Embed Size (px)
Citation preview
HSC Chemistry Syllabus Summary
Huon Wilson
June 28, 2011
This document is provided ‘as is’, the author gives no assurance of quality. It can be
freely modified and distributed, in any form, as long as: it is not used for commercial
or monetary gain; this notice remains visible; and, the author is attributed in some clear
manner.
Preface
I wrote these notes over the course of 3 or 4 months as preparation for my trials and the HSC. I triedto go through the whole syllabus1 and give an answer for each point of it (and thought that I might aswell do in such a way that I could use it for revision, hence this document), just so that I know that Iknow everything. So most of the syllabus is ‘answered’, although some points were particularly easy (inmy opinion at least), or were already ingrained in my head from years of repetition in the junior schoolscience courses, and I skipped them so someone using these might want to just check that they’ve goteverything under control. Also, since these are my notes for me, I only did what was applicable to me,as in, only the option that we did at school (Industrial Chemistry).
They are personal notes, but I cleaned them up a very little bit (like, the first chapter’s formattingis still completely different to the rest) and decided to share them2, which would be why you are readingthis. However, being personal notes means that they are imperfect in depth of content (see above),spelling, formatting, exposition. . . everything, really. So, if you see a mistake, please don’t throw yourcomputer around, or eat the piece of paper in a fit of rage. If there is a particularly bad mistakein the content, or an unclear section of writing, you could email me3, preferably with a informativesubject, although I will not answer an email that is asking something along the lines of “teach mechemistry”. If I do correct a mistake (no guarantees of it, though), the latest version will be availablevia sites.google.com/site/somehscsciencenotes, and the date on the front cover will change, soyou can check you’ve got the latest and greatest version (if you care about those sort of things).
There are images of questionable copyright status included, but everything that I didn’t makehas a link to where I got it from, which will have the image in greater context and with more detailedinformation, so have a look, if you are inclined, and the Conquering Chemistry: HSC Course textbookwas pretty helpful as well. If you want to print this, it works best with one-page-per-sheet (otherwisethe text is very small) and double-sided (the layout of the sections and whatever is designed so it workswell as a booklet).
Anyway, have fun (and good luck!) with your HSC.
1Available at: boardofstudies.nsw.edu.au/syllabus_hsc/chemistry.html2If anyone wants the LATEX source, and it isn’t on the website, just email me3 and I will probably send it to you3At [email protected] (No, seriously, I have that email)
3
Contents
Contents 4
1 Production of Materials 51.1 Addition polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51.2 Condensation Polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71.3 Renewable sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81.4 Redox reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91.5 Nuclear chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2 The Acidic Environment 132.1 Indicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.2 Le Chatelier & Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.3 Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152.4 Acids: take two . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162.5 Esterification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
3 Chemical Monitoring and Management 213.1 Chemists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.2 Haber process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.3 Manufactured stuff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.4 The atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.5 Waterways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
4 Industrial Chemistry 334.1 Synthetic replacements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.2 Equilibrium reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.3 Sulfuric acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344.4 Sodium hydroxide and electrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374.5 Saponification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 394.6 The Solvay process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Index 45
4
1 Production of Materials
1.1 Fossil fuels provide both energy and raw materials such as ethylene forthe production of other substances
• Alkanes (CnH2n+2)
– Catalytic cracking: Large alkane (15 – 25 carbons) alkane + alkeneC15H32 C10H22 + C5H10
– Steam cracking: Alkane collection of small alkanes + H2
C11H24 4 C2H4 + C3H6 + H2
– Combustion: Alkane + O2 CO2 + H2Oe.g. C3H8 + 5 O2 3 CO2 + 4 H2O
– With Cl and Br in UV: Alkane + Cl2 or Br2 chloro/bromo-alkane + hydro-bromic/-chloric acidC3H8 + Cl2 C3H7Cl + HClC4H10 + Br2 C4H9Br + HBr
• Alkenes (CnH2n)
– Combustion as per alkanes
– Substitution reactions (always around double bond)
∗ C2H4 + H2Ni
C3H6
∗ C2H4 + HCl C2H5Cl
∗ C2H4+Br2 CH2Br−CH2Br (note: can be used as a test for alkene vs. alkane, alkenedecolours bromine solution)
∗ C2H4 + HOBr CH2Br−CH2−CH2OH
∗ C2H4 + H2OH2SO4 CH3−CH2OH (alkene to alkanol)
– Industrial conversion ethene to ethanol:
C4H2 + H2OH3PO4 @ 300◦C
C2H5OH
– Forming Ethylene glycol (1,2-ethanediol, however note its structure:CH2 CH2
O ):
C2H4 + 12 O2
Ag @ 250◦CCH2CH2O
– Forming vinyl chloride (chloroethane):
2 C2H4 + Cl2 + 12 O2
CuCl2 @ 150◦C2 CH2CHCl + H2O
Cracking
Most polymers start with ethene, thus it is needed. A large proportion of a refinery’s production ispetrol, and other large chains that are not otherwise needed, thus they are cracked to give smaller moreuseful molecules. It can be either catalytic or thermal/steam cracking.
5
1. Production of Materials
Catalytic Catalytic cracking splits long chain alkanes (15 – 25 carbons) into two smaller ones (analkane and an alkene). Controlling reaction conditions gives different amounts of ethene. Catalystsused are zeolites which are crystalline aluminosilicates, i.e. aluminium, silicon, oxygen and some othermetal ions. It is done at 500 ◦C without air, at pressure >1 atm. Catalytic is not enough for fulldemand.
Steam Steam cracking breaks alkanes are split into lots of different small alkenes (1–4 carbons),and hydrogen. Uses very hot pipes (700 – 1000 ◦C) and pressures just >1 atm. The steam is a diluentthat doesn’t react, but allows for higher pressure while maintaining optimum reactant concentrations.
Ethene
Ethene is a major part of most polymers, since it can be reacted with various side chains to form differentpolymers with vastly different properties (e.g. all chlorine gives Teflon, and styrene gives polystyrene).The double bond means it is easy for the molecule to react with copies of itself to create an additionpolymer of which ethene is the monomer.
Polyethene
Polyethene is an addition polymer (known as PE). The double bond breaks open and attaches to theend carbon of another ethene atom. The Gas phase process uses a initiator molecule with an O2
group, which can be bonded to, to start each hydrocarbon chain, as well as high pressure (1000 – 3000atm), and temperature (300 ◦C). Made this way the polymer has high chain branching, which createsLDPE , low density polyethene (the initiator is contained within the final polymer). The Ziegler-Natta process uses lower pressure (a few atm) and temperature (60 ◦C), and a catalyst (mixtureof TiCl3 and a trialkylaluminium like (CH3CH2)3Al). It forms chains with very little chain branching,which is HDPE , high density polyethene. In both, the chains are cumulatively grown from one end,where the initiator/catalyst is. Chains can collide, and exchange a H atom, which means both arestable (one with a ethene on the end, and the other ethane on the end) and the process stops for thesetwo chains. LDPE is used for:
• milk bottles
• soft toys
• cling wrap
HDPE is used for:
• kitchen equipment (utensils, containers)
• rigid toys
• rubbish bins
• carry bags
Vinyl chloride (chloroethane)
Vinyl chloride is CH2−−CH−Cl (ethene with Cl replacing one H). It makes PVC (poly(vinyl chloride)),which is very cheap and very common (maybe second behind PE). PVC is chemically and biologicallyresistant. Used for:
• electrical insulation
• hoses
• pipes (including drainage, sewerage, guttering, downpipes)
6
1.2. Condensation Polymers
Styrene (phenylethene)
Styrene is CH2−−CH−C6H5, ethene with a phenyl side group (which is a benzene ring with a single Hmissing where the ethene is attached). Creates Polystyrene, which is very hard (thus brittle) and clearin its solid form, it can be aerated to form a foam (with closed cells) which has low density, low thermalconductivity. It is used for:
• Car battery cases
• Handles (of tools)
• Furniture
• CD cases
• Drinking cups (as a foam and as a solid)
• Packaging material (as a foam)
Other Addition polymers
• Polypropylene (polypropene); CH2−−CH−CH3; car bumpers, ropes, household goods (inc. chairs,carpets)
• Polyacrylonitrile (acrylics); CH2−−CH−C−−−N; wool substitute (clothing, carpets, blankets etc.)
• Poly(vinyl acetate) (PVA); CH2−−CH−O−CO−CH3; vinyl coatings, paint, adhesives
• polytetrafluroethylene PTFE (Teflon); CF2−−CF2; electrical insulation, non-stick surfaces in cook-ware, screw thread sealant
Correspondence between structure and properties
• Molecular weights. The longer the molecules (and smaller the spread of lengths) the higher theM.P., and hardness of the polymer
• Chain branching. More branching creates an amorphous structure since the molecules cannotpack as closely, thus low density, softer etc. Less branching allows for a crystalline structure,which is harder, more dense, high M.P.
• Chain stiffening. Large side groups on the ethene molecules reduce the flexibility of the polymerby restricting the movement of the chains. Small side chains like the CH3 of polypropylene havea small effect but the large styrene side-group of polystyrene makes it much harder.
• Cross linking. Links between chains can increase the hardness and elasticity of a polymer.
1.2 Some scientists research the extraction of materials from biomass toreduce our dependence on fossil fuels
• Fermentation of glucose to ethanol: C6H12O6
yeast2 C2H5OH + 2 CO2
Need
There is a limited amount of petrochemicals available in the form of fossil fuels, and thus they will runout. Thus new sources (or alternatives) for these chemicals need to be found. Ethanol is very similarto ethene, and can be converted into ethene, it can be produced by fermentation of starch/sugars foundin crops. Cellulose is also an alternative, which forms condensation polymers, and can be turned intopetrochemicals. Cellulose is also more abundant than ethanol in plants.
7
1. Production of Materials
Condensation polymers
Condensation polymers are polymers which involve an elimination of another molecule (usually H2O) intheir formations. For example Glucose (C6H12O6) forms Cellulose (X−O−C6H10O4−O−C6H10O4−O−X)by elimination of an OH from one glucose and an H from the other, which forms water.
Nylons (polyamides) Nylons are synthetic condensations polymers. Form between an aminegroup (Y−NH2) and a carboxylic acid ground (X−COOH), when the OH of the acid and one H fromthe amine are eliminated, making a molecule of the form X−CO−NH−Y. The link is called an amineor peptide link. (Proteins form from amino acids which have an amine and a carboxylic acid at oppositeends). Used in clothing as a fibre, as packaging.
Polyesters Polyesters are also synthetic condensation polymers. Formed when a carboxylic acid(X−COOH) and an alcohol (Y−OH) react, eliminating H2O, forming X−CO−O−Y. Used in clothingas a fibre, as PET bottles.
Cellulose
Cellulose is a natural polymer, formed form glucose (see above). Cellulose is very linear, because ofthe geometry of the bonds and the glucose ring. It makes up a vast majority of biomass, since it is themain component of plants. It can be converted to glucose by acid digestion or enzymes, glucose canbe converted to ethanol, which can make ethene. (Starch can also undergo the same process and is aisomer of cellulose).
Biopolymers
Biopolymers are those made by living organisms (cellulose, and those made by modified bacteria etc.)
Polyhydroxybutyrate (PHB) PHB is made naturally by some bacteria (Alcaligenes eutrophusand Bacillus megaterium). This is slow but can be accelerated by modifying the bacteria and makingthem more like catalysts. PHB is biodegradable (eaten by bacteria), so can be used for medical applica-tions where the material is left within the body and will slowly disappear. PHB is also water insoluble,and resistant to UV (although not to acids or bases). It is not toxic, has similar tensile strength topolypropylene, has a similar melting point. It is very expensive to produce, although improved methodsare being developed.
