OxfordAQA International GCSE Chemistry: Factors affecting equilibrium, and production of ammonia and sulfuric acid (9202) – Revision Notes

These revision notes condense sections 3.8.2 Factors affecting equilibrium and 3.8.3 Production of ammonia and sulfuric acid of the OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards. They cover statements 3.8.2 a to e and 3.8.3 a to d; neither section has a required practical. The qualification is untiered, so every point is for every candidate. For full explanations and worked examples, use the study guide for this unit.

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Key definitions

Term	Meaning
Reversible reaction	Goes both ways: the products can turn back into the reactants (symbol ⇌)
Closed system	Nothing can enter or leave, so no substance escapes
Equilibrium (3.8.2 a)	In a closed system, the forward and backward reactions occur at exactly the same rate
Yield	How much of a product is present in the equilibrium mixture
Exothermic	Transfers energy to the surroundings
Endothermic	Takes in energy from the surroundings
Catalyst	Speeds up a reaction without being used up; at equilibrium it speeds up both directions

At equilibrium: both reactions continue; amounts stay constant; amounts are usually not equal.

3.8.2 b: the relative amounts of all the reacting substances at equilibrium depend on the conditions.

3.8.2 c: temperature rules

You change	Endothermic direction	Exothermic direction
Raise temperature	yield increases	yield decreases
Lower temperature	yield decreases	yield increases

Method in steps

1. Read which direction is exothermic. The other direction is endothermic.
2. Raising temperature → the endothermic direction gains.
3. Lowering temperature → the exothermic direction gains.
4. Name the substance whose amount goes up or down.

Worked reminder. 2NO(g) + O₂(g) ⇌ 2NO₂(g), forward exothermic. Cooling increases the exothermic (forward) yield, so more NO₂. Heating gives less NO₂.

3.8.2 d: pressure rules

Gases only. Count molecules, not atoms, from the balanced equation.

You change	Favoured reaction
Increase pressure	The one producing the fewest molecules
Decrease pressure	The one producing the most molecules
Same number of molecules on both sides	No change in yield

Worked reminder. PCl₅(g) ⇌ PCl₃(g) + Cl₂(g): 1 molecule left, 2 right. Increasing pressure favours the backward reaction, so more PCl₅ remains.

Worked reminder. N₂(g) + O₂(g) ⇌ 2NO(g): 2 molecules each side, so pressure does not change the yield.

3.8.2 e: optimum industrial conditions

Industry balances four things:

• Yield: what the equilibrium rules favour.
• Rate: higher temperature, higher gas pressure and a catalyst all make the reaction faster.
• Cost: high pressure needs strong equipment and energy for compressing gases.
• Safety: very high pressures are more dangerous.

When the forward reaction is exothermic, a low temperature gives the best yield but is too slow. A compromise temperature gives a reasonable yield quickly. A catalyst lets equilibrium be reached faster; it never changes the yield.

3.8.3 a to c: the Haber process

Feature	Detail
Product	Ammonia
Word equation	nitrogen + hydrogen ⇌ ammonia
Symbol equation	N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
Nitrogen from	The air
Hydrogen from	Natural gas or other sources
Pressure	About 200 atmospheres
Catalyst	Iron
Temperature	About 450 °C
Forward reaction	Exothermic; 4 molecules → 2
Separation	Cooling: ammonia liquefies and is removed
Unreacted gases	Nitrogen and hydrogen recycled

Method in steps: explaining the Haber conditions

1. Temperature: forward exothermic → low temperature, higher yield → but too slow → 450 °C is a compromise.
2. Pressure: 4 → 2 molecules → high pressure, higher yield and faster rate → limited by cost and safety → about 200 atmospheres.
3. Catalyst: iron speeds up reaching equilibrium; yield unchanged.
4. Recycling: unreacted gases return to the reactor, so raw materials are not wasted.

Why cooling separates ammonia: ammonia condenses at a much higher temperature than nitrogen or hydrogen, so it becomes liquid while they stay as gases. It can then be run off and stored, while the gases go back to the reactor.

3.8.3 b: fertilizers and food

• Ammonia is a raw material for fertilizers; it is used to make ammonium salts.
• Crops need nitrogen to make proteins and grow; most cannot take nitrogen from the air.
• Harvesting removes nutrients, so soil loses fertility.
• The global population is rising and farmland is limited.
• So fertilizers are needed to maximise food yields from each area of land.

3.8.3 d: the contact process

Stage	What happens	Equation
1	Sulfur burned in air	S(s) + O₂(g) → SO₂(g) (the specification prints this with ⇌)
2	Sulfur dioxide + more oxygen; exothermic, reversible	2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
3	Sulfur trioxide + water	H₂O(l) + SO₃(g) → H₂SO₄(aq)

Stage 2 conditions: vanadium(V) oxide (V₂O₅) catalyst, around 450 °C, atmospheric pressure.

