As Chemistry Note1 Final

8/8/2019 As Chemistry Note1 Final

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AS CHEMISTRY NOTES

1.3 Formulae, equations and amounts of substances

Atoms are made up of Protons, Neutrons and Electrons

y An atom is the smallest particle of any element that retains the chemical propertiesof the element

y All substances are made out of atoms. Atoms are indivisible and indestructibley The mass and charge of the atoms subatomic particles are really small, so relative

mass and relative charge are used instead

Subatomic particle Relative mass Relative charge

Proton 1 +1

Neutron 1 0

Electron 1/2000 -1

y Electrons - circle around the nucleus in orbitals. The orbitals take up most space ofthe atom

y Nucleus- Most of the mass of the atom is concentrated in the nucleus- The diameter of the nucleus is really tiny compared to the whole atom- The nucleus is where you find the protons and the neutrons

Elements

y A substance that contains only one type of atom y A substance that cannot be broken down (by chemical means) into anything simpler

Molecules

y Two or more atoms chemically bonded together (can be atoms of the same elemente.g. Br2 or different elements e.g. H2O)

Compounds

y Two or more elements chemically bonded in a fixed ratioy A compound is a substance formed by two or more elements which have chemically

reacted with each other

Ions have different numbers of protons and electrons

y If an atom loses or gains electrons it is no longer an atom. It is called an ion. An ionhas a full shell of outer electrons. An ion has an electrical charge

y Negative ions (anions) have more electrons then protons


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y Positive ions (cations) have more protons then electronsThe Empirical formula and Molecular formula

y A molecular formula tells you how many atoms of each element there are in amolecule of the compound

y The empirical formula is the simplest whole ratio number of elements in thecompound

Balancing ionic equations and molecular equations

y Sodium hydroxide + Magnesium chlorideSodium Chloride + Magnesium Hydroxide2NaOH (aq) + MgCl2 (aq) 2NaCl (aq) + Mg(OH)2 (s)

y This is a molecular equation , because it shows the complete formula of everysubstance. Both reactants dissolve in water to form ions, so an ionic equation gives

more of a accurate picture:

2Na+

(aq) + 2OH-

(aq) + Mg2+

(aq) + 2Cl-

(aq) 2Na+

(aq) + 2Cl-

(aq) + Mg (OH)2 (s)y The sodium ions and chloride ions appear in exactly the same way on each side of

the equation. Ions like this are often called spectator ions, and can be left out of the

equation. This produces the simpler overall ionic equation:

Mg2+

(aq)+ 2OH

-(aq) Mg(OH)2 (s)

Relative atomic masses of atoms are compared to Carbon -12

y The relative atomic mass of an element is the average mass of the atoms (taking intoall of its isotope as their abundance) of the element relative to 1/12 the mass of one

carbon-12 atom

y There are two isotopes of chlorine 35Cl and 37Cl. If they had the same abundance, theAr would be 36. But

35Cl has a 75 percent abundance and 37Cl has a 25 percent

abundance. This means that for every 100 atoms, 75 have a relative isotopic mass of

35 and 25 have a relative atomic mass of 37

y The Ar is calculated as follows:


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Amount of substance

y Amount of substance is the number of moles of that substance (measured in moles) y The Avogadro constant is the number of carbon atoms exactly in 12 grams of the

carbon-12 isotope. Its value is 6.02 x 1023 mol-1

y One mole of a substance is the amount of that substance that contains 6.02 x 1023

particles of that substance

Number of moles= Number of particles you have

Number of particles you have in a mole

Molar mass is the mass of one mole

y The molar mass of a substance is the mass (in grams) of one moley Molar mass is just the same as the relative molecular mass, Mr

Parts Per Million is used for really small quantities (gases in the atmosphere, water

pollution, exhausts)

y Parts per million (ppm) is used for measuring concentration of material present insmall quantities, such as pollutants in the air or in water supplies

y Xenon makes up only 0.000 009% of the atmosphere. Numbers this small are a painto work with

y This is why another type of measurement is used- parts per million ppmy If theres 0.000 009 parts of xenon in every one hundred parts of air, you can

multiply both quantities by 10,000 to make the quantity large enough to work with

y ppm= mass of component/ mass of solution x106In a Solution the Concentration is measured in mol dm

-3

y The concentration of a solution is how many moles are dissolved per 1dm 3 ofsolution

y A solution that has more moles per dm3 is more concentrated. A solution that hasfewer moles per dm3 is less concentrated

y To find concentration in g dm -3= amount of solute (grams)/ volume of solution (dm 3)

Number of moles= mass of substance

molar mass

Number of moles= Concentration x volume of solution (in dm3)


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Reacting mass equations

y First , write a balanced equation for the reactiony Use the equation amount of substance= mass/molar massy Worked example: Calculate the mass of sodium hydroxide required to reaction with

1.23 g silicon dioxide Equation: SiO2 + NaOH Na2SiO3 + H2O

1. Amount ofSiO2 = 1.23/60.01= 0.0205 mol 2. Amount of NaOH= 0.0205 x 2= 0.0205 mol3. Mass of NaOH= 0.0410 x 40.0= 1.64 g

y Follow the route

Gas volume calculations

y One mole of gas occupies 24 dm3 at 25C (298 K) and 1 atmosphere pressure1. For reactions where a gas is produced from a solid or a solution follow:

y For step 1 use the relationship: moles of A= mass of A/molar mass of Ay For step 3 use the relationship: volume of gas= no. of moles x molar volume (24dm 3)y Worked example: Calculate the volume of carbon-dioxide evolved at r.t.p. when 7.8

g of sodium hydrogen carbonate is heated. The molar volume of a gas is 24dm3

at

the temperature and the pressure of the experiment

Equation: 2NaHCO3

Na2CO3 + H2O + CO2 (g)1. Amount of NaHCO3= 7.8/84.0= 0.09286 mol2. Amount of CO2= 0.09286 x 0.5= 0.0464 mol3. Volume of CO2= 0.0464 x 24 dm3 mol-1= 1.1 dm3

2. For calculations involving gases a shortcut can be used. The volumes of the twogases are in the same ratio as their stoichiometry in the equation

y Worked example:What volume of oxygen is needed to completely burn with15.6cm

3of ethane?

