1.6Physical Chemistry

Kinetics — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

5 min read 9 parts Lesson 6 of 13

📚 Oxford AQA International A Level Chemistry Hub / Topical Revision

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Lesson summary

Complete PDF with figures and original page layoutPage 1 Oxford AQA International Your notes A Level (IAL) Chemistry Kinetics Contents Collision Theory Maxwell–Boltzmann Distribution Effect of Temperature On Reaction Rate Effect of Concentration & Pre…

Part 1 of 7Collision theory

Collision theory
- particles must collide with the correct orientation and with enough energy for a reaction to happen

	Effective collision	Ineffective collision
Orientation	Correct	Incorrect
Energy	At least E_a	Less than E_a
Reaction	Yes	No

Ineffective collision

Ineffective collision particles bounce off each other without reacting

Collision frequency

Collision frequency the number of collisions per unit time,
- and more frequent collisions mean more particles with energy of at least E_a,
- so a faster rate

Requirement

Requirement both correct orientation and energy equal to or above E_a are needed

Part 2 of 7Activation energy

Activation energy, E_a = the minimum energy reactant particles need to overcome for a reaction to occur

Exothermic reactions

Exothermic reactions reactants are higher in energy than products, so the barrier for reactants is relatively small

Endothermic reactions

Endothermic reactions reactants are lower in energy than products, so E_a is relatively large

Meaning

Meaning it is the energy needed to activate the reactant particles for effective collisions

Part 3 of 7Maxwell–Boltzmann distribution

Definition

Definition a graph of the distribution of molecular energies at a given temperature

Features

It starts at the origin because no particles have zero energy.
It never meets the x axis, as there is no maximum energy.
The peak shows the most common energy, and the mean energy is just to its right.
The area under the curve equals the total number of particles.

Distribution

Distribution a few particles have very low energy, a few have very high energy, and most are in between

Meaning for reaction

Meaning for reaction only a small proportion of molecules have energy above E_a

Key point

Key point the increase in the proportion with energy above E_a affects rate more than the increase in collision frequency

Part 4 of 7Effect of temperature

Higher temperature

Higher temperature particles gain kinetic energy and move faster, so collisions are more frequent

Proportion

Proportion a greater proportion of particles have energy of at least E_a, so more collisions are successful

Curve

Curve it flattens and its peak shifts to the right

Lower temperature

Lower temperature the peak is higher and to the left, as more molecules have average energy and the mean is lower

Result

Result a small temperature rise causes a larger rate increase, due to both more effective collisions and a greater proportion above E_a

Part 5 of 7Effect of concentration and pressure

Effect of concentration and pressure

Concentration

A more concentrated solution has more particles in a given volume.
Collision frequency increases, so the rate increases.
The rate falls over time because reactants are used up.

Pressure (gases)

Higher pressure squeezes the same particles into a smaller volume.
Collision frequency increases, so the rate increases, just like raising concentration.

Part 6 of 7Catalysts

Catalysis

Catalysis a catalyst increases the rate by providing an alternative reaction pathway with a lower activation energy

Homogeneous catalyst

Homogeneous catalyst it is in the same phase as the reactants, such as concentrated sulfuric acid in esterification

Heterogeneous catalyst

Heterogeneous catalyst it is in a different phase from the reactants
Aluminium oxide in cracking.
Iron in the Haber process.
Catalytic converters in car exhausts.

Part 7 of 7Catalysts and the Boltzmann distribution

Lowered Ea

Lowered Ea a greater proportion of molecules have the energy needed for an effective collision

Graph

Graph the area to the right of the new, lower E_a is much larger than the area to the right of the original E_a

Result

Result the catalysed reaction is faster, although the curve itself is unchanged

Key terms

Term	Meaning
Collision theory	Reactions need collisions with the correct orientation and energy of at least E_a.
Activation energy	The minimum energy for an effective collision.
Maxwell–Boltzmann distribution	A graph of molecular energies at a given temperature.
Homogeneous catalyst	A catalyst in the same phase as the reactants.
Heterogeneous catalyst	A catalyst in a different phase from the reactants.
Collision frequency	The number of collisions per unit time.

Avoid these mistakes

Saying temperature only raises the number of collisions — it mainly raises the proportion with energy above E_a.

Saying a catalyst changes the distribution curve — it lowers E_a and the curve stays the same.

Starting the Boltzmann curve on the y axis — it starts at the origin.

Drawing the curve touching the x axis at high energy — it never meets the axis.

Last updated: 18 October 2026