AQA GCSE Chemistry 4.6 The rate and extent of chemical change: a complete overview

What topic 4.6 actually demands

The rate and extent of chemical change explains how fast reactions go and where reversible reactions settle. Topic 4.6 rewards confident graph work, clear collision-theory explanations, and careful Le Chatelier reasoning. It builds directly on energy changes (activation energy) and connects to the industrial chemistry of later topics.

This guide walks through all three dot points of the topic in specification order, then sets out the exam patterns AQA repeats. Each dot point has a matching page with practice questions; this overview ties them together.

Rate of reaction and measuring it

The rate of reaction is how fast reactants are used or products are made. It is measured by mass loss on a balance, gas volume in a gas syringe, or by timing a fixed change. The mean rate is the quantity changed divided by the time, and the rate at a point is the gradient of a tangent to the curve. The rate is fastest at the start and falls to zero as reactants run out.

Factors affecting rate

Collision theory says reactions occur when particles collide with at least the activation energy, and rate depends on the frequency of successful collisions. Increasing concentration, pressure or surface area all make collisions more frequent. Increasing temperature makes collisions more frequent and gives more particles the activation energy. A catalyst provides an alternative pathway with a lower activation energy and is not used up.

Reversible reactions and equilibrium

A reversible reaction goes both ways (shown by $\rightleftharpoons$), with the forward and backward reactions having opposite energy changes. In a closed system it reaches dynamic equilibrium, where both reactions occur at equal rates and the amounts stay constant. Le Chatelier's principle predicts the shift when a change is made: concentration, temperature and pressure each move the equilibrium in a defined direction.

How topic 4.6 is examined

A typical AQA profile for this topic:

• Graph work. Calculating mean rate, drawing tangents and comparing curves for different conditions.
• Explanation. Using collision theory to explain the effect of each factor on rate.
• Practical. The rate required practical, including methods, variables and results.
• Equilibrium. Predicting Le Chatelier shifts and explaining dynamic equilibrium.

Check your knowledge

A mix of recall and explanation questions covering topic 4.6. Attempt them under timed conditions, then check against the solutions.

1. State two ways to measure the rate of a reaction that produces a gas. (2 marks)
2. A reaction makes $60$ cm$^3$ of gas in $30$ s. Calculate the mean rate. (1 mark)
3. Explain, using collision theory, why increasing concentration increases the rate. (2 marks)
4. Explain why increasing temperature increases the rate. (2 marks)
5. Define a catalyst. (2 marks)
6. Explain what is meant by dynamic equilibrium. (2 marks)
7. State Le Chatelier's principle. (1 mark)
8. In an equilibrium where the forward reaction is exothermic, state the effect of increasing temperature on the yield of products. (2 marks)