Equation: 2C2H6 (g) + 7O2 (g) 4CO2 (g) + 6H20 (l)

Calculation:


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Percentage yield

y The yield of a reaction is the actual mass of the product obtainedy There are many reasons why the theoretical yield is not achieved

Some of the product may be lost during the purification procedure

Reversible reactions may not go to comp letionSome of the products may react in an unexpected way

y The theoretical yield is the maximum amount obtainable if all the reactants areconverted into products

y First calculate, the theoretical yield from the equ ation using the reacting massmethod as above (page 4)

y Then the % yield

Atom economy

y Atom economy is a measure of the proportion of reactant atoms that become partof the desired product in the balanced equation

y Atom economy=

High atom economy is better for profits and the environment

y It is important for sustainable development and for economical reasons to usereaction with a high atom economy

y Pretty obviously, if youre making lots of waste, thats a problemy Reactions with low atom economy use up resources very quickly. At the same time,

they make lots of waste materials that have to be disposed of somehow.That tends

to make these reactions unsustainable- the raw materials will run out and the waste

has to go somewhere

y For the same reasons, low atom economy reactions arent usually profit able. Rawmaterials are expensive to buy, and waste product can be expensive to remove anddispose of responsibly

y Sustainable development involves balancing the need for economic development,decent standards of living and respect for the environment, so t hat resources are

available for future generations


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Calculation of number of particles

y Number of molecules=moles x Avogadro constanty Number of ions=moles x Avogadro constant x number of those ions in the formula y Worked example: Calculate the number of carbon-dioxide molecules in 3.3 g of CO2

Answer:Amount of CO2= 3.3/44.0= 0.075 mol

Number of molecules= 0.075 x 6.02 x 1023= 4.5 x 1023

y Worked example: Calculate the number of sodium ions in 5.5g of Na 2CO3Answer:

Amount of Na2CO3= 5.5/106.0= 0.0519 mol

Number of Na+

ions= 0.0519 x 6.02 x 1023

x 2= 6.2 x 1022

Finding and confirming and equation

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How to calculate the empirical formula and molecular formula


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Salts can be hydrated

y All solid salts consist of a lattice of positive and negative ions. In some salts, watermolecules are incorporated in the lattice too (its called water of crystallisation)

y A solid salt containing water of crystallisation is hydrated y For example, hydrated copper sulphate has five moles of water for every mole of the

salt. So its formula is CuSO4.5H2O

Double salts

y Double salts are crystals that contain two different salts in a 1:1 ratio. Ammoniumiron (II) sulphate, (NH 4)2Fe(SO)4.6H2O is a double salt because it has two cations

y Preparation:1. Add mass of iron fillings to excess warm sulphuric acid and stir till they have

reacted. Youve now got iron (II) sulphate solution

2. Add just enough ammonia solution to react completely with the iron3. Leave the solution to evaporate- blue green crystals of the salt will form.Some

solution will remain

4. Collect the crystal by filtering, then was them using distilled water 5. To dry crystals, press them between two pieces of filter paper to absorb as much

water as possible

Percentage yield of the doubl e salt

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1.4 Energetics

Chemical reactions often have Enthalpy Changes

y When chemical reactions occur bonds are broken or formed - this causes a change inenergy

The enthalpy change, H is the measure of the transfer of energy into or out ofreacting system at a constant pressure. The units of H are kJ mol

-1

Hreaction= the sum of Hformation of products the sum of Hformation of reactantsReactions can either be Exothermic or Endoth ermic

An exothermic reaction gets hot, so that heat is then given out to the surroundings.For all exothermic reactions H is negative. This means that chemical energy is being

converted into thermal (heat) energy. (oxidation)

y The heat in one such pack comes from the oxidation of iron an exothermic reaction.A damp mixture of iron fillings with salt and charcoal is contained in a perforated

bag. The bag is activated by breaking the seal, allowing the oxidation reaction to take

place, speeded up by the presence of the salt and charcoal.

4Fe (s) + 3O2 (g) 2Fe2O3 (s)

An endothermic reaction gets cold, so that heat is then taken in from thesurroundings. For all endothermic reactions H is positive. (photosynthesis)

y Cold packs for treating sports injuries by gaining heat from your body. One type usesthe endothermic reaction that happens when ammonium nitrate and water are

mixed

NH4NO3 (s) NH4+

(aq) + NO3-(aq)

y Enthalpy Profile diagrams show you how the enthalpy (energy) changes duringreactions


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Standard conditions and standard states

y You write H to show that elements were in their standard states and that themeasurements were under standard conditions

y Enthalpy changes are affected by temperature and pressure- using standardcondition means that everyone can know exactly what the enthalpy change isdescribing

y Standard conditions are: 1. A pressure of 1 atmosphere (100kPa)2. A temperature of 25C (298 K)3. Solutions if any, at a concentration of 1.00 mol dm -34. Substances at their most stable state e.g. carbon as graphite not

diamond, water at 298K as a liquid

There are different types of H depending on the reaction

Standard enthalpy change of reaction, Hr, is the enthalpy change when thereaction occurs in the molar quantities shown in the chemical equation, under

standard condition in their standard states

Standard enthalpy change of formation, Hfis the enthalpy change, under standardconditions when one mole of a compound is formed from its elements in their

standard states. The enthalpy change of formation of an element (in its stable state)

is zero e.g. Hf for ethanol is the enthalpy change for the reaction:

Standard enthalpy change of combustion, Hc is the enthalpy change, understandard conditions when one mole of a substance is completely burned in oxygen

e.g Hc for ethanol is the enthalpy change for the reaction:

Standard enthalpy change of neutralisation, Hneut is the enthalpy change, understandard conditions, when an acid and a base undergo a neutralisation reaction to

form one mole of water e.g. Hneut of sulphuric acid, by sodium hydroxide solution, is

the enthalpy change for the reaction:

Standard enthalpy change of atomisation, Hat, is the enthalpy change when 1 moleof gaseous atoms is formed from the element in its standard state, e.g.


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Experimental results always include errors

y Systematic errors are repeated every time you carry out the experiment, and alwaysaffect your result in the same way. Theyre due to the experimental set up, or

limitations of the equipments

y Experimental problems with the calorimetry generally:Some heat will be absorbed by the container, rather than going towards

heating up the water

Some heat is always lost to the surroundings during the experiment (however

well you insulate the container)

y Experimental problems with flammable-liquid calorimetry:Some combustion may be incomplete- which will mean less energy will be

given out

Some of the flammable liquid may escape by evaporation (theyre usually

quite volatile)

y Random errors always happen. The best way to deal with them is to repeat youexperiment, and take an average of all the readings (except anomalies)

y The calorimeter is used to determine the energy change during a reaction accuratelyis known as a bomb calorimeter

y However good the insulation, some heat is transferred to the surroundings whichadds, an element of inaccuracy and unreliability to results. This heat loss can be

estimated and corrections can be calculate to compensate for it

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Hesss Law- the Total Enthalpy Change is independent of the route taken

y Hesss law states that:The total enthalpy change for a reaction is independent of the route taken

y i.e. the total enthalpy change of a reaction will always be the same, no matter whichroute is taken

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Pg 23 cgp


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Breaking bonds: Bond enthalpy

Bond enthalpy is the energy required to break a bond between two atoms in agaseous molecule