1.3 Other resources such as ethanol, are readily available from renewableresources such as plants
• Dehydration of ethanol (with heating):
CH3CH2OHconc. H2SO4 or H3PO4−−−−−−−−−−−−−−−→ C2H4 + H2O
• Hydration of ethene (with heating):
C2H4 + H2Odilute H2SO4 CH3CH2OH (note: these apply to any alkene ↔ alkanol reaction)
• Combustion of ethanol:C2H5OH + 3 O2 2 CO2 + 3 H2O ∆H = −1360 kJ/mol
Ethanol
8
1.4. Redox reactions
# Carbons Prefix1 meth-2 eth-3 prop-4 but-5 pent-6 hex-7 hept-8 oct-
Table 1.1: IUPAC carbon count prefixes
Figure 1.1: Zn∣∣Zn 2+
∣∣∣∣∣∣Cu 2+∣∣Cu
Source: en.wikipedia.org/wiki/File:
Galvanic_Cell.svg
Ethanol is often used as a solvent, since it can dissolve both non-polarand polar substances. Ethanol has a polar region, the hydrogen bond ofthe OH group which can dissolve polar things (and means it is misciblein water in any proportion), and a non-polar region, the CH3CH2 re-gion, which can use dispersion forces to dissolve non-polar substances.Ethanol is used as a solvent in cosmetics, food colourings/flavourings,antiseptics, industry. Ethanol can be used as a fuel since it combustseasily, e.g. it is used in small stoves, it can be used in small proportionswith petrol ( < 20%) without engine modifications, it can be made fromglucose e.g. in sugar cane.
Advantages over fossil fuels Ethanol is far more renewable thanfossil fuels, and can maybe reduce net CO2 emmissions.
Disadvantages Large agricultural areas taken over for fuel pro-duction, disposal of fermentation by-products. It was used in the 1970’sand 80’s in Brazil, but the program petered out after a few decades;modern petrol often contains upto 10% ethanol.
1.4 Redox reactionsare increasingly important as a source of energy
N.B. Oxidation is loss, Reduction is gain of electrons. Electrodes: Anode is where oxidation happens, Cathode is where reductionhappens. These are placed in an electrolyte which is a solution whichconducts electricity.
Displacement
A displacement reaction is an exchange of electrons from a solid metal to one in solution so that thedissolved one comes out of solution and the solid goes into solution (e.g. Cu(s)+2 Ag+
(aq) 2 Ag(s)+
Cu 2+(aq)). The solid is oxidised (it loses electrons) and the dissolved one is reduced (it gains electrons).
More reactive metals will displace less reactive ones.
Galvanic cell
A galvanic cell (See Figure 1.1) is a separated redox reactions. The oxidation half-reaction releaseselectrons into the circuit at the anode, which travel to the cathode were they reduce the ion in solution,which deposits on the cathode. The Salt bridge of the galvanic cell is to allow ions to migrate,allowing electrical neutrality to be preserved, otherwise there is a net charge created by the movement
9
1. Production of Materials
of electrons, which will eventually build up into a potential large enough to stop current flow (theelectrolyte in the salt bridge should not form precipitates with other substances, like KNO3 or NaCl ifthere is no Ag).
Figure 1.2: Lechlanche cell
Source: diracdelta.co.uk/science/source/l/e/
leclanchecell/source.html
Dry cell (Leclanche cell)
The dry cell is a basic cell that is very cheap and widely used(Illustrated in Figure 1.2). The zinc is oxidised around thecase (anode) (Zn(s) Zn 2+
(aq) + 2 e – ), and the reduction
reaction on the carbon rod (cathode) is NH+4 (aq)+MnO2(s)+
H2O + e – Mn(OH)3(s) + NH3. That is, zinc goes intosolution, and thus the cell is slightly acidic because of NH4Cl.The cell’s casing is eaten away in operation, and by the acidthat forms, and it is not particularly energy dense, nor is itcapable of delivering large currents. However, it is cheap, ro-bust, has a long shelf life, and is not particularly environmen-tally damaging. The dry cell was the first cell which could beused as a portable source of power and has thus changed theway humans interact with their environment, for example,allowing communication breakthroughs like small transistorradios.
Gratzel cell
Figure 1.3: Gratzel cell
Source: energyer.com/Know_How/
dye-sensitized-solar-cell.html
The Gratzel cell (See Figure 1.3) is a solar cell, also known asa Liquid junction photovoltaic device, or a dye-sensitized so-lar cell. The cell consists of a transparent electrode throughwhich light comes and hits the thin layer of TiO2 dopedwith a photosensitive dye which increases the spectrum ab-sorbed by the TiO2. The light releases a photoelectronfrom the dye which is conducted through the front elec-trode into the circuit. This electron is replaced by one fromthe I – in solution as an electrolyte. The resulting triiodidethen receives an electron returning along the other electrode.TiO2 +hf TiO+
2 +e – and 3 TiO+2 +3 I – 3 TiO2 +I –3 .
The Gratzel cell is a robust device, which can be made verythin (thin enough to be transparent). It is constructed fromfairly common materials and is thus fairly cheap, as well ashaving a very low, even negative environmental impact, sinceit replaces fossil fuels in energy generation. The Gratzel cellalong with other forms of renewable energy will revolutionisesociety by freeing it from dependence on fossil fuels, and re-ducing the environmental impact of energy generation.
1.5 Nuclearchemistry provides a range of materials
Nuclear instability
Elements with too few neutrons and those with too manywill be unstable, as well as those with more than 83 protons(i.e. past bismuth on the periodic table). The stable pro-
10
1.5. Nuclear chemistry
ton/neutron ratio goes from 1 at helium to 1.5 at uranium.Too many neutrons = β decay, Too few neutrons = α decay.
Transuranic elements
Transuranic elements are produced by bombarding large nu-clei with neutrons so that they undergo β decay so their atomic number increases (even largertransuranic elements can be made by bombarding with larger particles like helium or carbon nuclei).
Detection
Radiation can be detected by:
• Photographic film. The film is developed by radiation
• Cloud chamber. A supersaturated vapour (of water or alcohol), which is acted upon by air ionisedby the radiation, to form trails of condensation
• Geiger-Muller counter. Normally measures β particles but can do the others. It works by a gasmolecule being ionised by radiation and the electron flying off is directed and accelerated, givingit enough energy to ionise other gas molecules, cascading until the electrons hit an electrode,generating an electrical pulse that can be detected.
• Scintillation counter. When some materials are irradiated with a specific type of radiation theyemit light, which can be collected, amplified and detected.
Uses/Production
Radioisotopes are produced in nuclear reactors (like technetium-99m and cobalt-60), by bombardmentwith neutrons, and cyclotrons (like iodine-123 and fluorine-18), by bombardment with helium nuclei.Benefits of radiation: more sensitive equipment for industry, reliability (e.g. sterilisation) and newpossibilities for things (like weld fault detection; non-invasive diagnostic and treatment procedures,much more effective treatments). Disadvantages: tissue damage, genetic mutation (including cancers).
Industry Sodium-24. Made by neutron bombardment of sodium-23, decays by β Made by neutronbombardment of sodium-23, decays by β emission into magnesium-24 with a half life of about 15 hours.It is used for leak detection in pipes, since the radiation can easily be picked up, and the short half lifemeans that the fluid the sodium-24 is placed in quickly ceases to be radioactive.
Medicine Cobalt-60. Made by netron bombardment of cobalt-59. Decays by β emission intonickel-60, releasing γ rays in the process, which can penetrate deep enough to kill cancer by destroyingsome specific molecules. It has a half life of about 5 years, which is long enough to allow for extendeduse, but also short enough to have a useful intensity of radiation.
11
2 The Acidic Environment
2.1 Indicators were identified with the observation that the colour of someflowers depends on soil composition
Acidity
A substance is either acidic (pH < 7), neutral (= 7) or basic (> 7). An acid tastes sour, burns the skin,conducts electricity, turn blue litmus red. Alkalis are taste bitter, feel soapy, conduct electricity well,turn red litmus blue.
Indicators
An indicator is a substance that changes colour based on the pH of a substance. They can be used totest and thus regulate soil pH, which plants require to be in certain ranges for best growth. Similarly,they can be used to make sure swimming pools are appropriately basic. Also, they can be used tomonitor waste, since it is of
Colour pH rangeIndicator Low High Low High
methyl orange red yellow 3.1 4.4bromophenol blue yellow blue 3.0 4.6bromocresol green yellow blue 3.8 5.4
methyl red pink yellow 4.4 6.0litmus red blue 5.0 8.0
bromothymol blue yellow blue 6.2 7.6phenol red yellow red 6.8 8.4
thymol blue yellow blue 8.0 9.6phenolphthalein clear red 8.3 10.0
Table 2.1: Common indicators
2.2 While we usually think of the air around us as neutral, theatmosphere naturally contains acidic oxides of carbon, nitrogen andsulfur. The concentrations of these acidic oxides have been increasingsince the Industrial Revolution
Oxides
Oxides can be either acidic or basic. Acid oxides are normally those of non-metals, such as CO2, andP2O5. An oxide is acidic if it reacts with water to create an acid, and/or if it reacts with a base to makesalts (e.g. CO2, NO2, P2O3, SO2,Cl2O, normally non-metals). Basic oxides are those which react withacids to make salts and do not react with alkali solutions (e.g. Na2O, K2O, MgO, normally metals).
13
2. The Acidic Environment
An oxide which makes salts in acids, but also react with alkalis is called an amphoteric oxide (e.g.ZnO, Al2O3, PbO, SnO). Neutral oxides react with neither acids nor bases (e.g. CO, NO, N2O).
Le Chatelier’s principle
Le Chatelier’s principle is the statement that a system will react to disturbance in a way that minimisesthe disturbance. E.g. if a system is heated it will move in an endothermic direction, to absorb energy;if a gaseous system is increased in pressure it will move to reduce the pressure (equivalent to volume)by reducing the number of molecules.
Equilibrium reactions
An equilbrium reaction is a reaction in which does not go to completion, a balance between productsand reactants is reached when the rate of forward reaction and the rate of reverse reaction is equal.An equilibrium reaction is denoted X Y. The position of equilibrium can be anything, not justhalf-half, and for a given set of products it can change depending on factors like temperature, pressureetc.
Carbon dioxide
Carbon dioxide undergoes an equilibrium reaction in water
CO2(g) + H2O(l) H2CO3(aq)
This reaction is exothermic, so increasing the temperature will drive it left to absorb heat, so CO2
has higher solubility at lower temperatures. Increased pressure of CO2 pushes the equilibrium right(forcing CO2 into solution). Adding OH – into the solution increases solubility of CO2, since H2CO3 +2 OH – 2 H2O + CO 2 –
3 , so dihydrogen carbonate is removed from solution, forcing the equilibriumto move to the right to counteract the changing conditions (by Le Chatelier’s principle).
Sulfur dioxide
Sulfur dioxide is mostly generated by volcanoes and other geothermal activity, although some comesfrom human activities like burning fossil fuels, and extracting metals from ores (which often containsulfur). e.g.
2 ZnS + 3 O2 2 ZnO + 2 SO2
Sulfur dioxide is a respiratory irritant which is particularly bad for asthma and emphysema (even whenonly at 1 ppm). Sulfur dioxide forms acid rain (along with nitrogen dioxide), and it also gives air a badsmell.
