbon triple bond and can be polymerised to form a polymer chain comprising alternating C=C double and C–C single bonds.

i Name an element that can be added to polyethyne to increase its electrical conductivity. [1] ii Explain how the incorporation of this element brings about the desired e� ect. [2]

9 The addition polymer polyethene can exist in a number of di� erent structural forms of varying density, rigidity and strength, depending on the extent of branching. Two of these, low-density polyethene (LDPE) and high-density polyethene (HDPE), have speci� c uses dependent on their properties. It is therefore important to be able to manufacture them individually.

a i State three ways (not including mechanistic detail) in which the polymerisation processes that produce LDPE and HDPE are di� erent. [3]

ii State how the polymerisation of ethene to form LDPE and HDPE di� ers mechanistically. [2]

b Describe the movement of electrons during the polymerisation of ethene to form LDPE. [4]

10 Semiconduction is an extremely useful and important property of a small number of elements and compounds. It has been pivotal in the development of silicon chips, computer microprocessors and miniaturised circuitry. Semiconductors are able to conduct an electrical current under certain conditions.

a i Explain, with the aid of a diagram, how sunlight interacts with pure silicon to enhance its electrical conductivity. [4]

ii Why do many other pure non-metals, such as sulfur, not act as semiconductors, but rather insulators, in the presence of sunlight? [3]

b Although silicon possesses semiconducting properties in the presence of sunlight, it is possible to increase its conductivity by doping it with small quantities of other elements.

i Name two elements present in di� erent groups of the periodic table that can be added to silicon to increase its electrical conductivity. [2]

ii Identify and explain which of these elements is used to form a p-type semiconductor and which forms an n-type semiconductor. [6]


11 Many common molecules surprisingly possess the ability to act as liquid crystals due to their chemical structure. Although most of these naturally occurring liquid crystals do not � nd much use in modern liquid-crystal devices, this property is sometimes crucial for their role. Cholesterol is found in animal cell membranes and is important in maintaining the membrane’s integrity across a range of temperatures.

a Describe the structural features that allow cholesterol to behave as a liquid crystal within a certain range of temperatures. [4]

b Synthetic liquid crystal molecules have been designed for modern applications such as liquid crystal displays (LCDs). However, there are still limitations on the conditions under which these devices will operate. With reference to the twisted nematic cell present in LCDs, explain why the screen of a pocket calculator will not function correctly if it is too cold. [4]

12 The chlor-alkali industry is a lucrative and important chemical industry responsible for the production of a number of chemicals used directly or indirectly in a wide spectrum of applications. Chlorine and sodium hydroxide are produced from brine (concentrated sodium chloride solution) irrespective of the particular electrolytic cell used.

a Compare the environmental impact of the mercury, diaphragm and membrane cells. [3]

b State one way in which the diaphragm and membrane cells di� er and one way in which they are similar. [2]

c Suggest reasons why the chlor-alkali industry is so important to the world economy. [4]

Documents

C Chemistry in industry and technology - pedagogics.capedagogics.ca/cambridge/source_data/options/Options_C.pdf · 2012-02-20 · Properties and uses of steel and iron The properties