Bond enthalpy (Bond dissociation enthalpy), E, is the energy required to break onemole of bonds of the same type in gaseous molecules under standard conditions(298 K, 100 kPa)

y Consider hydrogen, which has an H -H bond:H-H (g) H (g) + H (g) H = +436

Reactions are all about breaking and making bonds

y Breaking bonds is an endothermic process. This is because we are putting i n energyto overcome the force of electrostatic attraction in the bond ( H is positive)

y Making bonds is exothermic, as energy is released when bonds are formed ( H isnegative)

y In an exothermic reaction, the energy released from forming new bonds is greaterthan the energy needed to break existing bonds

y If an endothermic reaction, the energy need to break existing bonds is greater thatthe energy released from forming new bonds

y In ionic bonding, positive and negative ions are attracted to each other. In covalentmolecules, the positive nuclei are attracted to the negative charge of the shared

electrons in the covalent bond

Bond enthalpies can also help you predict which bonds will break first in a reaction,and how easy it is to break the bond. The hig her the bond enthalpy, the more energy

is needed to break the bonds, and the less likely it is to break in a reaction Bonds with a relatively low bond enthalpies are the easiest and therefore usually the

first bonds to break

A reaction that involves breaking bonds with relatively low bond enthalpies is morelikely to take place at room temperature than a reaction between molecules held by

bonds with high bond enthalpies. This is because less energy is needed from the

surroundings to break the reactant bonds

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Average bond enthalpies are not exact

Mean bond enthalpy- the mean value of the bond dissociation enthalpy of aparticular bonds over a wide range of different compounds


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y There isnt just one bond enthalpy value between two particular types of atmos.Forexample in water (H2O) there are two O-H bonds

y The energy required to break the first O-H bond is not the same as the energyrequired to break the second:

H-O-H (g) H (g) + O-H (g) H = +502 kJ mol-1

O-H (g) O (g) + H (g) H = +427 kJ mol-1

y The first bond is in a molecular environment in which two O -H bonds exist, and moreenergy is required to break this bond.The second O-H bond is in a changed

environment, and clearly this has an effect on its bonds energy (extra electron

repulsion)

y The mean bond enthalpy= (502+427)/2= +464 kJ mol -1y The data book says the bond enthalpy of O-H is +463 kJ mol-1. Its a bit different

because its an average for a much bigger range of molecules, not just water

Pg 10 get the grade


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You can use Bond enthalpies in Hesss law cycle

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1.5 Atomic structure and the periodic table

Definitions

The relative atomic mass of an element is the average mass of the atoms (takinginto all of its isotope as their abundance) of the element relative to 1/12 the mass of

one carbon-12 atom

For a compound, the relative formula mass is the sum of the relative atomic massesof all the atoms in the chemical formula. For a molecular (covalent) compound this is

referred to as the relative molecular mass

The relative mass of an isotope (its relative isotopic mass) is the mass of one atomof that isotope compared to 1/12 the mass of one atom of carbon -12

Measuring the relative atomic mass of an atom using a mass spectrometer

Atoms are too small to measure their mass directly by weighing. However, aninstruments called the mass spectrometry provides an answer

1. Vaporisation- The sample being measured must be in the gaseous state for itsparticles to move through the machine. The sample is injected into the mass

spectrometer and is first vaporised using an electrical heater

2. Ionisation- The vapour is bombarded with high -energy electrons, whichcollide with atoms of the sample. They knock one or more electrons out of the

atoms within the sample particles to form positive ions. Because the particles

are now charged they can be accelerated in an electric field

3. Acceleration- The positive ions are accelerated by an electric field4. Velocity selector-They are passed through a velocity selector, which makes

sure they are all travelling at the same velocity. This means any differences in

the effect of the magnetic field will be due the different mass or charges of

the ions not their speeds

5. Deflection-The ions enters a magnetic field, which deflects them. The amountthey are deflected depends on the mass of the ion and the charge on it.

Heavier ions are deflected less than lighter ions, and ions with a small positive

charge are deflected less than ions with a bigger positive charge. The magnetic

field is slowly increased. At any particular setting of the magnetic field, only

ions with a particular mass/charge will make it to a detector- any other ions

will be deflected to much or too little to pass through6. Detection-The detector detects how many ions pass through the machine at

each magnetic field setting and each setting of the velocity selector. A mass

spectrum is produced. It show how many ions of each mass: charge ratio there

are in the sample


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Pg 52 edexcel and page 26 cgp


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Ionisation is the complete removal of an electron from an atom

Ionisation is an endothermic process since work must be done on an electron inorder to overcome the attractive force between it and the nucleus

The amount of energy need to remove an electron from an atom is known as theionisation energy

The energy needed to remove one electron from an atom is called the first ionisatioenergy:

The energy required to remove one mole of electrons from one mole of gaseous

atoms to form one mole of ions with a single positive charge

y First ionisation of oxygen: O (g) O

+(g) + e

-1

stionisation energy= +1314 kJ mol

-1

y The lower the ionisation energy , the easier to form an ion The factors affecting ionisation Energy are:

y Nuclear charge- The more protons there are in the nucleus, the stronger the positivetherefore the electrons are more strongly attracted by the nucleus.This is the reason

why the ionisation energy increases as each successive electron is removed

y Distance- Attraction falls off very rapidly with distance, an electron close to thenucleus will be much more strongly attracted than one further away. Notice that

there is a large jump in the energy required to remove the third electron.This is

because we are breaking into the second shell which is closer to the nucleus (Mg)

y Shielding-as the number of electrons between the outer electrons and the nucleusincrease, there is an increase in shielding.This means that the outer electron are less

strongly attracted to the nucleus


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There are trends in the first ionisation energies

y The first ionisation energies of elements down a grou p of the periodic tabledecreases. As each element down the group has an extra electron shell meaning an

increase in shielding and distance. Therefore re duce the attraction between the

nucleus and outer electron making it easier to remove y The first ionisation energies of elements across a period generally increase

Successive ionisation energies involve removing additional electrons

y Each time you remove an electron, theres a successive ionisation energyy The second ionisation energy is the energy required to remove 1 mole of electrons

from 1 mole of gaseous 1+ ions to form 1 mole of gaseous 2+ ions

O+ (g) O2+ (g) + e- 2nd ionisation energy= +3388 kJ mol -1

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First ionisation energies of elements (2,3,3) pattern


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Electron shells are Made up of Sub-Shells and Orbitals

y Each shell is given a number called the principle number called the quantum number y Quantum mechanics also tells you that each shell may contain a number of sub-

shells. This sub-shells are described by the letters: s, p, d, f, g

Shell Sub-shells1 1s

2 2s, 2p

3 3s, 3d, 3p

4 4s, 4d, 4p, 4f

y Shell 1 is the closest to the nucleus, therefore required the most energy to removeelectrons within this shell. Within a shell, the sub -shells have different energies.With

electron in the lowest energy sub -shells being closer to the nucleus:

s (lowest energy) < p < d

Orbitals have characteristic shapes

y Each type of sub-shell contains one or more orbitals. An orbital is the region wherethe electrons are most likely to be found. Orbitals in the same sub -shell have the

same energy

Sub-shell No. of

orbitals

Maximum

electrons

s 1 2

p 3 6

d 5 10

F 7 14

y s orbital are spherical. p orbitals are dumbbell-shaped. The three p orbits are atright angle to one another

y As with the s orbital, the size of p orbitals increases with the principal quantumnumber, so a 3p orbital is large than a 2p orbital

