IGCSE Chemistry: Chemical Reactions Practice Questions
The rate of a reaction depends on how frequently reacting particles collide with enough energy. Increasing concentration, temperature, surface area or pressure increases the rate, and a catalyst increases it by lowering the activation energy.
Topic 6 of Cambridge IGCSE Chemistry 0620 covers rates, equilibrium and redox, and every rate answer must be given in terms of collisions. Naming the factor without the collision explanation loses most of the marks. The questions below train the full explanation each time.
What you need to know for Chemical Reactions
- Collision theoryA reaction occurs when particles collide with energy equal to or greater than the activation energy. Rate depends on the frequency of these successful collisions.
- Effect of temperatureHigher temperature gives particles more kinetic energy, so they move faster and collide more frequently, and a greater proportion of collisions have enough energy to react.
- Effect of concentration, pressure and surface areaHigher concentration or pressure means more particles in the same volume, so collisions are more frequent. Smaller pieces give a larger surface area, so more particles are exposed and collisions are more frequent.
- CatalystA substance that increases the rate of a reaction and is unchanged at the end. It provides an alternative pathway with a lower activation energy, so a greater proportion of collisions are successful.
- Reversible reactions and equilibriumIn a closed system a reversible reaction reaches equilibrium when the forward and reverse reactions occur at the same rate, so the concentrations of reactants and products stay constant.
- RedoxOxidation is loss of electrons, reduction is gain of electrons. Remember OIL RIG. An oxidising agent takes electrons from another species and is itself reduced.
IGCSE Chemistry Chemical Reactions questions and answers
4 exam-style questions written to the 0620 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
Explain, in terms of particles, why increasing the temperature increases the rate of a reaction.
Show the worked answer
- Increasing the temperature increases the average kinetic energy of the particles.
- The particles therefore move faster and collide with each other more frequently.
- More importantly, a greater proportion of the particles now have energy equal to or greater than the activation energy.
- A greater proportion of collisions are therefore successful, so the rate of reaction increases. The second effect is much larger than the first.
Powdered calcium carbonate reacts faster with dilute hydrochloric acid than large lumps of the same mass. Explain why.
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- Breaking a solid into smaller pieces increases the total surface area for the same mass.
- More calcium carbonate particles are therefore exposed at the surface where the acid can reach them.
- The frequency of collisions between acid particles and the solid surface increases.
- More successful collisions occur per second, so the rate of reaction increases. The total volume of gas produced is unchanged, because the mass of calcium carbonate is the same.
Write the equation for the Haber process and state the conditions used, including the catalyst.
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- The reaction is reversible: N2 + 3H2 in equilibrium with 2NH3.
- Check the balancing: 2 nitrogen atoms and 6 hydrogen atoms on each side.
- The temperature used is around 450 degrees Celsius. This is a compromise, since a lower temperature would give a higher yield but too slow a rate.
- The pressure is around 200 atmospheres, and an iron catalyst is used to increase the rate without changing the position of equilibrium.
In the reaction Zn + CuSO4 gives ZnSO4 + Cu, state which species is oxidised and which is reduced, and explain your answer in terms of electrons.
Show the worked answer
- Zinc starts as a neutral atom and ends as Zn2+ in zinc sulfate, so it has lost two electrons: Zn gives Zn2+ + 2e-.
- Loss of electrons is oxidation, so zinc is oxidised. Zinc is therefore the reducing agent.
- Copper starts as Cu2+ in copper(II) sulfate and ends as neutral copper metal, so it has gained two electrons: Cu2+ + 2e- gives Cu.
- Gain of electrons is reduction, so copper ions are reduced. Copper(II) sulfate is therefore the oxidising agent.
Common mistakes in this topic
- Explaining a rate change without mentioning collisions.
- Saying a catalyst is used up. A catalyst is chemically unchanged at the end.
- Claiming a catalyst increases the yield of a reversible reaction. It only speeds up the approach to equilibrium.
- Saying equilibrium means the reaction has stopped. Both reactions continue at equal rates.
- Confusing oxidation with the gain of electrons. Oxidation is loss.
Exam tips
- Every rate answer needs three elements: what changes about the particles, the effect on collision frequency, and the effect on successful collisions.
- On a rate graph, a steeper initial gradient means a faster rate, and the same final volume means the same amount of reactant.
- For the Haber and Contact processes, learn the conditions as a compromise between rate and yield, and say so.
- Use OIL RIG for redox and always name the species, not the compound.
- Remember that a catalyst lowers activation energy but does not change the enthalpy change of the reaction.
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Chemical Reactions FAQs
Why does increasing temperature speed up a reaction?
Higher temperature gives particles more kinetic energy so they move faster and collide more frequently. More significantly, a greater proportion of particles have energy equal to or above the activation energy, so a greater proportion of collisions are successful. The second effect is the larger of the two.
How does a catalyst work?
A catalyst provides an alternative reaction pathway with a lower activation energy. This means a greater proportion of collisions have enough energy to react, so the rate increases. The catalyst is chemically unchanged at the end and does not alter the position of equilibrium or the enthalpy change.
What does equilibrium mean in a reversible reaction?
Equilibrium is reached in a closed system when the forward and reverse reactions are proceeding at the same rate, so the concentrations of reactants and products remain constant. Both reactions continue, which is why it is described as a dynamic equilibrium rather than the reaction stopping.
What is the difference between oxidation and reduction?
Oxidation is the loss of electrons and reduction is the gain of electrons, remembered as OIL RIG. Oxidation can also be described as gain of oxygen or loss of hydrogen. A species that is oxidised acts as the reducing agent, and a species that is reduced acts as the oxidising agent.
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Written to the published Cambridge IGCSE Chemistry (0620) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.