AQA GCSE Chemistry 2026 Topic 6: The Rate and Extent of Chemical Change

What is the rate of a chemical reaction?

How fast reactants are converted into products.

What is the formula for rate of reaction?

Rate = amount of reactant used or product formed ÷ time.

What are the common units for rate?

g/s, cm³/s, or mol/s.

How can the rate of reaction be measured?

Change in mass, volume of gas produced, or disappearance of a reactant.

What is collision theory?

Chemical reactions occur when particles collide with enough energy.

What is activation energy?

The minimum energy that reacting particles must have to react.

What four factors affect rate of reaction?

Temperature, concentration (or pressure), surface area, and catalysts.

How does increasing temperature affect rate?

Increases particle energy — more frequent and energetic collisions.

How does increasing concentration or pressure affect rate?

Particles are closer together — more frequent collisions.

How does increasing surface area affect rate?

More exposed particles — more collisions per second.

How does a catalyst affect rate?

Provides an alternative pathway with lower activation energy.

What is the effect of a catalyst on yield?

None — it only speeds up reaction.

How can rate be measured with gas production?

Using a gas syringe or measuring cylinder to collect gas over time.

How can rate be measured with change in mass?

Measure mass loss on a balance as gas escapes.

What is the gradient of a rate graph?

The rate of reaction — steeper gradient = faster rate.

How can you find rate at a specific point on a curve?

Draw a tangent and calculate its gradient.

What are reversible reactions?

Reactions that can go in both forward and backward directions.

What is meant by dynamic equilibrium?

In a closed system, forward and reverse reactions occur at the same rate, and concentrations remain constant.

When does equilibrium occur?

When the rates of the forward and reverse reactions are equal.

What happens to equilibrium when temperature, pressure, or concentration changes?

It shifts to oppose the change (Le Chatelier's Principle).

If temperature is increased, which direction is favoured?

The endothermic direction (absorbs energy).

If temperature is decreased, which direction is favoured?

The exothermic direction (releases energy).

If pressure is increased, which direction is favoured?

The side with fewer gas molecules.

If pressure is decreased, which direction is favoured?

The side with more gas molecules.

If concentration of a reactant is increased, what happens?

Equilibrium shifts to make more products.

If concentration of a product is increased, what happens?

Equilibrium shifts to make more reactants.

Example: N₂ + 3H₂ ⇌ 2NH₃ (Haber Process) — what happens if pressure increases?

Equilibrium shifts right (fewer gas molecules), more ammonia produced.

What happens if temperature increases in the Haber Process?

Equilibrium shifts left (endothermic), yield of ammonia decreases.

Why is a compromise temperature used in the Haber Process?

To balance reasonable yield with fast reaction rate.

What effect do catalysts have on equilibrium position?

None — they speed up both forward and reverse reactions equally.

Required practical linked to this topic?

Investigate how changes in concentration affect rate of reaction (e.g. sodium thiosulfate + hydrochloric acid).