The rate and extent of chemical change: collision theory and activation energy

What you'll learn

This revision guide covers collision theory and activation energy as required by the AQA GCSE Chemistry specification. You'll understand why chemical reactions occur at different rates, what conditions are needed for successful collisions, and how activation energy acts as an energy barrier to reactions. These concepts form the foundation for understanding rates of reaction and are essential for both Paper 1 and Paper 2.

Key terms and definitions

Collision theory — the principle that chemical reactions can only occur when reacting particles collide with sufficient energy and correct orientation

Activation energy — the minimum amount of energy that colliding particles must have for a reaction to occur

Successful collision — a collision between reactant particles that has both sufficient energy (equal to or greater than the activation energy) and the correct orientation to result in a reaction

Frequency of collisions — the number of collisions between reactant particles per unit time

Rate of reaction — the speed at which reactants are converted into products, measured as the change in concentration of a reactant or product per unit time

Energy profile diagram — a graph showing the energy changes during the course of a chemical reaction, with progress of reaction on the x-axis and energy on the y-axis

Exothermic reaction — a reaction that transfers energy to the surroundings, usually as heat, resulting in a temperature increase

Endothermic reaction — a reaction that takes in energy from the surroundings, usually as heat, resulting in a temperature decrease

Core concepts

Collision theory fundamentals

For a chemical reaction to occur, reactant particles must collide with each other. However, not all collisions lead to a reaction. Collision theory explains the conditions necessary for collisions to be successful.

According to collision theory, two conditions must be met for a successful collision:

• The colliding particles must have sufficient energy — they must possess at least the activation energy
• The particles must collide with the correct orientation — they need to approach each other in a way that allows bonds to break and new bonds to form

When both conditions are satisfied, bonds in the reactant particles break and new bonds form to create products. If either condition is not met, the particles simply bounce off each other without reacting.

The rate of a chemical reaction depends on:

• The frequency of collisions between reactant particles
• The proportion of collisions that have sufficient energy (greater than or equal to the activation energy)

Increasing either of these factors will increase the rate of reaction. This explains why changing conditions such as temperature, concentration, pressure, or surface area affects reaction rates.

Activation energy in detail

Every chemical reaction has a specific activation energy (Ea). This is the energy barrier that must be overcome for the reaction to proceed. Even if particles collide, they will not react unless they possess energy equal to or greater than the activation energy.

Key points about activation energy:

• Different reactions have different activation energies
• Reactions with low activation energies occur more readily because a greater proportion of particles have sufficient energy to react at a given temperature
• Reactions with high activation energies require more energy input and typically occur more slowly
• The activation energy is independent of whether a reaction is exothermic or endothermic

At any given temperature, particles in a substance have a range of energies. Only those particles with energy equal to or exceeding the activation energy can react when they collide. Increasing temperature increases the proportion of particles with energy above the activation energy, which is why reactions speed up when heated.

Energy profile diagrams for exothermic reactions

Energy profile diagrams (also called reaction profiles) show the energy changes that occur during a chemical reaction. For exothermic reactions, the products have less energy than the reactants because energy is transferred to the surroundings.

Key features of an exothermic energy profile:

• The reactants start at a higher energy level than the products
• An energy "hill" or barrier represents the activation energy
• The activation energy is measured from the energy level of the reactants to the peak of the curve
• The overall energy change (ΔH) is negative and is measured from reactants to products
• The energy difference between products and reactants represents the energy transferred to the surroundings

Example: The combustion of methane (CH₄ + 2O₂ → CO₂ + 2H₂O) is exothermic. Although activation energy is needed to start the reaction (breaking C-H and O=O bonds), more energy is released when new C=O and O-H bonds form in the products.

Energy profile diagrams for endothermic reactions

For endothermic reactions, the products have more energy than the reactants because energy is absorbed from the surroundings during the reaction.

Key features of an endothermic energy profile:

• The reactants start at a lower energy level than the products
• The activation energy is still measured from the reactants to the peak of the curve
• The overall energy change (ΔH) is positive
• The energy difference between products and reactants represents the energy absorbed from the surroundings
• The activation energy for an endothermic reaction is always larger than the overall energy change

Example: Thermal decomposition of calcium carbonate (CaCO₃ → CaO + CO₂) is endothermic. Energy must be continuously supplied to break the ionic bonds in calcium carbonate, and less energy is released when new bonds form in the products.

Linking collision theory to factors affecting rate

Understanding collision theory allows you to explain how different factors affect the rate of reaction:

Temperature increase:

• Particles move faster with more kinetic energy
• Frequency of collisions increases
• More importantly, a greater proportion of particles now have energy equal to or greater than the activation energy
• Both effects combine to significantly increase the rate of reaction
• This is why the rate doesn't just double when temperature doubles — the increase is usually much more dramatic

Concentration increase (solutions) or pressure increase (gases):

• More particles present in the same volume
• Particles are closer together
• Frequency of collisions increases
• The proportion of successful collisions remains the same, but more collisions occur per unit time
• Rate of reaction increases proportionally

Surface area increase (for solids):

• More surface is exposed to the other reactant
• More particles are available to collide at any given time
• Frequency of collisions increases
• Rate of reaction increases

Use of a catalyst:

• Provides an alternative reaction pathway with a lower activation energy
• More particles now have sufficient energy to react
• The proportion of successful collisions increases
• Frequency of collisions is unchanged
• Rate of reaction increases without being used up in the reaction