Oxides of nitrogen
Oxides of nitrogen come in three main types N2O (nitrous oxide), NO (nitric oxide) and NO2 (nitrogendioxide). Nitrous oxide is formed by some bacteria in soil, human fertiliser increase the amount ofnitrogen in the soil and so facilitate the release of this oxide. Much nitric oxide comes from lighting,where the high temperatures allow this reaction to happen N2 + O2 2 NO. It is also formed alongwith nitrogen dioxide in fossil fuel combustion (in cars and power stations), where the high temperatureagain helps. Nitric oxide reacts with oxygen to form nitrogen dioxide, which is an acidic oxide. (Themixture of NO and NO2 is called NOx). Like sulfur dioxide, nitrogen dioxide is a respiratory irritant,although it needs a concentration of about 3 ppm to inhibit breathing, which is very rarely reached.Nitrogen dioxide reacts in sunlight to form ozone in a Photochemical smog, which is very poisonouseven at low concentrations. Nitrogen dioxide also forms acid rain.
14
2.3. Acids
Acid rain
Acid rain is rain which has a pH less than 5 (normal rain has pH between 5 and 6 due to carbonic acidfrom CO2). It is formed when SO2 and NO2 dissolve in rain water, forming sulfuric and nitric acids.
2 NO2 + H2O HNO2 + HNO3
2 HNO2 + O2 2 HNO3
2 SO2 + O2 2 SO3
SO3 + H2O H2SO4
Acid rain can have a pH as low as 3. The acidity of the rain has many detrimental impacts:
• Increased acidity of waterways, which can kill animal and plant life not adapted to the pH, aswell as increasing the ability of the water to leach minerals out of surrounding rocks, increasingthe number of dissolved ions. (Observed in lakes in Scandinavia and North America)
• Damage to vegetation, acid rain can strip trees of foliage, destroying large swathes (Observed inEurope and North America). It can also completely denude an area of vegetation (Queenstownin Tasmania).
• Erosion of CaCO3 based buildings. Carbonate dissolves in acid, so the acid rain ends up dissolvingbuildings and statues, damaging them beyond repair, and sharply accelerating the weatheringprocess (seen in many old buildings and statues in Europe).
• Change soil chemistry, by changing the pH of the soil and the water running through it, acidrain can leach minerals out of soil, and kill bacteria and other micro-organisms, damaging the soilecosystem, possibly even killing larger trees.
Acid rain washes SO2 and NO2 out of the atmosphere so these substances rarely spread a long wayfrom where they were emitted, often restricting the atmospheric problem to a local one, although theeffect on waterways can have far reaching consequences.
2.3 Acids occur in many foods, drinks and even within our stomachs
Acids
An acid is a proton donor, it will react with water to form a hydronium ion H3O+. e.g.
HBr + H2O H3O+ + Br−
pH
pH is a measure of the concentration of hydronium ions, [H3O+].
pH = − log10[H3O+]
pH should have the same number of decimal places as the number of significant figures in the concen-tration of hydronium ion.
Strong & weak acids
An acid is described as strong if it completely ionises in water, there is no molecules retaining a hydrogenatom, e.g. hydrochloric acid. An acid is weak if it doesn’t completely ionise in water, the degree ofionisation is a measure of how much ionises, e.g. citric acid and acetic acid (although citric has a higher
15
2. The Acidic Environment
Acid Formula CommentsNaturalHydrochloric HCl Aids efficient operation of enzymes, as industrial
cleaner, base neutraliserAcetic CH3COOH Found in vinegar, used to synthesis organic com-
poundsCitric C6H8O7 Found in citrus fruit, used as a food additiveAscorbic C6H8O6 Vitamin C, good for healthSyntheticSulfuric H2SO4 Most common acid, used in fertilisers, detergents,
synthetic fibresNitric HNO3 Used in fertilisers and explosives
Table 2.2: Examples of Acids
Base Formula CommentsNaturalAmmonia NH3 Formed in anaerobic decay, used in fertilisers, clean-
ing agentsAmines CH3NH2 Smell like fish, occur in decomposition of organic
matterMetallic oxides Fe2O3 Used to extract some metalsCarbonates NaCO3 Used to make glass, paper, as a neutralising acidsSyntheticSodium hydroxide NaOH Used to make soap, synthetic fibre, as a powerful
cleaning agentCalcium oxide CaO Quick lime, made by heating limestone, used in ce-
mentsCalcium hydroxide Ca(OH)2 Slaked lime, used in mortar and plaster
Table 2.3: Examples of Bases
degree of ionisation). Dilute and concentrate are terms which describes the concentration of the acid,concentrate is more than 4 or 5 M, dilute is less than 2 or 3 M. The reaction of an acid with water isan equilibrium, in a strong acid the equilibrium is far to the right (dissociation), in a weak acid theequilibrium lies somewhere in the middle.
Acids as food additives
Acids are often used as food additives because they can improve the taste, and they can act as preser-vatives because they kill bacteria which can’t survive in acidic environments. Common additives areacetic, citric and phosphoric acids, along with propanoic acid in bread, and ascorbic acid added becauseit is vitamin C, for its nutrient value.
2.4 Because of the prevalence and importance of acids they have beenused and studied for hundreds of years. Over time, the definitions ofacid and base have been refined
Definitions of acids
The definition of an acid has changed over time, one of the first definitions was that of Lavoisier in1780 who said that acids were substances that contained oxygen, although this was disproved by basic
16
2.4. Acids: take two
NeutralNaCl KNO3 Na2SO4 CH3COONH4
AcidicNaHSO4 KHSO4 NH4NO3
BasicKNO2 NaHCO3 CH3COONa KCN Na2CO3
Table 2.4: Salts, by acidity and alkalinity
substances such as Na2O, and substances like HCl which were acidic without any oxygen.
Davy said in 1815 that acids contained replaceable hydrogen, i.e. substances with a hydrogenthat could be replaced by a metal, to form a salt. Bases reacted with acids to make salt and water.
Arrhenius said in 1884 that acids ionised to produce hydrogen ions. He made the distinctionbetween complete and equilibrium ionisation. His definition of a base was a substance that ionised tomake hydroxide ions, although this misses some substances.
Bronsted-Lowry theory
The Bronsted-Lowry theory of acid and bases says that acids are proton donors, and bases are protonacceptors. Thus, a substance can change from basic to acidic and vice-versa when it changes solvents,if the substance gives up protons to the solvent (as in a greater tendency to lose protons) the it is anacid. If the substance has a tendency to take protons from the solvent then it is a base.
Conjugate pairs
Bases and acids have what is called their conjugate acid and base, respectively. Conjugate pairs are thepairs of acids and bases where one has gained a proton and the other lost it (e.g. HCl and Cl – are aconjufate pair). The strength of a conjugate acid and base is approximately inverse, as in a strong acidwill have a weak conjugate, and a moderate base will have a similarly moderate conjugate acid.
Salts
Salts can be non-neutral, since the ions can be conjugates of acids or bases. If the ion is the conjugateof an acid, it will accept a proton, so it is acidic. If an ion is the conjugate of a base it will give up aproton so it is basic. The ions can be weak or strong acids or bases, if a salt consists of two ions of thesame strength (i.e. strong or weak) then it will be approximately neutral, if they are misbalanced thenthe strong ion will mean a solution of that salt will be this acidity.
Amphiprotic substances
An amphiprotic substance can donate and accept protons, i.e. it can act as a base or an acid. e.g.HCO3:
HCO−3 + H2O H3O+ + CO 2−
3
HCO−3 + H2O OH− + H2CO3
Thus, CO 2 –3 is the conjugate acid of HCO3, and H2CO3 is the conjugate base.
17
2. The Acidic Environment
Neutralisation
Neutralisation is a proton transfer reaction, where the proton given up by the acid replaces the protontaken from the solvent by the base. In water, the net ionic reaction is often H3O+ + OH – 2 H2O.Other reactions can occur, such as the neutralisation of ammonia H3O+ + NH3 NH+
4 + H2O. Allneutralisation reactions are exothermic, and the energy released is about 56 kJ mol–1.
Titration
Titration (also called volumetric analysis), is the process by which the concentration of a substancecan be determined by the use of another substance with known concentration. The substance withknown concentration is called the titrant, it is added to the other substance slowly so the volume atwhich all the added titrant and all the unknown substance are used can be accurately determined. Thispoint is called the equivalence point. The equivalence point can be detected by using an indicator,since the solution will suddenly undergo a large change in pH when the titrant reacts with all of thesubstance being titrated with. It is important the concentration of the titrant is accurately known,the volume used (the titre), and that the volume of the substance being analysed is also accuratelyknown. Equipment used in titration is: a burette (should be washed with the titre), a flask to holdthe substance being analysed (washed with pure water), a volumetric pipette to accurately measure avolume of the substance to be analysed (washed with the substance), an indicator.
Primary standard A primary standard is a substance that can be used to make an titrant ofaccurately known concentration. A primary standard has to not absorb water, must be stable, andmust be very pure, so that a measured weight of the substance is known to be almost completely thatsubstance. The purity of a primary standard is important so that volumetric analysis can be accuratelyand reliably carried out with it. Examples of primary standards are NaCO3 and NaHCO3, which canbe dried and measured with high purity.
Equivalence point The pH of the equivalence point is the pH of the salt which forms at thispoint, so an indicator must be chosen that has a colour change near this.
Buffers
A buffer solution is a solution which contains similar amounts of a weak acid and its conjugate. Thesesolutions can maintain a constant pH, since when either OH – or H3O+ ions are added one of theequilbriums will move (by Le Chatelier’s principle) to absorb these ions. e.g. for a solution of H2CO3
and HCO –3 :
H2CO3 + H2O H3O+ + HCO−3
So adding H3O+ will move this equilibrium left, and if there is enough HCO –3 in solution, all the H3O+
will be absorbed, so there will be no change in pH. Adding OH – will neutralise H3O+, which will forcethe equilibrium to the right to replace the hydronium, and if there is enough H2CO3 in solution, all theOH – will react before the H2CO3 runs out, so there will be no change in pH. It is important that bothacid and base from the conjugate pair are present so the solution can buffer against both increases anddecreases in pH. The pH a buffer maintains depends on the position of the equilibrium.
Example of buffered systems Buffers occur in some rivers and lakes. Rainwater has a naturalequilibrium with CO2 HCO –
3 , if there is another source of HCO –3 such as that dissolved out of rocks,
then the equilibrium above will be pushed left. This creates an equilibrium of H2CO3 and HCO –3 , which
acts as a buffer in the water system, so the pH is maintained constant (if lakes are not buffered, suchas those in Scandinavia, then acid rain will change the system’s pH). The same buffer reaction is usedin swimming pools to maintain a pH around 7. Blood is also buffered by this equilibrium, since
18
2.5. Esterification
Flavour Esterapple methyl butanoate & isopentyl pentanoate
banana isopentyl ethanoategrape ethyl methanoate & ethyl heptanoateorange octyl ethanoatepear pentyl ethanoate
raspberry butyl ethanoaterum ethyl methanoate
jasmine benzyl ethanoate
Table 2.5: Esters as flavours
many biological processes require specific pH’s, although other buffers like hæmoglobin are also neededto buffer against pH changing due to CO2.
2.5 Esterification is a naturally occurring process which can be performedin the laboratory
Alkanoic acids
An alkanoic acid is an alkane with a carboxyl group on one end:
R−COOH
It is basically an primary alkanol with an additional double bonded oxygen atom. They are named as“<parent alkane>-oic acid”, i.e. methanoic acid, ethanoic acid etc. (In an alkanoic acid ‘R’ can onlybe an alkyl, but in a carboxylic acid ‘R’ can be any side group). The melting and boiling points ofalkanoic acids are higher than alkanes and alkanols (which is also much higher than the alkanes) ofsimilar molecular weights, because both the C−O and the O−H bond is polar, (and the O−H bondforms hydrogen bonding) which means the intermolecular forces are strong.
