Paper 4 Chemical Reactions Structured Practice
5 structured exam questions on chemical reactions for IGCSE Chemistry Paper 4. Covers rate of reaction, collision theory, catalysts, reversible reactions, equilibrium, and redox. Total: 26 marks.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Five structured questions in the style of IGCSE Chemistry Paper 4, totalling 26 marks. Write your answers in full before checking the mark scheme.
Question 1 (6 marks)
A student investigates the rate of reaction between magnesium ribbon and dilute hydrochloric acid by measuring the volume of hydrogen gas produced over time.
Mg + 2HCl → MgCl₂ + H₂
The student repeats the experiment using the same mass of magnesium powder instead of ribbon.
(a) Sketch a graph to show how the volume of gas changes with time for both experiments on the same axes. Label the curves “ribbon” and “powder”. [3]
(b) Explain, in terms of collision theory, why the powder reacts faster than the ribbon. [2]
(c) State why the final volume of gas is the same in both experiments. [1]
Mark scheme
(a) Both curves start at zero [1]. The powder curve rises more steeply / reaches the final volume in less time [1]. Both curves level off at the same final volume / same plateau [1].
(b) The powder has a greater surface area (exposed to the acid) than the ribbon [1]. This means more acid particles can collide with the magnesium at the same time / collisions are more frequent, so the rate is faster [1].
(c) The same mass of magnesium and the same amount of acid are used, so the same total amount of hydrogen is produced / the same number of moles of Mg reacts [1].
Total: 6 marks
Question 2 (5 marks)
Explain the effect of each of the following changes on the rate of a reaction. Use collision theory in each answer.
(a) Increasing the temperature. [2]
(b) Increasing the concentration of a reactant in solution. [2]
(c) Adding a catalyst. [1]
Mark scheme
(a) Particles move faster / have more kinetic energy [1]. Collisions are more frequent AND a greater proportion of collisions have energy equal to or greater than the activation energy, so more collisions are successful per second [1].
(b) There are more reactant particles per unit volume [1]. Collisions between reactant particles are more frequent, so there are more successful collisions per second and the rate increases [1].
(c) A catalyst provides an alternative reaction pathway with a lower activation energy, so more particles have sufficient energy to react in a given collision / greater proportion of successful collisions [1].
Total: 5 marks
Question 3 (5 marks)
The Haber process is a reversible reaction:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (forward reaction is exothermic)
(a) State the meaning of dynamic equilibrium. [2]
(b) State and explain the effect on the position of equilibrium of: (i) increasing the pressure [1] (ii) increasing the temperature [2]
Mark scheme
(a) The rates of the forward and backward reactions are equal [1]. The concentrations of reactants and products remain constant (but are not necessarily equal) / both reactions continue to occur [1].
(b)(i) Increasing pressure shifts the equilibrium to the right / towards products / towards ammonia [1]. There are 4 moles of gas on the left and 2 on the right, so the system shifts to the side with fewer moles of gas to reduce the pressure.
(b)(ii) Increasing temperature shifts the equilibrium to the left / towards reactants [1]. The forward reaction is exothermic, so the system shifts in the endothermic direction (backward) to absorb the extra heat / reduce the temperature [1].
Total: 5 marks
Question 4 (5 marks)
When iron reacts with copper(II) sulfate solution, the following reaction occurs:
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
(a) State which substance is oxidised and which is reduced. [2]
(b) Write the ionic equation for this reaction. [1]
(c) Write the half-equation for: (i) the oxidation [1] (ii) the reduction [1]
Mark scheme
(a) Iron is oxidised [1]. Copper (in CuSO₄) is reduced [1].
(b) Fe(s) + Cu²⁺(aq) → Fe²⁺(aq) + Cu(s) [1]
(c)(i) Fe → Fe²⁺ + 2e⁻ [1] (c)(ii) Cu²⁺ + 2e⁻ → Cu [1]
Total: 5 marks
Question 5 (5 marks)
A student measures the rate of reaction between marble chips and dilute hydrochloric acid by recording the mass of the flask over time. The mass decreases because CO₂ gas escapes.
(a) Sketch a graph of mass loss against time for this reaction. [1]
(b) On the same axes, sketch the curve you would expect if the experiment were repeated with the same mass of marble chips but using acid of double the concentration. [2]
(c) State the effect of adding a catalyst on: (i) the rate of the reaction [1] (ii) the total mass of CO₂ produced [1]
Mark scheme
(a) A curve that starts steeply and then levels off to a constant value (maximum mass loss) [1].
(b) The curve should be steeper initially / reach the final mass loss in less time [1]. The final mass loss should be the same (same mass of marble chips, so same amount of CO₂ produced; acid is in excess in both cases, assuming this is the case) [1].
(c)(i) The rate increases / the reaction is faster [1]. (c)(ii) No effect --- the total mass of CO₂ is the same [1]. A catalyst affects only the rate, not the amount of product formed.
Total: 5 marks
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