IGCSE Chemistry: Rates of Reaction and Reversible Reactions — Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Rates of Reaction revision notes

Tier note: questions marked (0620 Extended, 5070 required) — collision theory explanations, equilibrium, and all of the Haber process (its equation, the sources of nitrogen and hydrogen, the conditions and the reasoning behind them) — go beyond 0620 Core. All other questions are answerable by a 0620 Core candidate; where an answer adds an Extended point, it is labelled and not needed for the marks.

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Questions

1. State four factors that affect the rate of a chemical reaction. [4]

2. (0620 Extended, 5070 required) Explain, using collision theory, why:

(a) powdered marble reacts faster than marble chips [2] (b) a more concentrated acid reacts faster [2] (c) raising the temperature increases the rate [3]

3. A student measures the volume of gas produced over time. The graph is steep at first and then levels off.

(a) Explain why the graph is steepest at the start. [2] (b) Explain why it levels off. [2] (c) Describe how the graph would differ if a catalyst were added, and explain. [3]

4. Explain what a catalyst does, and state one reason catalysts are important in industry. [3]

5. (0620 Extended, 5070 required) Consider the reversible reaction: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ mol⁻¹

(a) State what is meant by a reversible reaction at equilibrium. [2] (b) State and explain the effect on the yield of ammonia of increasing the pressure. [2] (c) State and explain the effect of increasing the temperature. [2] (d) State the effect of adding a catalyst on the yield. [1]

6. Hydrated copper(II) sulfate, CuSO₄·5H₂O, is blue; anhydrous copper(II) sulfate, CuSO₄, is white.

(a) Write the equation for the reversible reaction between them, including state symbols and colours. [2] (b) Explain why this reaction is described as reversible, and state how you would reverse it once it has gone one way. [2]

7. (0620 Extended, 5070 required) The Haber process is represented by N₂(g) + 3H₂(g) ⇌ 2NH₃(g), typically run at 450 °C, 200 atm, with an iron catalyst.

(a) State the source of the nitrogen and of the hydrogen used. [2] (b) Explain why a pressure of 200 atm is used rather than a much higher pressure, given that higher pressure would increase the yield. [2] (c) Explain why the iron catalyst is used, given that it has no effect on the position of equilibrium. [2]

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8. (0620 Extended, 5070 required) Use collision theory to explain why increasing the temperature increases the rate of a reaction. [3]

Answers

1. Any four: concentration, temperature, surface area of a solid, pressure of a gas, presence of a catalyst [1] [1] [1] [1]. (Light affects the rate of some reactions, e.g. photosynthesis, but it is not one of the factors listed in the 0620/5070 syllabus for this topic, so learn the five above.)

2. (0620 Extended, 5070 required) (a) Powder has a larger surface area [1], so more particles are exposed and there are more collisions per second [1]. (b) More acid particles per unit volume [1], so collisions are more frequent [1]. (c) Particles have more kinetic energy and move faster, so they collide more often [1]. More importantly, a greater proportion of collisions have energy above the activation energy [1], so more are successful [1].

3. (a) The concentration of reactants is highest at the start [1], so the rate of reaction is greatest then [1]. (Extended: because collisions between reacting particles are most frequent.) (b) A reactant has been used up — either it has run out or its concentration has fallen very low [1], so no more product is formed [1]. (c) The graph would be steeper initially [1] but level off at the same final volume [1], because the catalyst increases the rate but does not change the amount of product formed [1].

4. It increases the rate of reaction [1] and is unchanged at the end of the reaction (it is not used up) [1]. In industry, this means reactions can run at lower temperatures, saving energy and cost [1]. (Extended: it works by providing an alternative route of lower activation energy.)

5. (0620 Extended, 5070 required) (a) The forward and reverse reactions occur at the same rate [1], so the concentrations of all species remain constant in a closed system [1]. (b) Yield increases [1] — there are 4 moles of gas on the left and 2 on the right, so higher pressure favours the side with fewer gas moles [1]. (c) Yield decreases [1] — the forward reaction is exothermic, so raising the temperature favours the endothermic reverse direction [1]. (d) No effect on the yield [1].

6. (a) CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l) [1]; blue hydrated salt ⇌ white anhydrous salt [1]. (b) It is reversible because heating the blue hydrated salt drives off the water of crystallisation, leaving the white anhydrous salt [1], and adding water to the white anhydrous salt reverses this, turning it back to blue [1].

7. (0620 Extended, 5070 required) (a) Nitrogen is obtained from the air; hydrogen from methane (natural gas) [1] [1]. (b) Very high pressures are expensive to generate and maintain, and present safety risks, so 200 atm is a compromise between yield and cost/safety, not the highest pressure achievable [1] [1]. (c) The catalyst speeds up both the forward and reverse reactions equally, so it lets equilibrium be reached quickly at a moderate temperature [1], without needing the higher temperature that would otherwise shift the equilibrium away from ammonia, since the forward reaction is exothermic [1].

8. (0620 Extended, 5070 required) The particles have more kinetic energy and move faster [1], so they collide more frequently [1], and a greater proportion of the particles (collisions) have energy equal to or greater than the activation energy, so there are more successful collisions per unit time [1].

Examiner insight (June 2025): answers lost marks by saying “kinetic energy increases” without linking it to the particles, and by writing that “particles have energy greater than the activation energy”, which suggests all of them do. Only a greater proportion do. Try the real question next: Cambridge IGCSE Chemistry 0620, June 2025, Paper 42, Question 6.

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Where marks are usually lost

• Saying a temperature rise only makes collisions more frequent.
• Saying a catalyst increases the yield.
• Not stating that concentrations are constant, not equal, at equilibrium.
• Forgetting that a catalysed reaction reaches the same final amount of product.
• Describing the hydrated/anhydrous copper sulfate colour change without stating it goes both ways — this is what makes it reversible, not just a one-way colour change.
• Assuming higher pressure or higher temperature is always used in industry simply because it increases yield or rate — real conditions are a compromise that also weighs cost and safety.