Esters & esterification
An ester is a compounded formed by the reaction of a carboxylic acid with an alcohol. The −OH groupon each molecule reacts to eliminate H2O, leaving an −O− between the original acid and alcohol.
X−COOH + Y−CHOH X−CO−O−CH−Y + H2O
Esters are named as “<alcohol><acid>-oate” e.g. methanol and ethanoic acid forms methyl ethanoate.Esterification is the name given to the synthesis of an ester. The esterification reaction is an equilibriumwhich is slow, so H2SO4 is used as a catalyst, and it absorbs the water produced, so moves the equilib-rium to completion (if enough H2SO4 is used). Also, the reaction is carried out at a high temperature(just below the boiling point of the alcohol) to increase the rate of reaction.
Refluxing Refluxing is required to stop the loss of the reactants due to vaporisation. It works byusing an open flask with a long neck, which is surrounded by a water jacket, the hot vapours rise upthrough this, and are condensed by the cold water, and so cannot escape from the reaction vessel. Thisallows the reaction to be carried out at a high temperature without using a closed vessel, which wouldcause pressure build up and thus possible explosions.
Uses of esters
Esters are used for many things, including:
19
2. The Acidic Environment
• Food flavouring (see Table 2.5), esters are often found in natural foods giving them their flavours,and thus the synthetic flavours (and they are non-poisonous)
• Perfumes, since they have distinctive odours which can be used.
• Solvents, normally ethyl ethanoate (a.k.a. ethyl acetate), which is used in nail polish remover.
• Plasticisers, ‘heavy’ (therefore non-volatile) molecules are used in plastics like PVC to make itmore plastic.
20
3 Chemical Monitoring and Management
3.1 Much of the work of chemists involves monitoring the reactants andproducts of reactions and managing reaction conditions
An actual chemist
John Smith1 is a chemical engineer working for the Shell Chemical Company. He works as a seniorresearch engineer in the chemical development department. Smith develops washing machine detergents,attempting to improve and innovate on the products which are currently available. He has to improvethe efficacy of the detergents by improving properties of them such as solubility in both polar andnon-polar substances. The detergents have to be able to dissolve into the water used in the washingmachine, and then dissolve the dirt on the clothing. Since the water is polar, it can dissolve any polarsubstances on the clothing, leaving only non-polar dirt, which the soap has to be able to dissolve andthus lift from the textiles. These requirements mean the soap molecule has to have areas of polarityand areas of non-polarity, neither of which can be much larger than the other, or else it will reduce thesolubility of the molecule in one of the types of substances. Part of Smith’s work is maximising themolecule’s solubility in water and in the non-polar dirt, often performing tests, as well as attemptingto fine-tune a detergent’s properties for a specific type of washing machine, so that it performs as wellas is possible.
Collaboration
Collaboration is need in chemistry since the subject is so broad that people must specialise in specificareas. However, many problems require detailed knowledge of multiple areas of speciality, this forcesthe participating chemists to collaborate so that each person can build of other’s expertise, extendingthe solution to a problem. Also, many problems in chemistry come from, or involve, other areas of
1Not his real name
Figure 3.1: Branches of Chemistry
21
3. Chemical Monitoring and Management
Use Formfertilisers ammonium nitrate, urea etc.plastics (inc. fibres) rayon, nylon, acrylicsnitric acid makes fertilisers, dyes, TNT, dynamitecleaning products/detergents ammonia
Table 3.1: Uses of Ammonia
science, such as biology or physics, so collaboration with people with expertise in these areas is alsoneeded.
Monitoring
Many chemical process must be monitored so that the products are produced with the greatest yield,purity, speed etc. For example in the production of ethylene oxide, the reaction is
CH2−−CH2 + 12 O2
Ag cat. @ 250◦CCH2CH2O
but ethene can also react with oxygen in a combustion reaction, thus the conditions of the reaction toform ethene oxide must be monitored to restrict the amount of oxygen, and make sure the reaction isnot too hot. Also, properties such as concentration of CO2 and the pressure of the reaction would bemonitored.
3.2 Chemical processes in industry require monitoring and managementto maximise production
Synthesis of ammonia
Ammonia can be synthesised from N2 and H2 by the equilibrium (its exothermic, releasing 92 kJ mol–1):
N2 + 3 H2 2 NH3
Normally this lies to the left, but increasing the pressure and lowering the temperature pushes theequilibrium position to the right. However, cooling the reactants decreases the rate of reaction, becausea reaction happens when the reactants collide with enough energy to overcome the activation energy ofthe reaction, so heating the reaction will mean more of the collisions are high enough energy.
Haber process
The Haber process is the industrial process for the synthesis of ammonia, it is carried out at about400◦C and a pressure of 250 atm, along with magnetite (Fe3O4) with a pure iron surface as a catalyst.These conditions are a balance between reaction rate, and yield. With these conditions the equilibriumlies at about 45%, although the reaction is rarely carried to completion so yields are more usuallyaround 30%. The process consists of passing a stoichiometric mixture of N2 and H2 through a catalyticreactor. The products are passed into a coolant condenses the NH3 out of the mixture of ammoniaand the unreacted reactants (its boiling point is -33◦C, and the boiling points of the others are below-180◦C). The unreacted N2 and H2 are recycled through the catalytic reactor (the mixture always hasa ratio of 1:3 so this can be done without building an imbalance of either reactant).
Monitoring the Haber process
Some parts of the Haber process must be monitor to maximise the products, reaction rate etc and toensure safety. The temperature and pressure need to be monitored. Also, the ratio of N2 and H2 needs
22
3.3. Manufactured stuff
Figure 3.2: A flowchart to identify lone cations in a solution
Source: hsc.csu.edu.au/chemistry/core/monitoring/chem943/943net.html
to be maintained at 1:3 as accurately as possible. The presence of O2, CO2, CO and sulfurs need to bemonitored and minimized since O2 could cause an explosion, and the other three reduce the efficacy ofthe catalyst. Other gases like CH4 and Ar need to monitored because these reduce the efficiency of thereaction. The output ammonia should be monitored for purity as a double check.
History of the Haber process
Fritz Haber developed his method of synthesis of ammonia in 1908 in Germany. 6 years later this processwas industrialised by Bosch, also German. During WWI, this process allowed Germany independencefrom nitrates mined from guano deposits in bat caves in South America. This shielded the agriculturalsector of the German economy from the loss of trade routes, which would have caused a lack of fertilisers.Also, ammonia can be used to create explosives (TNT, dynamite etc.) so the development of the Haberprocess gave the German war effort cheaper and reliable access to the raw materials required for themanufacture of explosives.
3.3 Manufactured products, including food, drugs and householdchemicals, are analysed to determine or ensure their chemicalcomposition
Ion tests
The ions in a solution can be determined by precipitation tests, and by flame tests.
Identifying cations The cations in a solution can be identified by testing for precipitates, com-bined with flame tests to identify metals (see Figure 3.2). Different cations precipitate with differentanions, and often with different colours (see Appendix B for a chart). When some metals are passedthrough a flame, they give it a distinctive colour (see Table 3.2)
Identifying anions The anions in a solution can be identified by testing the formation of pre-cipitates, using the solubility rules found in Appendix B. A sample flowchart can be seen in Figure3.3. CO 2 –
3 can be identified because it releases CO2 gas in an acid. Phosphate ion doesn’t form aprecipitate with Ba 2+ in a neutral solution because it equilibriums with HPO 2 –
4 , so that there is very
23
3. Chemical Monitoring and Management
Figure 3.3: A flowchart to identify lone anions in a solution
Source: hsc.csu.edu.au/chemistry/core/monitoring/chem943/943net.html
little in a solution to form the precipitate. However, adding a base (such as ammonia) means that thereis sufficient phosphate to form insoluble Ba3(PO4)2.
Atomic Absorption Spectroscopy (AAS)
AAS is used to identify low concentrations of metals (in the range of a few ppm, to an accuracy of upto 0.01 ppm). Every element has a distinct series of wavelengths of light that it emits when excited,due to electrons moving shells and releasing a photon. When irradiated with the same wavelengths anelement will absorb it, and then reradiate it in any direction (as the electrons are pushed up a shell,and then fall back down).
AAS works on this principle (See Figure 3.4): an aqueous sample is atomised, light created byatomic emission of the element being detected is shone through the atomised sample, and the amountof each of the wavelengths absorbed is recorded. This gives a very accurate and precise measure of theconcentration of trace elements in a sample. AAS can be used to monitor the concentrations of tracemetals in the environment like lead, mercury, cadmium etc. It can also monitor soil health by recordingthe concentrations of nutrients in the soil.
Metal ColourCa Brick redBa Apple green
Cu(I) BlueCu(II) Green to blue-green
Fe GoldLi Dull redK LilacSr CrimsonNa Intense yellow
Table 3.2: Flame test colours of common metals
24
3.3. Manufactured stuff
Figure 3.4: A simple AAS device
Source: en.wikipedia.org/wiki/File:AASBLOCK.JPG
Need to monitor ions
Many ions in the environment are harmful to humans, and the environment, and so their concentrationsneed to be monitored and carefully controlled. Lead, for example, is poisonous, causing mental damage,and developmental problems. Lead in the environment can be absorbed into the food chain and workits way up to humans, so the concentration should be monitored to minimise the damage due to it.
Sulfate in lawn fertiliser
The sulfate content of lawn fertiliser can be determined by gravimetric analysis. The procedure couldbe: dissolve as much of the fertiliser as possible in an acid (HCl) and filter away remaining solids, heatthe solution until it is nearly boiling (e.g. place it in a water bath) and add excess BaCl2 while stirring,continue to heat for 30 minutes, and then leave to cool until the precipitate settles at the bottom, thencool with ice, filter this solution and precipitate through a sintered glass filter. The filter is washed toremove other ions as much as possible and its dry weight is compared to its original dry weight. Thisexperiment suffers from errors such as:
• Loss of BaSO4 due to dissolution, passage through the filter, remaining in the beaker, or spillage
• Contamination of the residue due to other ions or impurities attaching to the surface of precipitateparticles (adsorption)
• Measurement errors, such as not fully drying the filter when it is weighed
These errors can be reduced by:
• Forming the precipitate slowly at high temperatures so that the precipitate particles are as large aspossible (and allowing it to digest, as in, continue heating it, makes the particles larger) reducingadsorption (by reducing surface area) and losses through the filter
• Cooling with ice reduces the solubility of BaSO4 so reduces losses due to solubility
• Careful washing, with many repetitions with small volumes of liquid, and drying between
• The final drying-then-weighing stage should be done multiple times until the mass is constant
25
3. Chemical Monitoring and Management
160 180 200 220 240 260 280 3000
10
20
30
40
50
60
70
80
90
100
Temperature (K)
Hei
ght
(km
)
Troposphere
Stratosphere
Mesosphere
Thermosphere
Figure 3.5: The layers of the atmosphere and their temperatures
3.4 Human activity has caused changes in the composition and thestructure of the atmosphere. Chemists monitor these changes so thatfurther damage can be limited
Structure of the atmosphere
The atmosphere consists of 3 main elements nitrogen (78.1%), oxygen (21.0%) and argon (0.9%). Thereare other trace elements such as CO2 and Ne, and the concentration of water vapour varies from about0.5% to 5%. The atmosphere consists of layers (see Figure 3.5): the troposphere is about 15 km thickand is closest to the Earth’s surface; the stratosphere is about 35 km thick and is above the troposphere(they are separated by the tropopause; the stratopause separates this from the mesopause etc). Thetemperature falls as height increases in the troposphere as radiant heat from the Earth’s surface isthe main source of energy. In the stratosphere temperature rises with altitude due to heating by theabsorbtion of UV light, because of this the stratosphere is very stable, air doesn’t mix by convectionbecause the temperature gradient is backwards.