Representing activation energy graphically

On an energy profile diagram, you must be able to identify and label:

• Reactants — the starting energy level on the left
• Products — the final energy level on the right
• Activation energy (Ea) — the energy difference from reactants to the peak of the curve
• Overall energy change (ΔH) — the energy difference between reactants and products
• Progress of reaction — shown on the x-axis
• Energy — shown on the y-axis

For exam questions, you may be asked to:

• Draw and label energy profile diagrams
• Calculate activation energy from a diagram
• Identify whether a reaction is exothermic or endothermic from the profile
• Show how a catalyst affects the energy profile (by lowering the activation energy peak)

Worked examples

Example 1: Explaining reaction rate using collision theory

Question: Powdered calcium carbonate reacts faster with hydrochloric acid than large marble chips. Explain why, using collision theory. [3 marks]

Model answer:

• Powdered calcium carbonate has a larger surface area than marble chips [1 mark]
• This means more particles are exposed to the acid [1 mark]
• Therefore there is a greater frequency of collisions between reactant particles, increasing the rate of reaction [1 mark]

Examiner note: You must link the increased surface area to increased collisions. Simply stating "more surface area" without explaining the collision aspect will not gain full marks.

Example 2: Interpreting an energy profile diagram

Question: The diagram below shows an energy profile for a reaction.

[Imagine a diagram showing reactants at 50 kJ, peak at 120 kJ, products at 30 kJ]

(a) What is the activation energy for this reaction? [1 mark] (b) Is this reaction exothermic or endothermic? Explain your answer. [2 marks] (c) Describe what happens to the energy during this reaction. [2 marks]

Model answer: (a) 70 kJ (or 120 - 50 = 70 kJ) [1 mark]

(b) Exothermic [1 mark] because the products have less energy than the reactants / energy is transferred to the surroundings [1 mark]

(c) Energy must be supplied to reach the activation energy / to break bonds in reactants [1 mark]. More energy is released when new bonds form in products than was needed to break bonds / overall energy is transferred to surroundings [1 mark]

Examiner note: When calculating activation energy, always measure from the reactants to the peak, not from products. Show your working even for simple calculations.

Example 3: Effect of temperature on collision theory

Question: Use collision theory to explain why increasing the temperature increases the rate of a chemical reaction. [4 marks]

Model answer:

• When temperature increases, particles have more kinetic energy [1 mark]
• Particles move faster, so they collide more frequently [1 mark]
• A greater proportion of particles now have energy equal to or greater than the activation energy [1 mark]
• Therefore there are more successful collisions per unit time, increasing the rate [1 mark]

Examiner note: Both reasons (increased collision frequency AND increased proportion with sufficient energy) should be mentioned for full marks. The second factor (energy) is more significant but both contribute.

Common mistakes and how to avoid them

• Confusing activation energy with overall energy change — Remember that activation energy is always measured from reactants to the peak of the energy profile, while overall energy change (ΔH) is measured from reactants to products. A reaction can be exothermic (negative ΔH) but still have a high activation energy.

• Thinking all collisions are successful — Students often forget to mention that collisions must have sufficient energy AND correct orientation. Always refer to "successful collisions" when explaining rates of reaction using collision theory.

• Writing "particles move faster so they have more energy to react" — Be precise: particles have more kinetic energy, which increases the proportion of particles with energy above the activation energy. Don't use vague phrases like "more energy to react."

• Forgetting to link changes to collision frequency or energy — When explaining how factors affect rate, always complete the chain: factor changes → effect on particles → effect on collisions → effect on rate. Don't jump from "higher concentration" directly to "faster rate" without the intermediate steps.

• Drawing incorrect energy profiles — For exothermic reactions, products must be lower than reactants. For endothermic reactions, products must be higher. The activation energy peak must always be above both reactants and products.

• Confusing catalysts with increasing energy — Catalysts do not give particles more energy. They provide an alternative pathway with lower activation energy, meaning particles need less energy to react successfully.

Exam technique for "The rate and extent of chemical change: collision theory and activation energy"

• "Explain" questions require detailed answers — When asked to explain using collision theory, you must state what happens to the particles, what this does to collisions (frequency and/or energy), and how this affects the rate. A three-step chain earns full marks: change → collision effect → rate effect.

• Use the mark allocation to guide detail — A 3-mark question needs three distinct points. Don't write a paragraph that makes the same point three different ways. Check the marks and count your distinct scientific points.

• Command words matter — "Describe" means state what happens; "Explain" means give reasons why; "Suggest" means apply your knowledge to unfamiliar contexts. For collision theory questions, "explain" appears frequently and requires cause-and-effect reasoning.

• Drawing energy profiles — Use a ruler for axes, label them clearly (Energy on y-axis, Progress of reaction on x-axis), mark reactants and products levels, and ensure the activation energy peak is visible above both. If asked to show a catalyst's effect, draw a second, lower peak with a dotted line.

Quick revision summary

Chemical reactions occur when particles collide with sufficient energy (equal to or greater than the activation energy) and correct orientation. The rate of reaction depends on collision frequency and the proportion of successful collisions. Energy profile diagrams show activation energy as the peak height above reactants; exothermic reactions have products lower than reactants, while endothermic reactions have products higher. Increasing temperature, concentration, pressure, or surface area increases collision frequency; temperature also increases the proportion of particles with sufficient energy. Catalysts lower activation energy without being consumed.