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What controls how fast a chemical reaction goes?

Collision theory, the Maxwell-Boltzmann distribution, the effect of temperature, concentration, pressure, surface area and catalysts on rate, and how catalysts lower activation energy.

A focused answer to AQA A-Level Chemistry 3.1.5, covering collision theory, the Maxwell-Boltzmann distribution, the effects of temperature, concentration, pressure and surface area on rate, and how catalysts work.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
Collision theory
The Maxwell-Boltzmann distribution
Factors affecting rate
Catalysts
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What this dot point is asking

AQA wants you to use collision theory to explain reaction rates, sketch and interpret the Maxwell-Boltzmann distribution, explain the effects of temperature, concentration, pressure and surface area, and explain how a catalyst increases rate by providing an alternative route with a lower activation energy.

Collision theory

For a reaction to happen, particles must collide with energy greater than or equal to the activation energy ( $E_a$ ) and with the correct orientation. Most collisions do not lead to reaction because they are too weak or wrongly aligned. The rate depends on the frequency of successful collisions.

The Maxwell-Boltzmann distribution

This curve shows the distribution of kinetic energies among molecules in a gas at a given temperature.

When temperature increases, the curve shifts right and flattens; the peak is lower and at higher energy, and the area to the right of $E_a$ (the proportion that can react) increases sharply.

Factors affecting rate

Temperature: more molecules exceed $E_a$ and collisions are more frequent, so rate increases.
Concentration (or pressure for gases): more particles per unit volume, so more frequent collisions and a higher rate.
Surface area: breaking a solid into smaller pieces exposes more particles, increasing collision frequency.

Catalysts

On a Maxwell-Boltzmann diagram, the catalyst moves $E_a$ to a lower value, so a larger proportion of molecules (a bigger area to the right of the new $E_a$ ) now exceed it and can react. The catalyst does not change the enthalpy change of the reaction or the position of any equilibrium; it only speeds up the approach to equilibrium. Heterogeneous catalysts (a different phase, e.g. solid iron in the Haber process) work by adsorbing reactants onto active sites, while homogeneous catalysts (the same phase) form an intermediate species; both lower the activation energy by offering a new pathway.

Worked example: explaining a rate change to full marks

Question. A reaction between two gases speeds up when the pressure is increased at constant temperature. Explain this in terms of collision theory.

State what increasing pressure does to the particles

At constant temperature, increasing the pressure squeezes the same number of gas molecules into a smaller volume, so the concentration (number of molecules per unit volume) increases.

Link to collision frequency

With more molecules in each unit of volume, molecules collide more frequently, so the frequency of collisions that have energy at or above $E_a$ also rises.

Conclude on rate

More successful collisions per second means a higher rate of reaction. Note the proportion of molecules above $E_a$ is unchanged (temperature is constant); only the collision frequency changes.

Try this

Q1. State the two conditions needed for a successful collision. [2 marks]

Cue. Energy at or above $E_a$ , and correct orientation.

Q2. Explain how a catalyst increases the rate of reaction. [2 marks]

Cue. Provides an alternative pathway with lower $E_a$ , so a greater proportion of molecules can react.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20194 marksUsing the Maxwell-Boltzmann distribution, explain why a small increase in temperature can cause a large increase in the rate of reaction.

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Raising the temperature increases the average kinetic energy of the molecules, so the distribution shifts to the right and flattens, with the peak lower and moved to higher energy.

A greater proportion of molecules now have energy equal to or greater than the activation energy ( $E_a$ ), so a larger fraction of collisions are successful.

Molecules also move faster, so the collision frequency increases, but the dominant effect is the increased proportion of molecules exceeding $E_a$ . Because this proportion rises sharply with temperature, even a small temperature rise can roughly double the rate.

Markers reward reference to the proportion of molecules above $E_a$ and the shift of the distribution to higher energy.

AQA 20173 marksSketch the Maxwell-Boltzmann distribution for a gas and use it to explain how a catalyst increases the rate of a reaction. Mark on the activation energy of the catalysed and uncatalysed routes.

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The sketch should start at the origin, rise to a single peak, then fall asymptotically towards (but never touching) the energy axis, with the area under the curve representing the total number of molecules.

Mark the uncatalysed activation energy $E_a$ on the energy axis and the catalysed activation energy $E_{cat}$ to its left (lower energy).

A catalyst provides an alternative route with a lower activation energy, so a larger area under the curve lies to the right of $E_{cat}$ , meaning a greater proportion of molecules have enough energy to react and the rate increases.

Markers reward the correctly shaped curve (origin, peak, asymptotic tail), $E_{cat}$ to the left of $E_a$ , and the link to the larger proportion of molecules exceeding the activation energy.

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Sources & how we know this

AQA A-level Chemistry (7405) specification — AQA (2015)