Pollutant SourceCarcinogens Unleaded petrol, vinyl chloride from plastics, dioxins from chlorine com-
poundsCFCs Refrigeration, air-con, fire extinguishers
Carbon monoxide Combustion (cars, fires etc.)Hydrocarbons Solvents
Lead Lead smelters, paint dust, leaded petrolNO & NO2 CombustionParticulates Combustion, mining, other industrial process, asbestos
O3 Photochemical smog, printers/photocopiersSO2 Combustion, metal smelting
Table 3.3: Pollutants in the lower atmosphere
26
3.4. The atmosphere
O· O2 O3
Formation UV Photosynthesis UV & electric dischargeAppearance — Colourless gas, pale blue
liquidColourless gas, blue liquid
Smell — Odourless Strong, distinctive smellEffect on life Easily reacts in living
cellsVital in diluted form Poisonous (even as low as
0.1 ppm)Boiling point — −183◦C −111◦C
Density — Similar to air 1.5× airSolubility — Slightly soluble more than O2
Reactivity Very high Moderate oxidant, reactswith most things into ox-ides
More reactive, strongeroxidant than O2
Uses — Medical, rocket fuel,steel-making, cuttingtools (oxy-acetylene)
Sterilisers, water puri-fier, paper/textile bleach-ing agent
Table 3.4: Comparison of properties of allotropes of oxygen
Co-ordinate covalent bonds
A co-ordinate covalent bond is just like a normal covalent bond except both electrons are supplied bythe one atom, i.e. it is sharing both electrons in a pair to make up for another atom missing 2 electrons.Examples of co-ordinate covalent bonds are the one in ozone (O−−O→O), the one in C←−=O, and the onein ammonium (H3N→(H+)).
Ozone
Ozone is molecule made from three oxygen atoms, it is normal O2 with an additional O co-ordinatecovalently bonded to form a bent shape. Ozone is a poison to life, and as such is a pollutant in thetroposphere, but it is involved in the absorption of UV light in the stratosphere so it is vital for lifewhen it is in the stratosphere.
Chlorofluorocarbons & Halons
A chlorofluorocarbon (CFC) are compounds with only chlorine, fluorine and carbon. A halon is acompound with carbon and bromine, possibly with other halogens. CFCs come from refrigeration,aerosol cans and foaming agents in the making of plastics like polystyrene. CFCs were also used toclean electronic circuit boards. Halons are used in fire extinguishers (BCF fire extinguishers, as inbromine, chlorine, fluorine). CFCs are very stable, and are not destroyed by sunlight, so they last fora long time in the atmosphere, long enough to spread into the stratosphere by dispersion, and they areinsoluble in water so are not washed out by rain.
Naming haloalkanes
The prefixes for F, Cl, Br and I are fluoro-, chloro-, bromo-, iodo- respectively. It’s positions andmultiplicities are given in the form “<carbon numbers>-<number prefix><element prefix>”, e.g. 1,2,3-trichloro· · · . The numbers are counted from the end that minimises their sum. The elements are listedalphabetically. (If there is multiple ways of counting to minimise the sum, then it is counted to minimisethe most electronegative element).
27
3. Chemical Monitoring and Management
Problems with CFCs
CFCs destroy ozone in the stratosphere. When a CFC is hit by high energy UV light, a chlorine atomis broken off. This Cl· reacts with O3, converting it into an ozone and a ClO· radical.
CFCl3UV light
CFCl2 + Cl
Cl·+ O3 ClO·+ O2
ClO·+ O3 Cl·+ 2 O2
Thus, chlorine is a catalyst, and so can destroy many ozone molecules. Thus CFCs are detrimentalto the atmosphere, so measures have been taken against them. The Montreal Protocol was signed in1987 which agreed to phase out halons, CFCs, and HCFCs in a short period of time (although somecountries were given extensions). So far there has not been any improvement in the concentration ofozone in the stratosphere, but the slow speed with which particles move in the stratosphere mean thatthis is not too surprising.
Measuring ozone
Ozone is measured in Dobson Units (DU), and is a measure of the thickness of the ozone if it wereat ground pressure. Ozone can be measured by observing the amount of the frequencies it absorbscompared to surrounding frequencies in the light coming through the atmosphere. This is done with aUV spectrophotometer. Satellite based instruments can also be used to give worldwide map of ozoneconcentration (as well as ozone concentration at specific altitudes). A balloon can also be used to obtaina concentration profile at a specific location on the Earth’s surface .
Alternatives for CFCS
There are several alternatives to CFCs for almost all the uses:
• Hydrochlorofluorocarbons (HCFCs), these are more volatile and thus a larger proportion breaksdown in the lower atmosphere, well away from the ozone layer. However, a significant proportionstill reaches the stratosphere, where it breaks down, releasing a chlorine atom, resulting in similardamage to a CFC.
• Hydroflurocarbons (HFC) are even more volatile that HCFCs and very few reach the stratosphere,although they contain no C−Cl bond to be broken and so cannot destroy ozone any way. HFCsare used in almost all the areas where CFC’s were used, including refrigeration (some plainhydrocarbons are also used, and these have similarly little impact on ozone levels) and aerosols,although they are slightly less efficient compared to CFCs.
However where they were used in laboratories for research, CFCs have had to be replaced by othermethods, since CFCs are often used to deplete ozone in a laboratory environment. Also, all thesealternatives are greenhouse gases so more research is still required to find more environmentally friendlyalternatives.
3.5 Human activity also impacts on waterways. Chemical monitoring andmanagement assists in providing safe water for human use and toprotect the habitats of other organisms
Water quality
Water quality describes its appropriateness for uses, e.g. for drinking, washing, agriculture, recreation,environmentally etc. Thus, a definition of good water quality depends on its use, but a measure ofquality can be obtained by measuring a few properties:
28
3.5. Waterways
• Acidity, as in pH, it should be around 6.5–8.5
• Concentration of ions the each ion has a different safe range (e.g. [NO−3 ] < 0.1 ppm &
[PO 2−4 ] < 0.03 ppm)
• Biological oxygen demand is a measure of the amount of dissolved oxygen that would be usedto decompose all the organic material in the water, measured in ppm, and should be less than 5ppm. A high BOD means that water will use more dissolved oxygen as the stuff in it decomposes.
• Dissolved oxygen is a measure of the amount of oxygen dissolved in the water, which is necessaryfor life, it should be 7-9 ppm
• Hardness, a measure of the concentration of Ca 2+ and Mg 2+
• Total dissolved solids, the mass of solids dissolved in the water, (can be either mg L–1 orppm), should be less than 500 ppm for drinking, it includes the mass of any salts (sea water has aTDS of 35,000 ppm), which often make up the bulk of the solids, so conductivity is often a goodapproximation.
• Turbidity is a measure of the cloudiness of water, as in, the amount of suspended particles. Itis measure in NTU (nephelometric turbidity units), and should be less than 3 NTU. It can bemeasured with a sechhi disk or a graduated calibrated tube, both processes involve adding morewater until the disk can’t be seen, or a cross at the bottom of the tube disappears.
Factors affecting ion concentrations
There are several factors which can affect the concentration of ions in water bodies. Rain is pretty muchfree of ions (a small amount of CO3 and sea spray etc.) but it can pick up ions as it runs on the ground,picking up nitrate, phosphate, calcium and magnesium. If the water enters an underground aquiferit will pick up many more ions from the rocks and soil (calcium, magnesium, sulfate, chloride, carbonateetc.). If it goes to an artesian basin, it will pick up other ions like iron, manganese, copper, and zinc,which can push TDS to over 1000 ppm. Acid rain can leach metal ions like calcium, magnesium andiron from the soil better, pushing TDS up again. Human activity such as land clearing and otheragriculture can also affect TDS, land clearing increases turbidity and helps the dissolution of ionslike sodium, potassium, calcium, magnesium, sulfate, chlorine and carbonate. Agriculture puts morenitrate and phosphate into the environment (in fertilisers) and thus increases the concentration of theseions in run off. Discharged sewage contains high levels of phosphate and nitrate and many otherions, even if it has been treated well sewage can have TDS of 200 ppm. Industrial waste can containmore exotic ions, like lead, mercury, cadmium, chromium, copper and zinc. Rubbish tips can containhigh concentrations of common ions (nitrate, phosphate), and others like lead, cadmium, mercury (inbatteries), zinc (from galvanised iron), if the tip is not properly designed then water running though itcan leach these minerals out.
Treatment
Water can be treated (see Figure 3.6) to improve its quality and often is, in a multistage process basi-cally involving clarification (making it look/taste better) and sanitisation (making it safe for humans).Clarification can be done by precipitating the particles out of the water, this process is called Floccu-lation or Coagulation. It works by adding FeCl3 to the water, which allow the suspended solids togroup into large and larger particles, eventually getting large enough to be filtered by a coarse filter likesand or anthracite (which also filters out organic matter), for better results though, a membrane filtercan be used. The FeCl3 needs to be added to an alkaline solution (so the pH is often adjusted withNaOH) so that the insoluble Fe(OH)3 can form. (Sometimes Al(OH)3 is used instead of Fe(OH)3). Thewater is sanitised by dissolving Cl gas in the water, this kills bacteria and some viruses, with carefulmonitoring, enough is added to last for long enough to keep the water disinfected until it reaches the
29
3. Chemical Monitoring and Management
Catchment Add NaOH
Add FeCl3to coagu-
late/precipitatesuspended
solids
Filter toremove pre-
cipitate (usingsand/anthracite
or a mem-brane filter)
Add Cl2 gasto sanitise
Add F – fortooth health
StorageConsumer
Figure 3.6: A process to treat water
consumer. Fluoride is often also added (at 1 ppm) as it is good for tooth health, it has no effect onwater quality/safety. Monitoring has to be used to make sure the concentrations of everything is withthe required ranges. Most water suppliers uses sand or anthracite filters even though membrane filtersare much superior, since the current filters are much cheaper.
Membrane filters
A membrane filter consists of a thin film of a polymer (polypropylene, teflon, polysuplone) with verysmall holes in it (these holes are uniformly sized). This film can be pleated and placed in a cylinder,the area outside the membrane is filled with dirty water, and the clean water comes out on the insideof the membrane. Another method is to make the film into a large number of thin capillaries (calledhollow fibre membrane filter), which is hollow, so that dirty water flows over the outside and cleancomes out of the passage in the capillary. A fibre filter is made from many of these so that it has alarge surface area. The filtrate is left on the outside of the membrane/capillary in each method, and itcan be cleaned by back flushing, by blowing air through it from the clean side. A membrane filter canbe made with holes of a custom size, so they can filter different things. A membrane filter can standpressure so they can be sped up by forcing the water onto them.