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Pg 28/29 cgp


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The Periodic Table arranges Elements by proton numbers

y The modern periodic table arranges the elements in order of their atomic numbery Vertical columns are called groups and horizontal rows are called periodsy All elements within a group have the same number of electrons in their outer shell -

so they have similar propertiesy The modern periodic law states that:

The properties of the elements are a function of their atomic numbers

y Elements in the s block have out er electron in the s sub shells, those in the p blockhave outer electrons in the p sub-shells

y The chemical properties of an element are determined by the number of electrons inits highest energy level

y A periodic property is a physical or chemical property of elements that changesgradually as you move across a period, and the gradual change is repeated in each

period

Trends in the periodic table

The atomic radius generally decreases across a period: as you move across a periodthe nuclear charge becomes increasingly positive as the number of protons in the

nucleus increases. Although the number of electrons also increases, the outer

electrons are all I the same shell. This means that they are attracted more strongly to

the nucleus, thus reducing the atomic radius across a period

The atomic radius generally increases down a group : the outer electron enter newenergy levels passing dong a group, so although the nucleus gains positive protons,

the electrons are both further away and screened by more electron shells. As aresult they are not held so tightly and the atomic radius incre ases

Periodic trends in ionisation energy

Ionisation energy generally increases across a period - it becomes harder to removean electron. This is the result of an increasing positive nuclear charge across the

period without the addition of any electron sh ells to screen the outer electrons.The

atomic radius gets smaller and the electrons are held more firmly, so it required

more energy to bring about ionisation. Noble gases have the highest ionisation

energy due to their stable electronic structure therefore are relatively unreactive

Ionisation energy generally decreases down a period- it becomes easier to lose anelectron

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Pg 34 gcp and page 77 edexcel


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1.6 Bonding

Ionic Bonding

Ions have different numbers of protons and electrons

y If an atom loses or gains electrons it is no longer an atom. It is called an ion. An ionhas a full shell of outer electrons. An ion has an electrical charge

y Negative ions (anions) have more electrons then protons.They ar e called anionsbecause they are attracted to the anodes (positive electrode)

y Positive ions (cations) have more protons then electrons. They are called cationsbecause they are attracted to the cathodes (negative electrode)

Ionic bonding is when ions are stuck together by electrostatic attraction

y Ionic compounds are usually formed when metals bond to non -metalsy Oppositely charge ions are formed, which are held together by an extremely strong

electrostatic force of attraction. This force is call the ionic bond

y Ionic bonding can be thought as the net electrostatic attraction between the ionsIonic crystals are giant lattices of ions

y The lattice structure of a particular ionic compound is the arrangement of the ions ina way that maximises the attractive forces between the oppositely charges ions and

minimises the repulsion between the similarly charged ions

y The forces exerted by the ions in a giant lattice act equally in all directions, holdingthe ions together tightly

y Giant lattices of ions from ionic crystals y Forces between ions of an ionic crystal depends on charge and size of ions:

The larger the charge, the stronger the force of attraction

The smaller the sum of atomic radii, the stronger the force of attraction

y In sodium chloride the Na + and the Cl- ions are packed together in a cubic structurePg36 cgp

Ions are smaller than atoms for metals but larger for non -metals

y The ionic radius is the radius of an ion in a crystalsy The radius of a positive (metal) ion is smaller than the elements atomic radius

because the remaining electrons are more strongly attracted to the positive nucleus :


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The positive charge of the nucleus is greater than the negative charge in theelectron cloud

The outer electron shell is emptied so the new outer shell is closer to the

nucleus and theres less electron shielding

y Negative (non-metals) ions are larger than the atom that formed them because theadditional charge means all the electrons are bound less tightly to the nucleus :

The negative charge is greater in the electron cloud, which means theres

greater repulsion between the electrons and the electron cloud expands a

bit

y Negative ions are larger than positive ions and increase in size as charge increasesThe size of an ion depends on its atomic number and charge

y The ionic radius increases down a group of the periodic table, as the atomic numberincreases. This is because the number of electron shells increase

y Isoelectronic ions are ions of different atoms with the same number of electronsy The ions N3-, O2-, F-, Na+, Mg2+ and Al3+ are Isoelectronic, they all have the same

electron structure (2,8)

y The ionic radius of a set of isoelectronic ions decreases as the atomic numberincreases. This is because the number of electrons stay the same, but the number of

protons increases meaning that the electrons are attracted to the nucleus more, so

the ionic radius decreases

Dot and cross diagrams

y Cations are groups 1, 2, 3 and the Anions are groups 5,6, and 7 have the electronconfiguration of a noble gas. Ions of the d-block metals do not have noble gas

electron configurations

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The theory of ionic bonding fits the evidence from physical properties

y The physical properties of ionic compounds provide evidence for the existence ofions

y They have high melting points- tells you that the atoms are held together by astrong attraction (model fits the evidence)


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y They are soluble in water but not in non-polar solvents- this tells you that theparticles are charged. The ions are pulled apart by polar molecules like water, but

not by non-polar molecules (model fits the evidence)

y Ionic compounds dont conduct electricity when theyre solid- but they do whenmolten or dissolved. (as the ions are free to move around and conduct charge) This

supports the idea that there are ions, which are fixed in position by strong ionic

bonds in a solid, which care unable to move to conduct electricity. When the ionic

compound melts or is dissolved in water, the attraction between the ions is

overcome (ionic bonds are disrupted) and the ions are free to move and conduct

electricity

There are different kinds of evidence for the existence of ions

y The electron density map of an ionic crystal shows that there are spaces betweenthe ions where the density of electrons is zero. This shows that the atoms have no

shared electrons- the bonding electrons have moved from one atom to another

y The bigger the ion the more electrons it has and the brighter the spots it produces.By analysis of the positions and intensities of the spots, experts can work out of the

charge density (electrons per cubic nanometre) of the electrons

y This diffraction ofX-rays is clear and repeatable pattern is further evidence of thepresence of ions in crystals

y Migration of ions on wet filter paper is evidence for the presence of ions:When you electrolyseagreen solution ofcopper (II) chromate (VI) the filter

paper turnsblue at the cathode and yellow at the anode

Copper ions are blue in solution and chromate ions are yellow. Copper

chromate solution is green because it contains both ions

When you pass a current through the solution, the positive ions move to the

cathode and the negative ions move to the anode

y This demonstration of electrolysis shows the presence of ions in solutionThe formation of the lattice involves a release of energy- lattice energy

y The Hlat, is the enthalpy formation of one mole of an ionic compound from itsgaseous under standard conditions

y Lattice energies are negative (exothermic) because they relate to forming the lattice(this releases energy)

y The electron affinity is the energy change when mole of ions is formed from onemole of atoms:

First electron affinity is the energy change per mole for the addition of one

electron to a gaseous atom to from a singly charge negative ion

Second electron affinity is the energy change per mole for the addition of an

electron to a singly charged negative gaseous ion


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Born-Haber cycles can be used to Calculate Lattice Energies

y Born-Haber cycles show enthalpy changes when a solid ionic compound is formedfrom its elements in their standard states. They show two routes - direct and indirect

y The direct way to from sodium chloride from its elements is the standard enthalpy offormation. The indirect route involves adding up all the enthalpy changes

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Born-Haber cycles can Show why some Compounds dont Exist

y If lot of energy is released during the formation of a compound, the compound isnice and stable- doesnt want to break up again

y Some compounds either dont form in the first place, or else they break up veryquickly to from more stable elements or compounds. For example, NaCl is found allover the place, but you never get NaCl2. Heres why:

The Born-Haber cycle for NaCl gives a negative enthalpy of formation- the

formation of NaCl from its eleme nts is an exothermic process- energy is

released overall. So the NaCl is stable

However, to from NaCl2, you need to form Na2+

ions

Therefore a second electron would have to be removed from the sodium.

