IGCSE Chemistry — Chemical Reactions: Notes, Practice Questions & Worked Solutions

IGCSE Chemistry 0620 · Topic 6: Chemical Reactions

Chemical Reactions

Reaction rate is all about how often and how energetically particles collide. Some reactions can also run in reverse, settling into a dynamic balance called equilibrium — and every reaction can be described in terms of electron transfer (redox).

1. Rate of Reaction and Collision Theory

For a reaction to happen, particles must collide with enough energy (at least the activation energy) and in the correct orientation.

Factor increased	Effect on rate	Why
Concentration	Increases	More particles per volume → more frequent collisions
Temperature	Increases	Particles move faster → more frequent AND more energetic collisions
Surface area	Increases	More particles exposed → more frequent collisions
Catalyst	Increases	Provides a pathway with lower activation energy, so more collisions succeed

2. Reversible Reactions and Equilibrium

A reversible reaction (shown with ⇌) can go forwards and backwards. In a closed system, it reaches dynamic equilibrium — the forward and reverse reactions continue at equal rates, so the concentrations of reactants and products stay constant.

The Haber process N₂ + 3H₂ ⇌ 2NH₃, using an iron catalyst, at about 450°C and 200 atmospheres pressure. A compromise temperature and pressure are used to balance yield against reaction rate and cost.

The Contact process 2SO₂ + O₂ ⇌ 2SO₃, using a vanadium(V) oxide catalyst, as part of the industrial production of sulfuric acid.

3. Redox Reactions

OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons)

Oxidation can also be described as gain of oxygen or loss of hydrogen; reduction as loss of oxygen or gain of hydrogen. A reaction where both oxidation and reduction happen together is a redox reaction.

Worked example — identifying oxidation and reduction In the reaction Fe₂O₃ + 3CO → 2Fe + 3CO₂, identify what is oxidised and what is reduced.
Iron(III) oxide loses oxygen to become iron — it is reduced. Carbon monoxide gains oxygen to become carbon dioxide — it is oxidised.

Common mistakes to watch for

Thinking equilibrium means the reaction has stopped — it’s dynamic, both directions are still happening, just at equal rates.
Forgetting a catalyst speeds up BOTH the forward and reverse reactions equally, so it doesn’t shift the equilibrium position, only how quickly it’s reached.
Confusing “oxidation” (losing electrons) with the everyday idea of “oxidising” only meaning gaining oxygen — both definitions are valid and often interchangeable.

Key Vocabulary Recap

Catalyst — a substance that speeds up a reaction without being used up

Dynamic equilibrium — forward and reverse reactions occurring at equal rates

Redox reaction — a reaction involving both oxidation and reduction

Syllabus Reference

This note covers IGCSE Chemistry 0620 Topic 6, Chemical Reactions: physical vs chemical change, rates of reaction, collision theory, catalysts, reversible reactions, equilibrium, the Haber process, the Contact process, and redox reactions.

Practice questions

IGCSE Chemistry 0620 · Topic 6: Chemical Reactions — Practice Question Bank (no answers)

Chemical Reactions — Practice

Covers: rates of reaction, collision theory, equilibrium, and redox. 10 questions.

Q1 Core

State four factors that affect the rate of a chemical reaction.

Q2 Core

Using collision theory, explain why increasing the temperature increases the rate of a reaction.

Q3 Core

Using collision theory, explain why increasing the surface area of a solid reactant increases the rate of reaction.

Q4 Core

Explain how a catalyst increases the rate of a reaction.

Q5 Extended

Explain what is meant by dynamic equilibrium in a reversible reaction.

Q6 Extended

State the equation, catalyst, and typical conditions used in the Haber process.

Q7 Extended

State the equation and catalyst used in the Contact process.

Q8 Core

Define oxidation and reduction in terms of electron transfer.

Q9 Core

In the reaction Fe₂O₃ + 3CO → 2Fe + 3CO₂, identify which substance is oxidised and which is reduced.

Q10 Core

Explain why a catalyst does not shift the position of an equilibrium.

Worked solutions

Show worked solutions

IGCSE Chemistry 0620 · Topic 6: Chemical Reactions — Solutions

Chemical Reactions — Practice: Solutions

Four factors affecting rate.

✓Concentration (of a solution), temperature, surface area (of a solid), and the presence of a catalyst. (Pressure, for reactions involving gases, is also valid.)

Why does temperature increase rate?

✓Increasing temperature gives particles more kinetic energy, so they move faster. This means they collide more frequently, AND a greater proportion of collisions have at least the activation energy needed to react — both effects increase the rate.

Why does surface area increase rate?

✓Increasing the surface area of a solid exposes more particles at its surface to the other reactant, increasing the frequency of collisions between them, which increases the rate of reaction.

How does a catalyst increase rate?

✓A catalyst provides an alternative reaction pathway with a lower activation energy. This means a greater proportion of particle collisions have enough energy to react, increasing the rate — without the catalyst itself being used up.

Define dynamic equilibrium.

✓Dynamic equilibrium is reached in a closed system when the forward and reverse reactions are occurring at exactly equal rates, so the concentrations of reactants and products remain constant — even though both reactions are still continuously happening.

Haber process equation, catalyst, conditions.

✓N₂ + 3H₂ ⇌ 2NH₃, using an iron catalyst, at approximately 450°C and 200 atmospheres pressure.

Contact process equation, catalyst.

✓2SO₂ + O₂ ⇌ 2SO₃, using a vanadium(V) oxide catalyst.

Define oxidation/reduction (electron transfer).

✓Oxidation is the loss of electrons. Reduction is the gain of electrons (OIL RIG).

Fe₂O₃+3CO→2Fe+3CO₂: identify oxidised/reduced.

✓Fe₂O₃ is reduced (loses oxygen, becoming Fe). CO is oxidised (gains oxygen, becoming CO₂).

Q10

Why doesn’t a catalyst shift equilibrium?

✓A catalyst speeds up the forward and reverse reactions by exactly the same factor, since it lowers the activation energy for both directions equally. Equilibrium is reached faster, but the final equilibrium position (the ratio of products to reactants) is unchanged.

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