Heavy metals
The presence of heavy metals in a solution can be determined by testing with H2S, in acidic and inalkaline solutions. In acid the equilibrium
S 2− + 2 H3O+ H2S + 2 H2O
lies well to the right, but PbS and other very insoluble precipitates (like copper) can still form, thistakes out some S 2 – so the equilibrium removes to the left, providing more S 2 – until all the insolubleprecipitates come out of solution, however, more soluble precipitate, like zinc and iron, don’t formprecipitates. In an alkaline solution the equilibrium lies on the left, so there is enough S 2 – to formprecipitates with the more soluble elements. Once their presence is determined the actual elementspresent can be more accurately determined (see Table 3.5)
Eutrophication
Eutrophication is when a water body becomes so nutrient laden that an algal bloom is likely. Eu-trophication is accelerated by human activity which increases the concentrations of, most importantly,phosphate and, secondarily, nitrate. Human activity such as agriculture with fertilisers and sewage
30
3.5. Waterways
Metal Test
Ba 2+ White precipitate with SO 2 –4 , none with OH – or F –
Ca 2+ White precipitate with SO 2 –4 and F –
Cu 2+ Blue precipitate with OH – , which dissolves to deep blue in NH3
Fe 2+ Green or white precipitate with OH – that may go brownFe 3+ Brown precipitate with OH – , deep red with SCN –
Pb 2+ Yellow precipitate with I –
Table 3.5: Tests for heavy metals
(including water used to wash clothes) are the major contributors to this. Eutrophication can be de-tected by monitoring the concentration of phosphate, in a still water body like a lake the concentrationshould be less than 0.05 ppm, but in moving water (river, stream) it can be as high as 0.1 ppm.The concentration of phosphate can be determined colorimentrically: a known volume of ammoniummolybdate is added to the solution, then solid ascorbic acid. The solution will have a blue colour, theabsorbance of this solution can be compared against a reference solution given the same treatment sothat a quantitative measure is obtained.
Town water supply
The catchment of a water supply is the area from which water drains into the dams of that town. Acatchment should have minimal human activity, no agriculture, settlement, mining etc since these willcontaminate the water supply. However, contaminants will still find their way into the supply, but thesecan be detected (e.g. ions can use precipitation/flame tests) and measured (AAS) to determine whetherthey are in a safe range. After the water has made it to the catchment dam, it can be purified andclarified, using filters, flocculation etc to increase its quality. Afterwards, chemicals like chlorine (as adisinfectant) or fluorine (for tooth health) can be added.
31
4 Industrial Chemistry
4.1 Industrial chemistry processes have enabled scientists to developreplacements for natural products
Rubber s a natural resource derived from rubber trees that has had synthetic replacements developed.Rubber is used in tyres, and in fabrics, among others. The most important properties of rubber areits elasticity, as well as the way in which the chemical structure allows the properties to be modifiedsignificantly (e.g. vulcanisation). Natural rubber is time consuming and effort intensive to obtain, sowith increased demand due to car tyres and population increase alternative had to be developed to keepprices low. Also, people can have allergies to latex which is a natural form of rubber, so alternativeswith hypo-allergenic properties are needed.
Alternatives to rubber include styrene butadiene and polyurethane, which have taken over mostof the applications where rubber was used. The synthetic compounds often have more suitable propertiesfor the specific applications they are being used for since the manufacturing process can be fine tunedto optimise certain things. For example, rubbers that retain their elasticity better in cold weather, orthat remain elastic for a longer period of time, or even are just cheaper for everyday applications.
Overall, research to replace natural rubber has lead to myriad substances, with different prop-erties which can be used in different situations. These include as in tyres, as a straight replacement fornatural rubber (styrene butadiene), or as foams in shoes, mattress (polyurethane). The production ofnatural rubber is very labourful in that each tree has to be tapped individually and the latex collected,after which it needs to be adjusted (e.g. vulcanised) to give the desired properties. Rubber substitutesare manufactured by chemical reactions (normally polymerisation) which allows it to be ’automated’ sothat the process can occur without much human interaction, which reduces costs, and the conditionscan be controlled to maximise the efficiency of production of the rubber, unlike natural rubber. There-fore, research has progressed the rubber manufacture process so that natural rubber is now a minorconstituent of the global rubber consumption.
4.2 Many industrial processes involve manipulation of equilibriumreactions
Equilibrium constant
The equilibrium constant is a quantitative measure of the equilibrium point of a reaction, it is denotedK. A value of K between 0.1 and 10 means the reaction has what could be called an equilibrium. If Kis very small it means that the reaction barely starts. If it is very large, then the reaction goes almostto completion. The value of K for any given reaction can only be changed by the temperature at whichthe reaction occurs, all other variables do not change K. K is a quotient of the concentrations of theproducts (P ) and reactants (R), for a reaction a1R1 +a2R2 + · · · b1P1 +b2P2 + · · · , the equilibriumconstant is given by:
K =[P1]b1 × [P2]b1 × · · ·[R1]a1 × [R2]a1 × · · ·
33
4. Industrial Chemistry
Use CommentsIn manufactureFertiliser Used to break down some compounds to form superphosphateEthene Used as a catalyst to dehydrate ethanolSynthetic fibres Used to neutralise the NaOH used in the manufacturing process (and some other
things as well)Direct usesCar batteries Used as the electrolyte in lead acid batteriesMetal cleaning Used to pickle steel before applying a surface coatingMetal extraction Used to dissolve metals out of their ores
Table 4.1: Some uses for sulfuric acid
Factors affecting equilibrium reactions
Pressure, volume and concentration Changing the pressure or volume of an equilibrium reac-tion will cause the reaction to move to counteract this change, by ‘absorbing’ moles, for example. Thischange does not affect the final ratios of products to reactants, i.e K remains constant, as the wholesystem moves equally. Similarly, changing the concentration of a single reactant or product will causethe system to move to either provide more of this compound (if it was removed) or to convert it intoother things (if more was added), but K will still be the same at equilibrium, because the concentrationof every participant in the reaction will change.
Temperature Changing the temperature of an equilibrium reaction can move the equivalencepoint, by Le Chatelier’s principle, the reaction will move to absorb added heat, or release removed heatto try to minimise the disturbance to the system.
4.3 Sulfuric acid is one of the most important industrial chemicals
Extraction of sulfuric acid
The Frasch process is the predominant method of extracting sulfur from deposits of pure sulfur. Whena deposit is located underground, three pipes are connected to it (see Figure 4.1), the external pipepushes superheated water (160◦C) down to the deposit to melt the sulfur. Compressed air is forceddown the centre pipe, so that the combination of the water pressure and the air pressure pushes themolten sulfur (along with the air and the water) up the third pipe. This method works with sulfurbecause the melting point of sulfur is only 113◦C, and it easily forms an emulsion (when molten) with
Sulfur deposit
Superheated water
Compressed air
Molten sulfur/water emulsion
Figure 4.1: A simplified diagram of the Frasch process
34
4.3. Sulfuric acid
Oxygen
Combustion
Sulfur
SO2
Catalyticconversion
SO3
Passed throughsulfuric acid
Oleum(H2S2O7)
Reactedwith water
H2SO4
Figure 4.2: A similified flow chart of the contact process
water. However, it is insoluble so, when it cools, it solidifies out of the emulsion leaving behind verypure (>99%) sulfur. Sulfur is also low density so the the compressed air is an effective way of bring itto the surface. The fact that the only materials used to extract the sulfur are water and air means thatthe whole process is fairly cheap.
However, the Frasch process has a variety of environmental issues. The water used cannot bedischarged into the environment without being processed because it will dissolve impurities (such asmetal ions) in the sulfur and so becomes contaminated. Also, sulfur forms dangerous oxides, so oxidationof the sulfur must be kept to an absolute minimum. Significantly, the sulfur deposit occurs as a largeblock, so it leaves behind a large hole in the ground which can cause ground subsidence and thus possibledamage the environment above.
Production of sulfuric acid
Sulfur by itself is not particularly useful, so once it is extract it undergoes further processing to obtainmore useful compounds. One such compound is sulfuric acid, which is produced by the contact process.The contact process has three basic steps: oxidation of sulfur to sulfur dioxide; oxidation of sulfurdioxide to sulfur trioxide; and, the conversion of sulfur trioxide and water to sulfuric acid. The basicsteps of the contact process are outlined in Figure 4.2.
Production of SO2 Sulfur dioxide is produced by spraying molten sulfur into dry air so that itcombusts. Since it is combustion, the reaction is exothermic and it can heat the air up to more than 2times the desired temperature for the next stage, so it has to be cooled. The reaction for this stage is:
S(s) + O2(g) SO2(g)
Sulfur dioxide can also be produced by roasting metal sulfates, which is commonly done toextract metals such as zinc and copper from their ores.
35
4. Industrial Chemistry
Production of SO3 The mixture of air and sulfur dioxide is cooled to 400◦C and pushed into acatalytic converter. The reaction is:
2 SO2(g) + O2(g) 2 SO3(g)
It is an exothermic equilibrium reaction, so to obtain a commercially viable yield many factors need tobe considered and optimised. Since it is exothermic, lowering the temperature will increase the yieldof SO3, but this slows the reaction down, so a compromise between yield and rate is required. For thisreaction, the optimal temperature is about 400◦C. A catalyst (V2O5) is also used to increase the rateof reaction. This catalyst is a surface layer on a highly porous material, to increase its surface area.The mixture of air and SO2 is passed over a bed of this catalyst at a high temperature (550◦C) whichconverts the majority of the dioxide into trioxide, after this it is run over the another bed of catalystat 400◦C to try to maximise the yield. The mixture is then redirected into an absorber tower (whereH2SO4 is produced) to remove the SO3, and the remaining SO2 is run over another catalyst bed, whichconverts the almost all the remaining gas into SO3. This is then run through another absorber towerto extract the most sulfuric acid possible.
Production of H2SO4 The last stage of the process is to convert the SO3 into sulfuric acid.Sulfur trioxide reacts directly with water to form sulfuric acid, but this reaction is very exothermic, soit can not be done in a safe and economic way. However, SO3 reacts with sulfuric acid to form oleum(H2S2O7), which reacts with water to form 2 H2SO4. This second reaction can easily be controlled, sincewater is the limiting reagent, and so this method is both safe and economic, since the sulfur trioxide isdissolved into the sulfuric acid as it bubbles through, and the rate of heat release is controllable.
Reactions of sulfuric acid
Oxidation Sulfuric acid can oxidise other substances, since it contains a hydrogen atom. Thereaction when sulfuric acid acts as a oxidant is:
H2SO4 + 2 H+ + 2 e− SO2 + 2 H2O
Or, when the sulfuric acid is concentrated:
2 H2SO4 + 2 e− SO2 + SO 2−4 + 2 H2O
Dehydration Sulfuric acid is hygroscopic when it is concentrated, so it readily absorbs water fromits environment and other substances. Sulfuric acid even pulls H2O molecules out of other molecules(such as alcohols), and so is used as a catalyst for the production of ethene from ethanol, for example.Sulfuric acid dehydrates hydrated molecules, and can dry gases.
C2H5OHH2SO4 C2H4 + H2O
Sulfuric acid ionisation
Concentrated sulfuric acid is almost completely molecular, since it contains very little water (which iswhat ionises the molecules), and much of what water there is not in a state to ionise the acid (since itis in hydrate form, like H2SO4 ·H2O). Thus, when sulfuric acid is diluted it will be completely ionised,from H2SO4 to H2SO –
4 and SO 2 –4 . This ionisation reaction is very exothermic, which accounts for the
huge amounts of energy released (∆H = −90 kJ/mol). The reason the dilution of sulfuric acid is muchmore exothermic than the dilution of other acids is the fact that concentrated sulfuric acid is molecular,while other acids are at least partially ionised, even in concentrated form.
36
4.4. Sodium hydroxide and electrolysis
Safety considerations with sulfuric acid
Sulfuric acid is a strong acid, so the normal safety precautions apply to it, such as appropriate pro-tection etc. However, sulfuric acid is a dehydrating agent, so especial care needs to be taken to avoidcontact with skin and eyes as it can cause serious damage to living tissues. Also, the dilution ofconcentrated sulfuric acid (from almost 100% molecular to almost 100% ionic) is highly exothermic(H+ + H2O H3O+ + energy), so dilution should be done by adding the acid to the water, so thatthere is more thermal mass of water to absorb any heat, and so that the reaction cannot get out ofcontrol, since it is regulated by the amount of acid added, which can be easily stopped. The storagevessel of the sulfuric acid is also important, the vessel must be contained in a second vessel, or on atray, which can hold the whole volume of sulfuric acid in the vessel. Also, dilute sulfuric acid must bekept in a glass container (since glass is very resistant to acids), but concentrated sulfuric acid can bekept in a metal container (since the acid is mostly molecular when concentrated).
