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

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — 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 and parts marked (0620 Extended, 5070 required) — collision theory explanations and equilibrium/Haber-process reasoning — go beyond 0620 Core. All other questions are answerable by a 0620 Core candidate.

---

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 + 3H₂ ⇌ 2NH₃ 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. 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) (0620 Extended, 5070 required) 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) (0620 Extended, 5070 required) Explain why the iron catalyst is used, given that it has no effect on the position of equilibrium. [2]

---

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. photography and photosynthesis, but is not part of the closed list of five factors given in the 0620/5070 subject content, so is not creditable here.)

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 collisions are most frequent [1]. (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] by providing an alternative route of lower activation energy [1], and it is not used up in the process. In industry, this means reactions can run at lower temperatures, saving energy and cost [1].

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. (a) Nitrogen is obtained from the air; hydrogen from methane (natural gas) [1] [1]. (b) (0620 Extended, 5070 required) 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) (0620 Extended, 5070 required) 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].

---

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.