TOPICS FROM THE REST OF THE PAPER

Acids/alkali/pH/indicators

In this section students can be examined on topics like Arrhenius theory and its limitations, Bronsted-Lowry theory of acids and bases, conjugate acids, conjugate bases, conjugate pairs, properties of acids/alkalis, “strong” and “weak”, amphoteric, salt, indicators, neutralisation, pH and its limitations, pH calculations, dissociation constants, ionic product of water. pH

pH of a solution is a measure of the H+ concentration and thus the acidity or alkalinity of the solution. pH = - log10 [H+] [ ] = moles/litre. →

pH is measured on a scale from 0 14.

Measurement of pH:

(i) pH paper - litmus paper or universal paper, (ii) pH meter

Limitations of the pH scale: only works for dilute solutions, restricted to aqueous solutions, only accurate for 25oC, scale only goes from 0 – 14.

Different types of pH calculations

(i) pH calculations of strong acids: Question: Calculate the pH of 0.025M solution of nitric acid ..... see 2009 Question 4 (f ). →

HNO3 H+ + NO3

0.025 0.025

= 0.025 moles of H+ pH = - log10 [H+] = - log10 [0.025] = 1.6.

Question: Calculate the pH of a solution of hydrochloric acid containing 2.74g of hydrochloric acid in 250cm3. 2.74g HCl in 250cm3 (x 4)

10.96g HCl in 1000cm3 =>

10.96 36.5 = 0.3M HCl (1 mole HCl = 36.5g) ÷

HCl 1H+ + Cl– → 0.3 0.3 pH = - log10 [H+] = - log10 [0.3] = 0.523.

(ii) pH calculations of strong alkali:

pOH = - log10 [OH-] and then pH = 14 - pOH.

Question: Calculate the pH of a 0.5M NaOH solution .... see 2012 Question 10(b).

NaOH Na+ + 1OH→

0.5 0.5 pOH = - log10 [OH-] = - log10 [0.5] = 0.301 pH = 14 - pOH = 14 - 0.301 = 13.699

(iii) Given pH and asked to calculate the H+ (or H3O+) concentration of the solution.

Question: Calculate the H3O+ ion concentration if the pH of the solution is 2.0 .... see 2014 Question 7 (c).

Remember [H+] = [H3O+] pH = - log10 [H+] (want just [H+] on its own)

2.0 = - log10 [H+] (x both sides by –)

-2.0 = log10 [H+] (get antilog)

Antilog (-2.0) = [H+]

0.01mol/l = [H+] [ H3O+] = 0.01 mol/l =>

Dissociation constants Ka, Kb are for weak acids/bases only and tell the extent to which the acid or the base ionises (dissociates) in an aqueous solution. Relationship between pH and dissociation constants: (iv) pH calculations involving weak acids/weak bases

Question: Calculate the pH of a 0.2M solution of a weak acid, HA, whose Ka = 6.3 x 10-5….see 2011 Question 7 (d).

Equilibrium

There is usually a full question or a half question on the paper on equilibrium. The following is just some of the material examined in this question.

Definitions are very important for example for reversible reaction, chemical equilibrium, dynamic equilibrium, Le Chatelier’s principle, equilibrium constant, law of chemical equilibrium.

The equilibrium constant K is a measure of how far the reaction has proceeded when equilibrium is reached Kc = Products Use[]as[]=moles/litre

Reactants and whatever the number of moles of reactants/ products raise it to that power.

The value of Kc is temperature dependent.

Students should also know the mandatory experiment ⇋ Fe3+ + CNS- Fe(CNS)2+ and the effect of changing concentration and/or temperature on the position of equilibrium,

- The industrial applications of Le Chatelier’s Principle (Haber process and contact process) and the conditions actually used in industry,

- Be able to work out the value of Kc if given concentrations of reactants and the concentration of a reactant/product at equilibrium. Be careful of units – set up table, initial, amount reacted, moles at equilibrium, moles per litre.

Water

There is usually a question on the paper on water (sometimes combined with acids, bases and pH). Students are asked about: – Hardness in water, its causes (dissolved calcium salts –

CaSO4 and Ca(HCO3)2, and to a lesser extent by the salts of magnesium) – The types ofhardness( temporary and permanent) and how they are removed, methods of softening water, advantages and disadvantages of hard water – The treatment of water for domestic use (screening, flocculation, sedimentation, filtration, chlorination, flouridation and pH adjustment)

– Water pollution, biochemical oxygen demand (B.O.D.), heavy metal ions (Pb+2, Hg+2, Cd+2), eutrophication (natural and artificial) – Sewage treatment – primary (screening, settlement), secondary (activated sludge, settling tank) and tertiary (removal of phosphates and nitrates. Phosphates removed by adding Al2(SO4)3 or FeCl3 or Lime)

– Water analysis (pH meter, atomic absorption spectrometry, colorimetry).