4.4 The industrial production of sodium hydroxide requires the use ofelectrolysis
Difference between electrolytic and galvanic cells
An electrolytic cell is basically a galvanic cell in reverse, by providing a negative voltage gradient (i.e.opposite to the direction of the voltage which the galvanic cell produces) the reactions at each electrodecan be reversed. Obviously, an external voltage source is required to overcome this natural voltagegradient and provide the energy to make the reactions happen in their unnatural directions. As such,the signs of electrodes change (since reduction always happens at the cathode, and oxidation at theanode) so the anode is positive and the cathode is negative in an electrolytic cell. It is as if thecathode is full of electrons (giving it a negative charge) which it pushes out into the electrolyte to reducethe ions in solution, while the anode is sucking electrons (since it has a positive charge, provided by theexternal voltage source) from the particles in the electrolyte, oxidising them.
Production of sodium hydroxide through electrolytic methods
Sodium hydroxide is an important industrial chemical: being a strong base it in (or to make) soaps,detergents, and other heavy-duty cleaners. Also it is important in the production of some materialssuch as paper, plastics and other synthetic fibres (rayon etc.). It is produced by the overall equation:
2 NaCl(aq) + 2 H2O(l) 2 NaOH(aq) + Cl2(g) + H2(g)
Thus, any process producing NaOH also produces chlorine and hydrogen, which can be (and are) usedfor other industrial processes. However, these gases also present a safety issue as they react violently(and form HCl).
Mercury process The mercury process uses liquid mercury as an electrode and as a catalyst. Theprocess consists of two chambers, each with a flow of mercury along the bottom. Highly concentratedsalt water (almost saturated) is fed into the first chamber, where plates of metal (such as titanium)act as the anode and remove the chlorine from solution. The cathode is the mercury, which createsa solution of sodium in mercury. This mixture is pushed into the second chamber where the sodiumnaturally reacts with the pure water in the chamber to form sodium hydroxide and hydrogen gas. Themercury can theoretically be reused indefinitely, but some mercury is lost into the water, so it needs tobe replaced.
Industrially, plants can use one mercury cell, which is very large, as the plates and the mercurycan be close together, and have a large surface area, so the current (i.e. the creation and movement ofions) can be very high (many thousands of amps). The sodium hydroxide produced by the mercuryprocess is very pure since the chlorine is completely separate from the sodium hydroxide (no chlorineis transported by the mercury). However, safety and environmental concerns surrounding the use of
37
4. Industrial Chemistry
mercury (which is released into the environment at a few hundred grams per tonne of NaOH) haveprompted legislation placing very very low limits on the release of mercury into the environment so thatmercury cells are becoming less and less common.
Diaphragm process The diaphragm process was the first industrial method of extracting sodiumhydroxide. A diaphragm cell consists of two chambers separated by a thin sheet of a porous material(normally asbestos since it is immune to hydroxide). Highly concentrated sodium chloride is contin-uously fed into one chamber, where there is a titanium electrode. This electrode acts as the anodeand so the NaCl is broken down, the sodium stays in solution, it migrates through the diaphragm intothe other chamber where hydroxide is produced by an iron mesh electrode. This solution of sodiumhydroxide is drawn out of the cell, where much of the water is evaporated off, leaving behind most ofthe small portion of chlorine that crosses the diaphragm. Some hydroxide also migrates into the firstchamber, but this is reduced by having a pressure gradient from the first to the second chamber.
Industrially, each cell is small so that the current (i.e. the production of the ions) is maximised,but a plant will have many individual cells, so that they can drawn thousands of amps of current.The diaphragm process is relatively cheap, but the NaOH it produces is contaminated by very smallamounts of chlorine so it is not suitable for applications requiring very high purity.
Membrane process The membrane process is very similar to the diaphragm process except itreplaces the diaphragm with a much better membrane that only allows the passage of cations (like Na+)and blocks anions (like Cl – or OH – ). The membrane is normally made from PTFE (Teflon) whichhas been modified so that has the properties stated above. The Teflon is very unreactive and so asimmune to the high pH of hydroxide solutions as asbestos. Other than the changing the diaphragm fora membrane, a membrane cell is identical to a diaphragm cell.
The membrane process produces almost completely pure NaOH, and there is no risk of the wastewater developing a high pH as no OH – can leach through the membrane (as it can in a diaphragm cell).A membrane cell is also much safer than either cell above, since there is neither mercury nor asbestos,which means that almost all new cells built are of this type.
Electrolysis of sodium chloride
When sodium chloride undergoes electrolysis there are three possible reactions, depending on the stateof the sodium chloride.
Molten NaCl If the sodium chloride is molten then electrolysis forms Cl2 gas at the anode (whichis oxidation) and pure Na (in liquid form since the temperature of NaCl(l) is above the melting pointof Na) at the cathode (it is reduction).
2 NaCl(aq) 2 Na(l) + Cl2(g)
Concentrated NaCl(aq) If the sodium chloride is aqueous, but concentrated, then the reaction atthe anode still forms Cl2(g) but the cathode splits water into hydroxide and hydrogen gas, the competereaction is:
2 NaCl(aq) + 2 H2O(l) Cl2(g) + H2(g) + 2 NaOH(aq)
Dilute NaCl(aq) If the sodium chloride is aqueous and dilute, then the ions are in very concen-trations, so they are replaced by water (at both electrodes), since water has comparable oxidability tochlorine (so the one with the most is most oxidised, i.e. water), and water has much higher reducabilitythat sodium, so it replaces sodium even when the sodium has a high concentration (as above). The
38
4.5. Saponification
reactions are:
Cathode 2 H2O(l) + 2 e− H2(g) + 2 OH−(aq)
Anode 2 H2O(l) O2(g)+4 H+
(aq) + 4 e−
Overall 2 H2O(l) 2 H2(g) + O2(g)
4.5 Saponification is an important organic industrial process
Saponification
Saponification is the process of making soap from lipids. The process is the reaction of an ester with anhydroxide ion (normally provided by sodium hydroxide, hence the need for it), which forms an alcoholand a carboxylate ion i.e. saponification is just the process of breaking the alcohol off an ester, leavinga carboxylic acid which is missing the hydrogen on the −COO group. An example of saponificationwith methyl propanoate:
CH3CH2COOCH3 + OH− CH3CH2COO− + CH3OH
In the school laboratory Saponification can be conducted in a laboratory by boiling the esterand sodium hydroxide solution together (since the ester is insoluble in water so another method ofcombining the two is needed). This will produce the carboxylate anion and the alcohol, which (in mostinstances in the laboratory) are soluble in water. The products can be separated by some form ofdistillation, depending on the ester. If the alcohol has a boiling point distant from 100◦C then normaldistillation suffices, otherwise more complicated fractional distillation is necessary. Also, since solubilityof alcohols decreases with temperature, the alcohol may separate from the water just by cooling thesolution.
Industrially Industrial saponification rarely just makes a carboxylate anion, the process is carriedthrough until soap is made. The soap is made from fats and oils (triglycerides) rather than esters.Tryglycerides consist of a backbone of 3 carbons, with carboxyclic acid side chain attached to each one.The reaction with hydroxide is almost identical for triglycerides and esters, except a single triglycerideforms one glycerol molecule (which is 1,2,3-trihydroxyl-propane) and three carboxylate anions. Theresulting solution of glycerol and carboxylate anions can be separated by cooling it and mixing insaturated salt water which separates sodium carboxylate into a solid layer on top of a glycerol solution(which itself is further purified by multiple distillations). In an industrial context the fats and oils usedare highly contaminated with things such as blood, muscle tissue etc. since many of the fats and soapsused are animal fats, which is very different to the pure reactants used in a laboratory.
Common oils and fats used in an industrial context for saponfication include: olive oil, which ischeap and produces a hard, durable soap; beef fat, from butchers and abbotoirs, although it is normallycontaminated, so care needs to be taken to remove the unwanted things; coconut and palm oil, whichare very cheap, but often cause environmental damage.
Explanation for the cleaning action of soap
Soap cleans things by allowing things that would otherwise be insoluble to be soluble in water, so thatthey can be washed away. Soap consists of a polar end (the end of the COO group) and a non-polarend (the long carbon chain), which means that it is soluble in both types of substance. When soapcomes into contact with a non-polar substance (like dirt) the non-polar end will ‘burrow’ into it, whilethe polar end will remain outside dissolved in the water. When there are enough soap molecules, thedirt will become soluble in water because it is surrounded by many polar charges. An substance whichdoes this, including soap, is called a surfactant.
39
4. Industrial Chemistry
N+
(a) Cationic detergent
−
(b) Soap, or anionic detergent(O C C
)n
OH
(c) Non-ionic detergent
Figure 4.3: Schematics of the molecular structures of different types of surfactants (Note: almost allhydrogens have been omitted, and a wavy line represents a hydrocarbon chain of arbitrary length)
Soap, water and oil is an emulsion
An emulsion is a mixture of two liquids where one liquid is suspended in the other in lots of little drops.Water and oil are insoluble normally and so don’t form an emulsion without an emulsifier. Soap actsas an emulsifier in a mixture of water and oil (by a process explained above).
Comparison between soaps and some types of detergents
Synthetic detergents can have very different properties to soap, which normally derives from the differentstructures of their molecules (visual representations of the structures can be seen in Figure 4.3). Eachtype of detergent has different properties, and so commercial products will contain multiple types ofdetergents.
Soap Soap is shaped like a tadpole, with a non-polar tail, and a negatively charged head. The‘tail’ is a normal hydrocarbon chain, and the ‘head’ is −COO – . In hard water (water with highconcentrations of Ca 2+ and Mg 2+ ions), soaps effectiveness is reduced because it precipitates out,removing soap molecules. Soap is normally used for personal hygiene.
Anionic detergents Anionic detergents are very similar to soaps, since they have a polar and anon-polar end. However, the polar end is caused by a sulfonate chain (−OSO2O – ). This means thatanionic detergents are much better at cleaning than soaps. They are used in most cleaning instances.However, the negative end means that anionic detergents are affected by hard water, although less thansoap.
Cationic detergents Cationic detergents are built around an ammonium ion, with hydrocarbonchains instead of just hydrogen atoms. Thus they can end up being a ‘X’ (although the branches canhave different lengths). Just like in the other detergents, the carbons chains are the bits that dissolvein non-polar substances, but the charged section is the N atom, which doesn’t have to be at the endof the molecule. Cationic detergents act as antiseptics, and also have good properties for use as hairconditioner or fabric softener. Cationic detergents cannot precipitate in hard water, as they are apositively charged ion, so they retain complete efficiacy in hard water.
Non-ionic detergents A non-ionic detergent is not an ion, but the molecule is still polar becauseof the large number of oxygen atoms towards one end and the hydroxide group at the same end. Since
40
4.6. The Solvay process
non-ionic detergents do not have as high a concentration of charge as the other types of detergents,they do not form stables bubbles, and hence foam, so easily. This tendency to not foam means thatnon-ionic detergents are useful for application which try to avoid foam, such as dishwashing. Also,non-ionic detergents are used in pesticides, glues, and paints. Since non-ionic detergents are not ionsthey cannot precipitate in hard water and so are still 100% effective in hard water.
