Key Stage 4 science · Chemistry – Rate and extent of chemical change

GCSE chemistry: rates of reaction and reversible reactions

This GCSE chemistry topic asks two questions: how fast does a reaction go, and how far does it go? Students investigate how temperature, concentration, surface area and catalysts change the rate of a reaction, and explain the pattern using particles and collisions. They also meet reversible reactions, which can run in both directions, and learn what changes the balance between reactants and products.

Rates of reaction and reversible reactions, explained

The rate of reaction is how quickly reactants are used up or products are formed. It is measured by following something that changes as the reaction goes, such as the volume of gas collected in a syringe, the mass lost from a flask as gas escapes, or the time for a cloudy mixture to hide a mark. A graph of product against time starts steep and flattens as reactants run out. The steeper the line, the faster the reaction, and a flat line means the reaction has finished.

The explanation comes from collision theory. For particles to react they must collide, and with enough energy, at least the activation energy. Raising the temperature makes particles move faster, so they collide more often and a bigger share of collisions have enough energy. Raising the concentration of a solution, or the pressure of a gas, packs more particles into the same space, so collisions are more frequent. Breaking a solid into smaller pieces increases its surface area, exposing more particles to collide with, so a powder reacts faster than a lump.

A catalyst speeds up a reaction without being used up. It provides an alternative pathway with a lower activation energy, so more collisions are successful. Because it is not used up, only a small amount is needed and it can be reused. Catalytic converters in cars and many industrial processes depend on this. Students are expected to know that a catalyst changes the rate, not the amount of product that can eventually be made.

Some reactions are reversible, shown by a ⇌ arrow, meaning the products can turn back into the reactants. Blue hydrated copper sulfate loses water on heating to become white, and the white powder turns blue again, and warms up, when water is added. The forward reaction takes in energy and the reverse gives out the same amount. In a closed system the forward and backward reactions eventually happen at the same rate. This is called dynamic equilibrium: the amounts stop changing, but both reactions carry on. Changing the temperature, pressure or concentrations can shift the balance towards more products or more reactants. For example, raising the temperature favours the direction that is endothermic, and adding more of a reactant favours making more product. The depth of this, including how the idea applies to ammonia production, varies between schools, so ask your child’s teacher.

Words your child will hear

Rate of reaction: How fast reactants are used up or products form, often measured in cm³ of gas per second.
Collision theory: The idea that a reaction happens only when particles collide with enough energy.
Surface area: The total area of a solid’s outside that is exposed to the other reactant. Crushing increases it.
Catalyst: A substance that speeds up a reaction by lowering the activation energy, and is not used up.
Reversible reaction: A reaction in which products can change back into the reactants, shown with ⇌.
Dynamic equilibrium: The point in a closed system where forward and reverse reactions go at the same rate.
Concentration: How much of a substance is dissolved in a given volume of solution.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about factors that influence the rate of reaction: varying temperature or concentration, changing the surface area of a solid reactant or adding a catalyst
Students should be taught about factors affecting reversible reactions

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

Marble chips (calcium carbonate) react with hydrochloric acid and give off carbon dioxide. A student collects the gas and records its volume: 0 s: 0 cm³, 30 s: 24 cm³, 60 s: 40 cm³, 90 s: 50 cm³, 120 s: 54 cm³, 150 s: 54 cm³. Find the mean rate over the first 30 seconds and the rate from 60 to 90 seconds, and explain the change.

Mean rate over the first 30 seconds is the volume of gas divided by the time taken.
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24 cm³ ÷ 30 s = 0.80 cm³/s
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From 60 s to 90 s, the volume rose from 40 to 50 cm³, which is 10 cm³ in 30 seconds.
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10 cm³ ÷ 30 s = 0.33 cm³/s
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The reaction slowed down. As the acid and marble were used up, the acid became less concentrated and there were fewer particles to collide, so successful collisions became less frequent.
The volume stops rising at 54 cm³ from 120 s onwards. One reactant has been used up, so the reaction has finished.
Predict a repeat with the same mass of powdered marble. The powder has a larger surface area, so the graph would start steeper and flatten sooner. It would still level off at 54 cm³, because the amount of reactant has not changed.

The mean rate is 0.80 cm³/s over the first 30 s and 0.33 cm³/s from 60 s to 90 s. The rate falls as reactants are used up. Powdered marble would give a faster start but the same final volume of 54 cm³.

Common mistakes and how to help

Thinking a faster reaction makes more product

Words such as “better” and “more effective” blur the difference between speed and amount in a student’s mind.

What helps: The final amount depends on how much reactant there is. Rate only changes how quickly you get there. On a graph the lines can differ in steepness while ending at the same height.

Saying particles at a higher temperature just hit harder

It is true, but it is only half of the explanation, and it is the half that is easiest to say.

What helps: Both effects matter: particles collide more often, and a larger fraction of collisions have at least the activation energy. A strong answer mentions frequency and energy together.

Believing a catalyst is used up or changes the products

In class, catalysts are added alongside the reactants and seem to join in.

What helps: A catalyst is recovered chemically unchanged at the end, and it does not change how much product forms in the end. In a reversible reaction it only speeds up how quickly equilibrium is reached.

Thinking reactions stop at equilibrium

The colours or amounts stop changing, which looks like nothing is happening.

What helps: In dynamic equilibrium, both directions carry on at the same rate. A bath with the tap running and the plug partly out can stay at the same level while water flows in and out.

How to help at home

Race two fizzing tablets. Drop one effervescent tablet whole into cold water and another, crushed, into the same volume of the same water. Time both with a phone timer. This shows surface area. Try cold and then tap-warm water to test temperature. Change only one thing at a time. Do not drink the result.
Compare sugar in different forms. Time how long a sugar cube and a spoonful of caster sugar take to dissolve in the same amount of water, stirred the same way. Dissolving is a physical change rather than a reaction, but the surface area idea works the same way. Ask your child to explain the result, then say why stirring counts as a variable.
Draw predictions before you test. Before each experiment, ask your child to sketch the graph they expect and say why. Then compare. Learning to predict with an explanation is the skill that GCSE questions on rate graphs reward.
Spot catalysts and reversible changes. Talk about how a car’s catalytic converter speeds up the changing of harmful exhaust gases, and about everyday changes that go both ways, such as water freezing and melting. The freezing example is a physical change, not a chemical one, but it makes “reversible” easy to picture. Ask your child how a reversible chemical reaction differs.

Builds on

Key Stage 3: Chemical reactions and equations
Key Stage 3: Acids, alkalis and neutralisation

Everything in Key Stage 4 science

Updated 20 October 2026.