Method in steps: explaining Stage 2

1. Catalyst: faster rate in both directions; no change to yield.
2. Temperature: forward exothermic, so lower temperature means higher yield but too slow; 450 °C is a compromise.
3. Pressure: 3 → 2 molecules, so high pressure would raise yield slightly. The yield is already high at atmospheric pressure, so the extra cost of high pressure is not justified.

Haber and contact at a glance

	Haber process	Contact process (Stage 2)
Reversible step	N₂ + 3H₂ ⇌ 2NH₃	2SO₂ + O₂ ⇌ 2SO₃
Molecules of gas	4 → 2	3 → 2
Forward reaction	Exothermic	Exothermic
Catalyst	Iron	Vanadium(V) oxide
Temperature	About 450 °C	Around 450 °C
Pressure	About 200 atmospheres	Atmospheric
Why that pressure	High pressure needed for a worthwhile yield	Yield already high, so high pressure not worth the cost

Must-know distinctions

• Rate vs yield: speed of reaction versus the share of product once equilibrium is reached.
• Constant vs equal: at equilibrium amounts are constant, not necessarily equal.
• Atoms vs molecules: pressure rules count molecules of gas.
• Haber vs contact: iron vs V₂O₅; 200 atmospheres vs atmospheric pressure; both around 450 °C.
• Catalyst vs condition change: temperature and pressure can change the yield; a catalyst cannot.

Quick self-test

1. What two things must be true for a reversible reaction to be at equilibrium?
2. A forward reaction is endothermic. State the effect on its yield of lowering the temperature.
3. 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). How many gas molecules are on each side?
4. CH₃OH(g) ⇌ CO(g) + 2H₂(g). Which direction is favoured by a decrease in pressure?
5. Name the catalyst in the Haber process and the catalyst in Stage 2 of the contact process.
6. Give the source of the nitrogen and of the hydrogen for the Haber process.
7. What happens to the unreacted nitrogen and hydrogen?
8. Explain why a catalyst does not change the yield of ammonia.
9. Give two reasons why the Haber process does not use a pressure far higher than 200 atmospheres.
10. Write the equation for Stage 3 of the contact process.
11. Calculate the masses of nitrogen and hydrogen needed to make 17 kg of ammonia. (Aᵣ: N = 14, H = 1)
12. Calculate the maximum mass of sulfuric acid made from 16 tonnes of sulfur. (Aᵣ: S = 32, O = 16, H = 1)

Answers

1. A closed system; forward and backward reactions at exactly the same rate.
2. The yield of the endothermic forward reaction decreases.
3. 2 + 1 = 3 on the left; 2 on the right.
4. The forward direction (1 molecule → 3 molecules), so more CO and H₂.
5. Haber: iron. Contact Stage 2: vanadium(V) oxide, V₂O₅.
6. Nitrogen from the air; hydrogen from natural gas (or other sources).
7. They are recycled back into the reactor.
8. It speeds up the forward and backward reactions equally, so equilibrium is reached sooner but the amounts at equilibrium are the same.
9. Higher pressure needs stronger, more expensive equipment and more energy to compress gases; it is also more dangerous.
10. H₂O(l) + SO₃(g) → H₂SO₄(aq).
11. N₂ + 3H₂ → 2NH₃ gives a mass ratio 28 : 6 : 34. Halve it: 14 kg nitrogen and 3 kg hydrogen make 17 kg ammonia.
12. 1 S gives 1 H₂SO₄, so 32 t gives 98 t; 16 t gives 49 tonnes.

Where marks are usually lost

• Writing that equilibrium means the reaction has stopped, or that the amounts are equal.
• Leaving out “closed system” when defining equilibrium.
• Stating “the equilibrium shifts” without naming what happens to the yield of the product asked about.
• Counting atoms, or ignoring the numbers in front of formulae, in pressure questions.
• Saying a catalyst increases yield, or that it is used up.
• Explaining 450 °C as “to increase the yield”; for an exothermic forward reaction, a higher temperature lowers the yield, so the reason is rate.
• Giving only one side of a compromise (for example “high pressure is expensive”) without the yield or rate benefit.
• Mixing up the two processes’ catalysts and pressures.
• Answering the fertilizer question with “plants grow better” without nitrogen, food yields or population.

Official syllabus

OxfordAQA International GCSE Chemistry (9202) specification, Version 6.2, for exams May/June 2018 onwards, published by OxfordAQA. These notes cover section 3.8.2 Factors affecting equilibrium and section 3.8.3 Production of ammonia and sulfuric acid.