Students need to know about other experiments including: – The estimation of suspended and dissolved solids ina

water sample

– Total hardness of a water sample (EDTA, suitable indicator is Eriochrome Black T, use a buffer solution of pH10 . The colour change at the end-point is wine red to blue. Total hardness is expressed as p.p.m. of CaCO3. – Dissolved oxygen (Winkler’s method) in a water sample.

Add concentrated manganese sulphate and alkaline potassium iodide to produce a white precipitate of manganese (II) hydroxide; the dissolved oxygen in the water sample oxidises this to manganese (III) hydroxide (brown

precipitate). Concentrated sulphuric acid is now added causing the manganese (III) hydroxide to oxidise the iodide ions to free iodine (red/brown) and it itself gets reduced back to manganese (II). The amount of I2 liberated can be determined by titration with standard sodium thiosulphate solution. Overall: 1O2 : 2I2 :4Na2S2O3).

ORGANIC CHEMISTRY

There are at least two Organic Chemistry questions in Section B. One of these questions is usually based on crude oil/fractional distillation and thermochemistry (except for 2003!), and the other question is a general organic question (functional groups, homologous series, etc).

The following is just some of the material examined in the general organic question:

– Draw/name functional groups, name members of different

homologous series (up to 4 carbons) and their isomers – Explain the relationship between the different homologous series, their boiling points/melting points and their solubility in polar (water) and non-polar (cyclohexane)

solvents

– Describe the tests to distinguish between different functional groups (eg. for aldehydes / ketones use dilute acidified KMnO4 or silver mirror test or Fehling’s solution), test for unsaturation

– Identify the five different types of reactions

– Describe the mechanisms for ionic addition and free radical substitution and the evidence for these mechanisms.

Chromatography – Paper chromatography, thin layer chromatography, column chromatography, gas chromatography, high performance liquid chromatography, infra red, ultra violet. The following is some of the material required for the crude oil/thermochemistry question.

Fractional distillation of crude oil (petroleum)

Fractional distillation involves heating the crude oil and the different mixtures (fractions) are separated on the basis of their boiling points.

* mercaptans are sulphur compounds that are added to natural gas to give an odour in an event of a leak. ** LPG - liquid petroleum gas.

Knocking (Auto-ignition)

This occurs when the petrol-air mixture in the engine explodes as it is being compressed instead of the explosion being caused by the spark.

Straight-chain alkanes, like heptane, ignite very easily on compression, thereby causing knocking whereas branchedchain alkanes, like 2,2,4-trimethyl pentane (iso-octane), do not tend to ignite on compression and do not auto-ignite.

The octane number of a fuel is a measure of the tendency of the fuel to resist knocking.

2,2,4-trimethyl pentane has an octane number of 100, Heptane has an octane number of 0.

Good petrol has an octane number of 97. Research has shown that the following structural features give rise to higher octane numbers:

(i) the shorter the alkane chain

(ii) the more branched the alkane chain

(iii) cyclic compounds.

Pb(C2H5)4, was added An “anti-knock additive”, tetraethyl lead, to petrol to help reduce the amount of knocking. However lead pollution from car exhausts causes environmental and health concerns as lead is toxic. Also lead poisons the catalysts in the catalytic converter.

Nowadays the octane number of petrol is increased by: (i) Isomerisation (ii) Catalytic cracking (iii) Reforming (dehydrocyclisation) (iv) Adding oxygenates such as methanol, ethanol and MTBE

(methyl tertiary butyl ether).

Thermochemistry This is the study of the heat changes which occur during chemical reactions.

Definitions are very important for: exothermic reaction, endothermic reaction, heat of reaction, heat of formation, heat of combustion, heat of neutralisation, kilogram calorific value, bond energy, Hess’s law, law of conservation of energy.

You must be able to write equations for the heat of formation and the heat of combustion of different substances. (Ensure you balance the equation).

Question: Write the equation for the heat of formation of ethanol (C2H5OH). 2C + 3H + ½O 1C H OH 2 2 → 2 5

Question: Write the equation for the heat of combustion of ethanol (C2H5OH). 1C2H5OH +3 ½O2 2CO2 + 3H2O →

Heat of combustion values are measured using a bomb calorimeter whereas the kilogram calorific value can be measured using a bomb calorimeter or can be found from heat of combustion values.

Heat of neutralisation experiment: use polystyrene cups, know procedure and precautions, and remember to use equimolar solutions.

Calculations: E=mxcx(t-t) 2 1

t –t1 = rise in temperature of the acid/alkali mixture 2 c = specific heat capacity of the acid/alkali mixture m =mass of the acid/alkali mixture (Kg) (density = 1)

That is the energy for the number of moles of acid/alkali neutralised, then work out the answer for one mole.

Remember: The heat of neutralisation of a strong acid by a strong alkali is almost constant at -57.2kJ/mol ∆H

H+ + OH- H O = -57.2kJ/mol → 2