S5 Chemistry: Industrial Processes (Paper 3: Challenge)

2026-11-12

Ten challenging items: the thermodynamics and economics behind the conditions, atom economy, mass to mole to heat conversions and relative falls, with answers.

Direct answers　Answers in order: the rate is too slow at room temperature so 400 to 450 degrees C is used, the yield at ordinary pressure is already high, the reaction itself does not need them, 100 per cent, 16.0 g, 6 g and 34 g, 2.04 per cent, about 65.7 per cent, about 2 706 kJ, recycling raises overall yield and raw material use.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	20
Difficulty mix	Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1
Chapter	Chapter: S5 Chemistry: Industrial Processes
Level	S5

02　Questions (10 items)

FoundationThe oxidation of sulphur dioxide in the Contact process has dH = -98 kJ mol-1. Why is 400 to 450 degrees C still used?
FoundationThe Contact process runs at only about 1 to 2 atmospheres. What is the key reason?
FoundationDo the conditions of 450 degrees C and 200 atmospheres mean the reaction itself needs high temperature and pressure to happen at all?
IntermediateWhat is the atom economy of the Haber process?
IntermediateWhat mass of sulphur trioxide is expected from 12.8 g of sulphur dioxide oxidised completely?
IntermediateWhen 28 g of nitrogen reacts completely, what mass of hydrogen is needed and what mass of ammonia is formed?
ChallengeRaising the temperature from 400 to 450 degrees C lowers the equilibrium yield from 98 to 96 per cent. What is the relative fall?
ChallengeWith a single pass conversion of 30 per cent, what is the overall conversion after three recycles?
ChallengeHow much heat is released per kilogram of ammonia made, taking 46 kJ per mole?
Error redoA student thinks recycling unreacted gas is only an environmental gesture with little effect on cost. What does that overlook?

03　Answers and one-line notes

#	Answer	One-line note
1	The rate at room temperature is far too slow, so the vanadium catalyst at 400 to 450 degrees C balances rate against an acceptable yield	A low temperature favours the equilibrium of an exothermic reaction, but the catalyst needs a certain temperature before it works well.
2	The equilibrium yield is already high at ordinary pressure, because the gas changes from 3 mol to 2 mol and extra pressure adds little	2SO2 + O2 -> 2SO3 has 3 mol of gas on the left and 2 mol on the right, yet conversion at ordinary pressure is already close to complete.
3	No. They are chosen for the best overall rate and yield; the reaction also proceeds at room temperature and pressure	The reaction is thermodynamically feasible anyway, and the plant conditions only lift the rate to an economically useful level.
4	100 per cent, because all 34 g of reactants becomes 34 g of product with no by-product	Atom economy is the relative mass of the wanted product divided by the total relative mass of all reactants.
5	16.0 g	Sulphur dioxide has a molar mass of 64 g mol-1, so 12.8 g is 0.2 mol, and at 80 g mol-1 that gives 0.2 times 80, which is 16.0 g.
6	6 g of hydrogen is needed and 34 g of ammonia is formed	28 g of nitrogen is 1 mol, needing 3 mol of hydrogen or 6 g, and giving 2 mol of ammonia or 34 g; mass is conserved since 28 plus 6 is 34.
7	2.04 per cent	2 divided by 98 times 100 gives 2.04; the absolute change is only 2 percentage points but the relative fall is measured against the original value.
8	About 65.7 per cent	One minus 0.7 cubed gives 0.657, which is about 65.7 per cent.
9	About 2 706 kJ	1 000 g divided by 17 g mol-1 is about 58.8 mol, and multiplying by 46 gives about 2 706 kJ.
10	Recycling directly raises the overall yield and raw material utilisation, and raw material is one of the largest running costs in such a plant	Single pass conversion is often only 20 to 30 per cent, so without recycling 70 to 80 per cent of the feed would simply be wasted.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

Use one number to show why a higher yield is not always better.Tutor Registration →

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