This electron would have to come from an inner shell a nd so a huge amount

of energy required. Therefore there is a high second ionisation energy

Even though, the lattice energy for NaCl2 would be more exothermic than

that of NaCl, this is not enough to compensate for the large endothermic

second ionisation energy. Thus the Hfof NaCl2 would be highly endothermic

and so it is not formed i.e. its energetically unfavourable - not stable

y Similarly, MgCl2 is more stable than either MgCl3 or MgCl:MgCl3 cant exist because the large endothermic third ionisation makes the

Hfpositive

With magnesium, the second electron comes from the outer shell and so the

second ionisation energy is only slight more endothermic than the first. This

is compensated for by the greater lattice energy ofMgCl 2 compared to MgCl

MgCl and MgCl2 both have negative enthalpies of formation, so both

compounds can from. However, more energy is released by formingMgCl 2,

so MgCl2 is more stable, so if any MgCl forms in a chemical reaction, it

immediately disproportionates to from MgCl and Mg. 2MgMgCl2 + Mg

Theoretical lattice Energies are based on the Ionic Model

y Two ways of calculating lattice energy:Experimental way- using experimental enthalpyvalues in a Born-Haber

Cycle

Theoretical way- doing some calculations based on the purely ionic model of

a lattice

y To work outtheoretical lattice energy, you assume that all the ions are sphericaland have their charge evenly distributed around them- a purely ionic lattice. Then

you work out how strongly the ions are attracted to another based on their charges,

the distanced between them and so on. That gives you a value for the energy change

when the ions form the lattice


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Comparing lattice energies can tell you How ionic an Ionic Lattice is

Pg 42

Polarisation of Ionic bonds leads to covalent character in ionic

y Polarisation is when ionic bonds can be distorted by the attraction of the positivecation for the outer electrons of the negative anion

y If distortion is great, it may lead to a chare cloud that begins to resemble a covalentbond

y Magnesium halides have more covalent character in their ionic bonds than sodiumhalides because Mg

2+is smaller and has a bigger charge (therefore higher charge

density, so it can pull electrons towards itself a bit, polarising the bond

y The greater the charge density of the cation the poorer the match between theexperimental and theoretical values for lattice energy

Small cations are very polarising and large ions can be polarised easier

y The polarising power depends on the charge density, and in turn this depends onboth the ionic radius and its charge. All ionic bonds have a degree of covalency due

to polarisation


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y Small cations with a large charge are very polarising because they have a highcharge density- the positive charge is concentrated in the ion.So they have a

stronger attraction for the outer electrons of the anion

y Large anions are polarised moreeasily because their electrons are further awayfrom the nucleus. So the electrons on large anions can be pulled away more easily

towards the cations High negatively charged ions are polarised more easily , as the

outer electrons are held more loosely by the positively charged nucleus)

y If a compound contains a cation with a high polarising ability and an anion which iseasily polarised, some of the anions electron charge cloud will be dragged towards

the positive cation. If the compound is polarised enough, a partially covalent bond is

formed

y When the difference of electronegative is smaller between the ions - the bondingcontains a degree of covalent character. Less attraction between the oppositely

charged particles and more electron sharing. When big-ionic bonding

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Covalent Bonding

y Many elements dont form ionic compounds. Therefore use another method toachieve a noble gas configuration- electron sharing (Covalent bonding)

y Molecules are held together by covalent bonds, which are strongy A covalent bond is the electrostatic attraction between the two nucl ei for a shared

pair of electrons

Covalent Dot and Cross diagrams

y A single covalent bond consists of a pair of shared electrons, with each atomsupplying one electron

y A dative covalent bond consists of a pair of shared electrons, with both electronssupplied from one atom

y A double bond consists of two shared pairs of electrons, each atom supplying two ofthe electrons


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Pg 44- 45


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Covalent bonds can from Giant Molecular structures

y Covalent substances may be:Giant atomic- diamond, graphite and quartz (SiO2)

Simple molecular- for example I2 and many organic substances

Non-crystalline- for example, polymers such as polyetheney In a giant atomic solid, the particles are atoms that are held together by strong

covalent bonds

y Giant atomic structures form distinctive atomic crystalsy Giant molecular structures have a huge network of covalently bonded atomsy One of the best know of these is diamond, with carbon atoms held together by

covalent bonds

y The crystals are very hard, with a high melting demonstrating the great strength ofthe covalent bonds

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Silicon-dioxide has a tetrahedral arrangement

y Silicon dioxide is a giant molecular structure y Each silicon atoms covalently bonds with four oxygen atoms in a tetrahedral

arrangement to from a big crystal lattice

Pg 44

The properties of Giant atomic structures provide evidence for covalent bonds

y They are all insoluble in polar solvents like water, which shows that they dontcontain ions. It requires a lot of energy to break the covalent bonds before the atoms

can move into the liquid


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y They from hard crystals with very high melting points. This is down to the theirnetwork of very strong covalent bonds- ( require a lot of energy to break them)

y They dont conduct electricity. all their bonding electrons are used to form covalentbonds, and they contain no charged particles (exception- graphite due to its

delocalised electrons within its sheets of atoms )

Metallic bonding

y Metals are good conductors of heat and electricityy The structure of a metal is a giant regular lattice of positive ions in a sea of

delocalised electrons. These delocalised electrons come from the outer shell of the

metal atoms- this leaves a positive metal ion

y Metallic bonding is the strong force of attraction between the positive ions and seaof delocalised electrons that surrounds them

y The smaller the radius of the metal ions, the stronger is the bond and the higher isthe melting point

The model of metallic bonding can be used to explain the properties of metals

1. Malleable (hammered in the shape) and ductile (drawn into wire)- Our modelsuggest that there are no bonds holding the specific ions together therefore the

positive metal ions can move (slide over ea ch other) within the sea of electrons, and

wherever they move are still surrounded by a sea of negative electrons

2. High melting and boiling point temperatures- The simple model of metallic bondingseems to provide an explanation of this, with a lattice of positive ions held tightly

together by negatively charged electrons.The strong attraction between the positive

ions and the negative electrons means a lot of energy is required to separate them