Examples of emulsiosn
There are many common emulsions. Things like milk, and mayonnaise are emulsions of oil and water(and other dissolved things). More solid things (like butter and margarine) are the reverse, i.e. littledrops of water dispersed through an oil. All water-oil emulsions need a emulsifier to mean that theyare stable, such as casein or lecithin in milk and mayonnaise respectively.
Environmental impact of surfactants
Soap is fair environmentally benign as it is readily broken down by bacteria. However, syntheticdetergents can cause much greater problems. Non-biodegradable detergents have caused a build upof foam in some water systems since they formed stable bubbles and were not quickly broken downby natural processes, however, the replacement of these by truly biodegradable detergents mitigatedthis problem. Phosphorus can also occur in detergents, not as a surfactant though, but as a ‘builder’.Builders are molecules which remove Ca 2+ and Mg 2+ from the water, allowing anionic detergentsto work properly. The most common builder is sodium tripolyphosphate, which, when released intothe environment, can cause eutriphication of water ways. This problem is serious enough to have lawscontrolling the phosphorus content of detergents in many countries. Another, although smaller, problemis the affect that the antibacterial nature of cationic detergents can have on the ‘good’ bacteria in sewageplants, although it only has a big impact at high concentrations.
4.6 The Solvay process has been in use since the 1860s
The Solvay process
The Solvay process is a industrial method of producing sodium carbonate. It takes limestone (80–90%CaCO3) and brine (30% NaCl(aq)), and produces sodium carbonate and wasted calcium chloride (usingammonia as a catalyst). The whole process is outlined in Figure 4.4. The overall reaction for the wholeprocess is (note the relative quantities of each reagent):
CaCO3(s) + 2 NaCl(aq) Na2CO3(s) + CaCl2(aq)
Brine purification The first stage of the process is to obtain brine that is pure and concentrated,i.e. 30% NaCl and the rest water. Any impurities could precipitate with the sodium hydrogen carbonate,which will result in the end produce being contaminated. Metal ion impurities such as calcium andmagnesium (which are common in brine obtained from underground reservoirs) can be precipitated out(with carbonate and hydroxide, respectively), flocculated, and removed before the brine is used Solvayprocess (the flocculation step will also remove other suspended solids, such as dirt).
Sodium hydrogen carbonate precipitation The process decomposes the limestone to get CO2
and CaO, the carbon dioxide is then used directly to produce Na2CO3 as it is bubbled through a thebrine saturated with ammonia, which causes carbonic acid (H2CO3) to form, which reacts with theammonia to produce hydrogen carbonate ions (HCO3) and ammonium. Sodium hydrogen carbonate isin soluble so it precipitates from solution. The net equation for this stage is:
NaCl(aq) + NH3(aq) + CO2(g) + H2O(l) NaHCO3(s) + NH4Cl(aq)
41
4. Industrial Chemistry
Limestone(CaCO3)
Limestonedecomposed
in a kiln
CaO
CO2
Reacted withwater in alime slaker
Ca(OH)2
Heated torecover NH3
(CaCl2 waste)
Carbon dioxidebubbled
through liquid
Saturatedwith NH3
Purifiedbrine
NaHCO3(s)
&NH4Cl(aq)
Separatedby filtration
NaHCO3
NH4Cl
Decomposedby heating
CO2Na2CO3
Figure 4.4: A flow chart outlining the key steps of the Solvay process
Sodium carbonate production This mixture of NaHCO3 and NH4Cl is filtered to separate thesolid sodium hydrogen carbonate from the rest. This then undergoes heating so that it decomposesinto sodium carbonate, carbon dioxide and water. The carbon dioxide produced at this step can be fedback into the carbonation step to reduce the materials cost. The net equation for this stage is:
2 NaHCO3(s) Na2CO3(s) + CO2(g) + H2O(g)
Ammonia recovery The ammonia used to precipitate the sodium hydrogen carbonate can berecovered and reused, to further reduce the cost of the method. The other product from the decompo-sition of limestone calcium oxide, is slaked, (reacted with water) to produce Ca(OH)2. This is mixedwith the NH4Cl solution from the previous step, and heated, which creates calcium chloride (the wasteproduct of the Solvay process), water and ammonia. The equation for this reaction is:
NH4Cl(aq) + Ca(OH)2(s) NH3(g) + CaCl2(aq) + 2 H2O(l)
Environmental issues The Solvay process produces two products, but only one is useful, themajority of the calcium chloride solution is discarded and disposed of. Normally, this disposal occurs intothe sea where the calcium/calcium carbonate equilibrium resists a significant change in the concentrationof Ca 2+, so this disposal is fairly harmless in these terms. However, the solution can not be dischargedinto a river, since the volume of water is not large enough to be able to dilute the increased concentration
42
4.6. The Solvay process
Use CommentsGlass-making Major use of Na2CO3, used to make normal glassSaponification Used as an alternative to NaOH since it is cheaperWater treatment Used to precipitate metal ions to soften water (including as an alternative
to phosphorus in washing powders)Paper making Used to make NaHSO3
Sulfur dioxide capture From power station waste gases, to comply with environmental regulationsAs a base It is cheap, and it is basic so it is often used as an industrial baseTo make NaHCO3 By bubbling CO2 through a solution of it, pure NaHCO3 can be obtained
(not contaminated by ammonia)
Table 4.2: Uses of sodium carbonate
of calcium and chlorine quickly enough (although the chlorine is the most damaging in fresh waterrivers), even if the output is diluted before disposal.
Another problem is the temperature of the waste liquids. The CaCl2 solution can have a hightemperature, and so it needs to be cooled or diluted to a point where it won’t cause thermal pollutionof the discharge point (even when discharged into the sea). This is another reason why plants distantto the ocean cannot discharge into rivers or lakes, and forces them to adopt other measures, such asartificial ponds.
Smaller, although not insignificant, problems include the damage that leaked ammonia can causeas air pollution, and the indirect impacts of the mining, transport, and storage of all the materialsrequired.
Location of a Solvay process plant
The location of a plant in relation to the reagents, as well as in relation to areas for disposal of the wasteproducts is an important part of minimising the cost and the environmental impact. For the Solvayprocess specifically, the cost of transport of the two reagents need to be weighed to decide whether, forexample, the plant should be situated nearer a source of brine, or nearer a limestone mine. Since thebrine is much more dilute (only 30% of its weight is important for the process versus above 80% or even90% for typical limestone), the amount need to be transported will be higher so its cost of transport islikely to be much greater which would make positioning the plant near the coast more favourable.
The environmental impacts of the position of the plant also must be considered. For example, thedisposal of the CaCO3 solution is a significant problem inland, so a plant would be both economicallyand environmentally better near the sea. Also, proximity to built up environments, where ammoniapollution is particularly bad should be considered, although, the availability of a work force is anotherfactor which should come into the decision of position.
Other factors could include: the cost and availability of energy (the Solvay process uses fairlylarge amounts of energy), the distance, cost and environmental impact of the transport of the productsto their markets (for example, a sodium carbonate plant could be situated next to a glass maker, toreduce costs).
43
Index
α decay, 11Acid rain, 15Acidity, 13Acids, 15
Conjugate, 17Definitions, 16Examples, 16Food additives, 16Salts, 17Strong & weak, 15Table of salts, 17
Addition polymersList of, 7Poly(vinyl acetate), 7Poly(vinyl chloride), 6Polyacrylonitrile, 7Polyethene, 6Polypropylene, 7Polystyrene, 7Teflon, 7
Alcaligenes eutrophus, 8Alkalinity, 13Alkanes, 5Alkanoic acids, 19Alkenes, 5Ammonia
Synthesis, 22Amphiprotic substances, 17Anode, 9Arrhenius, Svante, 17Atmosphere
Pollutants, 26Structure, 26
Atomic Absorption Spectroscopy, 24
β decay, 11Bacillus megaterium, 8Bases
Conjugate, 17Examples, 16Salts, 17Table of salts, 17
Biological oxygen demand, 29Biopolymers, 8
Polyhydroxybutyrate, 8Bosch, Carl, 23Bronsted-Lowry theory, 17Branches of Chemistry (largest branches are cir-
cled), 21Buffers, 18
Real-world example, 18
Carbon dioxideSolubility, 14
Cathode, 9Chlorofluorocarbons, 27
Alternatives, 28Problems, 28
Co-ordinate covalent bonds, 27Coagulation, 29Collaboration, 21Condensation polymers, 8
Cellulose, 8Nylon, 8Polyester, 8
Conjugate pairs, 17Cracking, 5
Catalytic, 6Steam, 6
Davy, Humphry, 17Detection of Radiation, 11
Cloud chamber, 11Geiger-Muller counter, 11Photographic film, 11Scintillation counter, 11
DetergentComparison to soap, 40Environmental impact, 41Types, 40
Diaphragm process, 38Displacement, 9Dissolved oxygen, 29Dry cell, 10
45
Index
Electrodes, 9Electrolytic cell
Comparison to galvanic cell, 37Emulsion
Examples, 41Emulsions, 40Equilibrium constant, 33Equilibrium reactions, 14, 33Equivalence point, 18Esters, 19
Esterification, 19Refluxing, 19Uses, 19
Ethanol, 8Ethene, 6Ethylene glycol, 5Eutrophication, 30
Flocculation, 29
Galvanic cell, 9Comparison to electrolytic cell, 37
Gas phase process, 6Glucose, 8Gratzel cell, 10
Haber process, 22History, 23Monitoring, 22
Haber, Fritz, 23Halons, 27Hardness, 29HDPE, 6Hydrochlorofluorocarbons, 28Hydroflurocarbons, 28
i, 33Indicators, 13Ion tests, 23
Anions, 23Cations, 23
Lavoisier, Antoine, 16LDPE, 6Le Chatelier’s principle, 14Leclanche cell, 10Liquid junction photovoltaic device, 10
Membrane filters, 30Membrane process, 38Mercury process, 37Mesosphere, 26Monitoring, 22
Need to monitor ions, 25Neutralisation, 18Nitrogen
Oxides, 14Di-, 14Nitric, 14Nitrous, 14
NuclearInstability, 10
Oxidation, 9Oxides, 13Oxygen free radical, 27Ozone, 27
Comparison to oxygen, 27Destruction, 28Measurement, 28
pH, 15Photochemical smog, 14Primary standard, 18Properties of addition polymers
Chain branching, 7Chain stiffening, 7Cross linking, 7Molecular weights, 7
RadioisotopesCobalt-60, 11Production, 11Sodium-24, 11Uses, 11
Reduction, 9
Salt bridge, 9Saponification, 39
Industrial, 39Secchi disk, 29Soap
Cleaning action, 39Comparison to detergent, 40Environmental impact, 41
Sodium carbonateUses, 43
Sodium hydroxideElectrolytic extraction, 37Production, 37
Solvay process, 41Environmental issues, 42
Stratosphere, 26Styrene, 7Sulfate, 25Sulfuirc acid
46
Index
As a dehydrating agent, 36Sulfur dioxide, 14Sulfuric acid, 34
As an oxidising agent, 36Extraction, 34Ionisation, 36Production, 35Safety, 37Uses, 34
Surfactant, 39
Titration, 18Total dissolved solids, 29Transuranic elements, 11Troposphere, 26Tryglycerides, 39Turbidity, 29
Vinyl chloride, 6Volumetric analysis, 18
Water quality, 28Acidity, 29Biological oxygen demand, 29Coagulation, 29Concentration of ions, 29Dissolved oxygen, 29Eutrophication, 30Factors affecting ion concentrations, 29Flocculation, 29Hardness, 29Heavy metals, 30Total dissolved solids, 29Treatment, 29Turbidity, 29
Zeolites, 6Ziegler-Natta process, 6
47
