Chemistry 0620 · syllabus 6.3

Cambridge IGCSE Chemistry 0620 — Reversible reactions and equilibrium topical past papers

Reversible reactions and hydrated salts, dynamic equilibrium, how conditions move its position, and the conditions chosen for the Haber and Contact processes.

Questions: 1,153
Years: 2009–2026
With mark scheme: 99%
Questions 2022–2026: 367

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What Reversible reactions and equilibrium covers

Some reactions can go either way, shown by the symbol ⇌. Core candidates meet two that change direction with the conditions: heating blue hydrated copper(II) sulfate drives off water and leaves the white anhydrous salt, and adding water turns it blue again; cobalt(II) chloride does the same between pink, hydrated, and blue, anhydrous, which is why both are used to test for water.

The rest of the topic is Extended content. In a closed system a reversible reaction reaches equilibrium when the forward and reverse reactions run at the same rate, so the concentrations of reactants and products stop changing even though both reactions carry on. Changing the conditions moves the position of equilibrium: a higher temperature favours the endothermic direction, a higher pressure favours the side with fewer gas molecules, adding a substance favours the direction that uses it up, and a catalyst does not move the position at all but brings the mixture to equilibrium sooner. You make these predictions from information given in the question.

Two industrial equilibria are named. The Haber process, N₂(g) + 3H₂(g) ⇌ 2NH₃(g), uses nitrogen from the air and hydrogen from methane at about 450 °C and 20 000 kPa (200 atm) with an iron catalyst. The Contact process step, 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), uses sulfur dioxide from burning sulfur or roasting sulfide ores and oxygen from the air, at about 450 °C and 200 kPa (2 atm) with a vanadium(V) oxide catalyst. Both forward reactions are exothermic, and you explain each set of conditions as a balance between rate, yield, cost and safety.

In the syllabus

The headings this topic has in the Cambridge International 2026–2028 syllabus, as the board prints them.

6.3 Reversible reactions and equilibrium

Read the syllabus on the board’s website

What you should be able to do

Describe how heating and adding water reverse the changes between hydrated and anhydrous copper(II) sulfate and cobalt(II) chloride, with the colours
State the two features of a reversible reaction at equilibrium in a closed system (Extended)
Predict and explain the effect of temperature, pressure, concentration and a catalyst on the position of equilibrium, from information given (Extended)
State the equations, raw materials and typical conditions of the Haber and Contact processes (Extended)
Explain those conditions in terms of rate, position of equilibrium, safety and cost (Extended)

Key ideas

CuSO₄·5H₂O (blue) ⇌ CuSO₄ (white) + 5H₂O and CoCl₂·6H₂O (pink) ⇌ CoCl₂ (blue) + 6H₂O, heating driving each to the right
Equilibrium is dynamic: forward rate = reverse rate, and the concentrations are constant, not necessarily equal
A higher temperature moves the position of equilibrium in the endothermic direction; a higher pressure moves it towards fewer moles of gas
Haber: 450 °C, 20 000 kPa, iron catalyst; Contact: 450 °C, 200 kPa, vanadium(V) oxide catalyst
A catalyst speeds up the forward and reverse reactions equally, so it does not change the position of equilibrium

How Reversible reactions and equilibrium has been examined

From 2022 to 2026, Reversible reactions and equilibrium was on Paper 4 · Theory (Extended) in 14 of 14 exam series, worth about 26% of that paper’s marks.

How Reversible reactions and equilibrium has been examined, by paper
Paper	Questions	Share of marks	Series
Paper 1 · Multiple Choice (Core)	124	10%	14 of 14
Paper 2 · Multiple Choice (Extended)	138	11%	14 of 14
Paper 3 · Theory (Core)	41	15%	14 of 14
Paper 4 · Theory (Extended)	47	26%	14 of 14
Paper 6 · Alternative to Practical	17	18%	9 of 14

Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 1. The bank files this topic together with neighbouring ones, so these figures count every question in its chapters, including some on those topics.

Filed in Momentum under: Energy Changes and Reversible Reactions, Some Non-metals and Their Compounds.

A worked example

In the Contact process, 2SO₂(g) + O₂(g) ⇌ 2SO₃(g), and the forward reaction is exothermic. Predict the effect on the equilibrium yield of SO₃ of raising the pressure, raising the temperature and adding a catalyst, and explain why about 450 °C and 200 kPa are used.

Moles of gas: 2 + 1 = 3 on the left and 2 on the right
A higher pressure moves the position of equilibrium towards fewer moles of gas, to the right, so the yield of SO₃ increases
The forward reaction is exothermic, so a higher temperature moves the position of equilibrium to the left and the yield decreases, although the rate increases
A catalyst speeds up the forward and reverse reactions equally: equilibrium is reached sooner, but the yield is unchanged
450 °C with a vanadium(V) oxide catalyst gives an acceptable rate with a reasonable yield; 200 kPa is enough because the position of equilibrium already lies well to the right, so a higher pressure would add cost and risk for little extra yield

Answer: Higher pressure increases the yield; higher temperature decreases it but increases the rate; a catalyst leaves it unchanged. 450 °C and 200 kPa balance rate, yield, cost and safety

Written by Momentum for this page, with its own numbers; it is not taken from a past paper.

Mistakes that cost marks

Saying the reaction stops at equilibrium, or that the concentrations are equal — both reactions continue at the same rate, and the concentrations are constant, which is not the same as equal
Claiming a catalyst increases the yield — it speeds up both directions equally, so equilibrium is reached sooner but its position does not move
Saying a higher temperature increases the yield of ammonia — the forward reaction is exothermic, so a higher temperature lowers the yield; 450 °C is used to get an acceptable rate
Forgetting to count moles of gas before a pressure question — the Haber process has 4 moles of gas on the left and 2 on the right, so a higher pressure moves the position to the right; equal numbers on each side would mean no effect
Mixing up the colours of the water tests — anhydrous copper(II) sulfate is white and turns blue; anhydrous cobalt(II) chloride is blue and turns pink

Exam technique

For 'predict and explain', give the direction of the shift and then the reason from the information given: 'the forward reaction is exothermic, so increasing the temperature moves the position of equilibrium to the left'
Write down the number of moles of gas on each side of the equation before answering any question about pressure
When explaining industrial conditions, cover both rate and yield for temperature, and yield together with cost or safety for pressure
Say 'the position of equilibrium moves', and use 'yield' only when the question is about the amount of product

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