The number of delocalised electrons per atom affects the mel ting point.The

more there are, the stronger the bonding will be and the higher the melting

points. Mg2+ two delocalised electrons per atom, so it has got a higher

melting point that Na+, which only has one. Mg is also harder

The size of the metal ions also affects the melting points, because it affects

the ions charge density. The higher the charge density, the strong the

bonding and the higher the melting point

3. Conducting electricity The model of metallic bonding explains this because thedelocalised electrons are free to move through the lattice under the influence ofelectric field i.e. the delocalised electrons can carry current

4. High thermal conductivity The model of metallic bonding with positive ions and asea of delocalised electrons explains this.The delocalised electrons move easily and

so can transmit kinetic energy rapidly though the lattice.When heated, the electrons

move rapidly from the areas of high temperature that have high kinetic energy,


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1.7 Introductory organic chemistry

About organic chemistry

y Organic chemistry is the study of carbon compounds, except simple ones (carbon -dioxide, carbon-monoxide and the carbonates)

y An organic compound consists of a chain of one or more carbon atoms and maycontain one or more functional groups

There are loads of ways of representing organic compounds

y General formula- An algebraic formula that can describe any member of a family ofcompounds. For all alcohols C nH2n+1OH

y Empirical formula- The simplest ration of atoms of each element in a compound(cancel the number downs if possible). So ethane, C 2H6 has an empirical formula of

CH3

y Molecular formula- The actual number of atoms in each element in a molecule, withany functional groups indicated. C 4H9OH

y Structural formula- shows the atoms carbon by carbon, with the attached hydrogensand functional groups e.g. CH 3CH(OH)CH3- propan-2-ol. It is advisable to show any

double bonds, so the structural formula of ethene is H2C=CH2

y Displayed formula- this shows all the atoms and all the bonds. The displayedformula of butan-2-ol is:

y Skeletal formula: this is normally used only for large molecules. The carbon skeletonis shown by zigzag lines, where each end of a line and each angle represents a

carbon atom. The skeletal formula of butan-2-ol is:

The Functional groups and Nomenclature

y Families of organic chemicals can be identified by the possession of a particularfunctional group

y A functional group is an atom or a group of atoms that is typical of a particularorganic family and which determines the chemical properties of the molecul e

y Aliphatic molecules contain straight- or branched chain carbon skeletons


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The difference between risk and hazard

y The hazard presented by a substance or an activity is its potential to do harm y A hazard is anything that cause harmy Risk is associated with a particular hazard is the change that it will actually cause

harmy Risk is the change that what youre doing will cause harm

Using organic chemical can be hazardous

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A Risk assessment can help to make lab work safer

y Before you do anything with chemicals in a lab, you should do a risk assessment. Arisk assessment looks at the hazards of all the reactants, products and procedures

involved in an experiment and consider how to make the risk from them as small as

possible. You can reduce risks by:

Work on a smaller scale e.g. use smaller quantities of chemicals

Take appropriate precautions- eye protection, plastic gloves. Also carry out

experiments that produce dangerous fumes (toxic) in a fume cu pboard

Carrying out the reaction using an alternative method that involve less

hazardous substances e.g. safer chemicals or lower concentrations

Changing the conditions under which a reaction takes place e.g. for instance,

lowering the temperature of a reacting mixture will slow the reaction down

y It is impossible to completely get rid of all risk. But the point of doing a riskassessment is to systematically to minimise the risks

Alkanes are saturated hydrocarbons

y Alkanes have the general formula CnH2n+1 and they occur as both straight andbranched chains. They are hydrocarbons because they only contain hydrogen and

carbon atoms

y Alkanes are saturated hydrocarbons that only contain carbon-carbon single bonds.Every carbon atom forms four single bonds with other carbon atoms. It is impossible

for alkanes to make more than four bonds-saturated


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y Cycloalkanes- a ring of carbon atoms with two hydrogens atoms attached to eachcarbon atom. Has the formula C nH2n but still are saturated e.g. cyclohexane

Structural isomers have different arrangements of the same atoms

y Structural isomers are molecules that have the same molecular formula but adifferent structural arrangement i.e. different structural formula

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Halogens react with Alkanes, forming Halogenoalkanes

y The alkanes react with chlorine, but only with an input of energy in the form ofsunlight or ultraviolet light

y The light provides the input of energy needed to break the hydrocarbon bonds y A hydrogen atom is substituted (replaced) by a chlorine or bromine in a

photochemical reaction (a reaction started byUV radiation). This is a free -radical

substitution reaction:

CH4+ Cl2CH3Cl + HCl

CH3Cl +Cl2 CH2Cl2 + HCL and so on

y For example chlorine and methane react with ultraviolent light as an input of energyto form chloromethane and hydrogen chloride

y The reaction mechanism has three stages:Initiation reaction-free radicalsare produced

1. The energy of the UV light causes the Cl-Cl bond to break- this isphotodissociation

2. The bond splits homylitically (equally and each atom gets to keep oneelectron-homolytic fission) producing two chlorine free radicals. The atom

becomes a highly reactive free radical, because of its unpaired electron

Propagation reactions- free radicals are used up and created in a chain

reaction. They produce another free radical

1. The chlorine radical (attacks) removes a hydrogen atom from the methaneproducing a methyl radical:

2. A methyl radical then removes a chlorine atom from a chlorine molecule:3. The new Chlorine radical can attack another CH4 molecule, until all the Cl 2 or

CH4 molecules are wiped out

Termination reactions (highly exothermic)- free radicals are mopped up.

There are reactions that remove radicals from the system without replacing

them with new ones:

1. If two free radical join together, they make a stable molecule2. Some products formed will be trace impurities in the final sample. The

presence of ethane is evidence for the mechanism

3. There are heaps of possible termination reactions:


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y If the supply of chlorine is limited (i.e. methane is in excess), there will be lots ofchloromethane, hydrogen chloride, and rela tively little ethane

y However, if there is a plentiful supply of chlorine (i.e. the chlorine is in excess) thenfurther substitutions of methane will take place to give di -, tri- and

tetrachloromethane as follows:

y CH3Cl was one of the first anaesthetics to be used for surgical operationsImportant definitions that should be learnt so far

y Homologous serious: a series of compound with the same functional group and thesame general formula in which one member differs from the next by CH2

y Homolytic fission: when a covalent bond breaks and one electron goes to each atomto form radicals

y Heterolytic fission: when a covalent bond breaks and both electrons go to one atomresulting in positive and negative charged ions

y Substitution: a reaction in which an atom or group of atoms in one molecule isreplaced by another atom or group of atoms

y Addition: a reaction in which two molecules react together to form a single product y Free radical: an atom or a fragment of a molecule with an unpaired electron e.g. Cl.y Electrophile: a species that seeks out electron-rich areas and accepts a pair of

electrons, forming a covalent bond (an electron pair acceptor) - (H+)

y Nucleophile : an electron pair donor. It may be a negative ions or it might be amolecule with a lone pair of electrons which can be donated to for a bond (Cl -)

Crude oil is a mixture of different sized hydrocarbon molecules

y Crude oil is formed from the buried remains of plants and animals - its a fossil fuel.Over millions of years with high temperature and pressure, the remains turn into

crude oil. It is extracted from the ground by drilling and pumping

y Crude oil is made out of mostly alkanes, with some alkenes and alkynesy Because crude oil is a mixture, the different hydrocarbon molecules arent

chemically bonded to one another- so they all keep their original properties, such as

their condensing points

-This means that crude oil can be split up into fractions by fractional distillation

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Heavy fractions can be Cracked to make smaller molecules

y The smaller fractions produced during the fractional distillation of crude oil are ingreat demand both as fuels and raw materials for the chemical industry, some of the

heavier fractions are of less use- make 50% of the products

yTo meet this demand, the less popular heavier fractions are cracked. Cracking isbreaking long-chain alkanes into smaller hydrocarbons (which can include alkenes). It

involves breaking the C-C bonds

y Cracking (thermal decomposition) is heating long chain alkanes to high temperaturescausing the molecule to split and form shorter -chain molecules. That are more useful

e.g. gasoline

y When alkanes are heated to high temperatures in the absence of air they split intosmaller molecules

Decane Ethene + Octane

C10H22 C2H4 + C8H18

y Producing high temperatures needed to crack the heavy petroleum fractions wouldbe very expensive. By using catalyst (zeolites) the process can be carried out at a

much lower temperature- It is known as catalytic cracking and is carried out a cat

cracker

y Cracking of ethane produce ethene, important raw material in the chemical industry,and hydrogen

C2H6 (g) C2H4 (g) + H2(g)

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Fuels contain a mixture of types of Alkane

y Most peoples cars run on petrol or diesel, both of which contain a mixture ofalkanes (as well as other hydrocarbons, impurities and additives)

y Some of the alkanes in petrol are straight-chain alkanes e.g. hexaney Petrol also contains some shorter, branched chain alkanes e.g. 2, 3-dimethlybutane,

Cycloalkanes and aromatic hydrocarbons. These types of alkanes makes the fuel

burn more efficiently

y Catalytic cracking produces aromatic hydrocarbons. Straight chain alkanes can alsobe reformed to make Cycloalkanes and more aromatic hydrocarbons

Alkanes can be reformed into Cycloalkanes and Aromatic hydrocarbons (arenes)

y Catalytic reforming is used to modify molecules to suit demand y In catalytic reforming, straight branched chain alkanes are converted in to arenes

and cycloalkanes (new isomers with branched chains)

y The catalyst used is a bimetallic catalyst- platinum rhodiumy An arene is a ring of hydrocarbons useful in the chemical industry

Pg 53

Alkanes are used as fuels

y When you burn an alkane in plenty of air, you end up with ca rbon-dioxide and water.Its an exothermic reaction. Alkanes make great fuels, because burning them

releases a lot of energy

Methanes used for central heating and cooking in homes

Alkanes with 5-12 carbon atoms are used in petrol

Kerosene is used as jet fuel. Its alkanes have 11-15 carbon atoms

Diesel is made of a mixture of alkanes with 15-19 carbon atoms and is used

as a car fuel

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Alternatives for crude oil

y Electricity can be generated by nuclear and wind powery Ethanol powered cars, hydrogen powered carsy Solar energy can be used to heat watery Most of the things are set up for crude oil fractions e.g. cars. Its ready availabley Crude oil fractions are often the easiest and cheapest to usey Crude oil fractions are more reliable e.g. solar power and wind power wont work

without the right weather conditions

Alternatives fuels are being used

y Ethanol can be produced by fermentation of plants and is used to power cars in thesome places. Its often mixed with petrol to make a better fuel

Pros- the carbon dioxide is released was taken in as the plant grew, so

its carbon neutral

Cons- engines need to be converted before theyll work with ethanol.

It isnt widely available

y Biogas is a mixture of methane and carbon dioxide. Its produced whenmicroorganisms digest waste material. Biogas is burnt and the energy is used for

cooking, heating, lighting

Pros- waste material is cheap and readily available. Its carbon neutral

Cons-Biogas production is slow in cool weather

y Hydrogen gas can also be used to power vehicles. You get the hydrogen from theelectrolysis of water. The electrical energy can come from a renewable source

Pros- it is very clean- only produces water and heatCons you need a special expensive engine. Hydrogen isnt widely

available. You need energy from a source, hydrogen is hard to store


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Alkenes are unsaturated hydrocarbons

y Alkenes have the general formula CnH2ny Alkene molecules at least have one C=C double covalent bond.Molecules with C=C

double bonds are unsaturated because they can make more bonds with extra atoms

in addition reactionsy Here are a few diagrams of alkenes:

y Compounds with benzene ring struc tures are called arenes, or aromatic compounds.All other organic compounds are called aliphatic compounds. Aliphatic molecules

contain straight- or branched- chain carbon skeletons

Alkenes are much more reactive than Alkanes

y In a carbon-carbon single bond (also known as an bond) the electron cloud issymmetrical about the central axis of the molecule. because it lies along the line

joining the two carbon atoms, so the two ends of the ethane molecules are free to

rotate relative to each other

y Each double bond in an alkene is made up of an bond and a bond. The bonddoes not allow rotation around the axis; this has a big effect on its structure and

properties. Rotation is not possible without breaking the bond

y Because there are two pairs of electrons in the bond, the C=C double bond has areally high electron density. This makes alkenes pretty reactive

y Also they are reactive due to the attraction of electrophiles to the electron rich bond within the double bond . The bond provides an electron pair to form a new

bond with an electron seeking group (electrophile)

y The bond is above a below the plane of the nuclei, the bond is more loosely heldby the carbon nuclei than the bond and therefore alkenes are more reactive thanalkanes

Eletrophilic addition reactions happen to alkenes

y An electrophile is an electron-deficient species than can form new covalent bond,using an electron pair provided by the carbon atom. The most common eletrophillic

agent is the proton, H+


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y In this reaction, the double bond opens up, and another atom is added to each of itscarbons

y Addition reactions happen because the double plenty of electrons and is easilyattacked by an electrophile (i.e. the double bond is electron rich)

y A couple example of an electrophile:Positively charged ions, like H+ and NO2+

Polar molecules- the + atom is attracted to places with lots of electrons

y The double bond is also nucleophilic - its attracted to places that dont have enoughelectrons

Addition reactions

Reaction of alkenes with hydrogen

y The alkenes do not react with hydrogen under normal conditions of temperatureand pressure

y However, with the presence of a nickel catalyst and a high temperature of around200 degrees, alkenes undergo an addition reaction with hydrogen to form the

corresponding alkane. The reaction of ethene with hydrogen is an example

Ni

C2H4 + H2 C2H6

Alkenes are oxidised by acidified Potassium Manganate (VII)

y The reaction of the alkenes with acidified potassium manganate (VII) solutioninvolves both addition across the double bond and oxidation (of the double bond)

y If you shake an alkene with acidified potassium manganate (VII), the purple solutionis decolourised. The alkene is oxidised and the products of the reaction are

alkanediols

y For example heres how ethane reaction with acidified potassium manganate (VII): Pg 59

y The reaction is another useful test for a double C=C bond, as alkenes will react andalkanes will not react

Use bromine water to test for C=C double bonds

y When you shake an alkene with orange bromine water, the solution decolourises .You can use this reaction as a test for C=C bonds


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y An addition reaction takes place across the double bond, which is why brominewater becomes colourless

y Bromine water is a dilute solution- it contains more water molecules than brominemolecules. The carbocation is more likely to react with H 20 than Br

-, so an OH group

adds to the second carbon rather than another Br

y The major product of the reaction is 2-bromoethanol because OH- ions from thewater take part in the reaction as well as Br

-ions

y However, some 1,2- dibromoethane is formed as wellPage 130 edexcel

Alkenes undergo Eletrophillic addition with halogens forming di -substituted

halogenoalkanes

y If you mix an alkene with bromine, the bromine adds across the double bond to fromdibromoalkane (bromine goes colourless)

y The ethene undergoes an electrophilic addition reaction with the bromine forming1,2-dibromoethane

y The Br-Br molecule is partially polarised by the elec tron-rich bond. The mechanisminvolves an electrophilic attack, follow by a nucleophilic attack on the resultingcarbocation. This is the mechanism reaction of all the halogens with ethene

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...and with Hydrogen Halides to form mono - substituted halogenoalkanes

y The double bond in the alkenes reacts readily with hydrogen halides, producing thecorresponding mono-substituted halogenoalkane

y For example ethene reacts with hydrogen bromide, to from bromoethane


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y The first stage of this reaction involves an electrophilic attack on the electron rich-bond

y As a result of this attack, one of the carbon atoms in the ethene molecule gains apositive charge, forming an ion called a carbocation

y The positively charge cation is then open to attack by nucleophiles (positive -charge-loving species) i.e. the bromide ion formed in the first stage of the reaction attacks

the carbocation, forming a new C -Br bond

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Adding Hydrogen Halides to Unsymmetrical Alkenes forms two products

y When HBr is added to ethene - there is only one possible product as it is asymmetrical alkene

y Propene in contrast is an asymmetrical alkene, with three carbon atoms and onlyone double bond

y If you react hydrogen bromide with propene, the electrophilic reaction has twopossible end products. The amount of each product formed depends on how the

stable the carbocation formed in the middle of the reaction

y Carbocations with more alkyl groups e.g. methyl -CH3 are more stable, because thealky groups feed electrons to the positive charge. The more stable carbocation is

much more likely to form

y The carbocation in (a) is more stable than the alternative in (b), because the twomethyl groups, donate electron density and stabilise the positive charge. So

carbocation (a) tend to be formed in preference

y This means that the major product of the reaction is CH 3CHBrCH3 (2-bromopropane),with less CH3CH2CH2Br (1-bromopropane)

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Double bonds cannot rotate

y Atoms cant rotate about the double bond, things can still rotate about any singlebonds in the molecule

y The restricted rotation around the C=C double bond is what causes alkenes to exhibitE/Z geometric isomerism

y The lack of free rotation around the double bond results in molecules calledgeometric isomers

y The double bond also causes alkenes to undergo addition reactions rather thansubstitution reactions of alkanes

E/Z isomerism is a type of stereoisomerism

y Stereoisomers have the same structural formula but a different arrangement of theatoms in space

y Because of the lack of rotation around the double bond, some alkenes can havestereoisomerism

y Stereoisomers occur when the two double-bonded carbon atoms each havedifferent atoms or groups attached to them. Then you get an E-isomer and a Z-

isomer

y They often show different physical properties boiling and melting point y They occur when the components of the molecule are arranged on different sidesy If two higher property groups are on the same side they are zusammen(together)-

Z-isomer

y If the two higher property groups are across the double bond then they areentgegen (opposite)- E isomer

E/Z Isomers Can Sometimes Be Called Cis -Trans Isomers

y cis means the Z-isomery trans means the E-isomery So E-but-2-ene can be called trans-but-2-ene and Z-but-2-ene can be called cis-but-2-

ene

y But if the carbon atoms both have totally different groups attached to them, the cis -trans naming system

y The E/Z system keeps on working though. This is because each of the groups linkedto the double-bonded carbons is given an priority


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y If two carbon atoms have their higher priority group on opposite sides, then its anE-isomer

y If the two carbon atoms have their higher priority group on the same side, then itsa Z-isomer

y In the E/Z system, Br has a higher priority (atomic number) than F, so the namesdepend on where the Br atom is in relation to the CH 3 group

Pg 61

Alkenesjoin up to form Addition polymers

y The double bonds in alkenes can open up and join together to make long chainscalled polymers. The individual, small alkenes are called monomers

y This is called addition polymerisation. For example, poly(ethene) is made by theaddition polymerisation of ethene . It is useful for packaging and utensils

y Ethene, the simples of the alkenes undergoes polymerisation to form poly(ethene).The double bonds and the carbon atoms of the repeating units link together to form

a long chain

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Biodegradable polymers

y Biopolymers are polymers which are made by modifying natural polymers such asstarch or cellulose

y This is a sustainable, renewable source which uses carbon -dioxide from theatmosphere during its growing time, reducing the carbon footprint of thesubsequent plastics

y Most polymers made from renewable sources can also be attacked and broken downby microbes- they are biodegradable

y Also renewable energy sources should be used in the manufacture of polymers, sothe environmental effect is greatly reduced e.g. electricity generated from wind

power

How are plastics recycled?

y Mechanical recycling involves physical changes to the plastics- they may be pelted,shredded or turned in to granules before use . Has to be done via hand + expensive

y Chemical recycling involves the breakdown of the waste plastic into its monomerunits which can be then used again in the formation of new polymers or in other

reactions in the chemical industry

Energy recovery

y Energy recovery- recovering some of the energy that was put into producing thepolymer e.g. burning waste polymer products, and used the energy released to

generate electricity. this reduces the use of fossil fuels to generate electricity

y Incinerators make sure that any pollutions such as dioxi ns are not released into theatmosphere

y Polymers contain a great deal of stored energy so they are a high -energy fuel source.The heat produced in the incinerators can be used to produce hot water for heating

in combined heat and power systems

The properties of polymers

y The average length of a polymer chain - tensile strength and melting temperatureincrease with the length of the chain until 500 units in a chain

yBranching of a chain- highly branched chains tend to have low -tensile strengths, lowmelting temperatures and low density

y The presences of intermolecular forces between chains - if there are strongintermolecular forces between the chains the polymer will be strong and tend to

have a high melting temperature

y Cross-links between chains- make a polymer very rigid, hard and brittle, usually witha very high melting temperature


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