6.1.5Chemical Change: Rate and Extent

Factors that Affect the Rate of Reaction — AQA GCSE Combined Science Trilogy 8464 - Chemistry Revision Notes & Topical Questions

5 min read 7 parts Lesson 6 of 16

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Part 1 of 5Measuring the rate

What rate means

What rate means The rate of reaction tells you how quickly reactants are used up or products are formed.
A fast reaction finishes in seconds; a slow one such as rusting can take years.

Mean rate equation

Mean rate equation Mean rate = quantity of reactant used ÷ time taken, or quantity of product formed ÷ time taken.
Quantity may be a mass in g, a volume in cm³ or an amount in mol, so units might be g/s, cm³/s or mol/s.

Reading a rate graph

Reading a rate graph On a graph of product against time, the steepest part of the line is the fastest rate.
The line becomes flat when one reactant has run out and the reaction has stopped.
To find the rate at a given moment, draw a tangent to the curve and calculate its gradient (change in y ÷ change in x) (HT only).

Method	What is measured	Suitable when
Mass loss on a balance	Decrease in mass as a gas escapes	A gas of fairly high density is released
Gas syringe or upturned measuring cylinder	Volume of gas collected over time	Any gas is produced
Timing a cross disappearing	Time for a precipitate to cloud the solution	A solid forms and turns the liquid opaque

Rate of reaction

Concentration (solutions)

Concentration (solutions) A higher concentration means more particles in each cm³, so collisions occur more often and the rate increases.

Pressure (gases)

Pressure (gases) Raising the pressure squeezes gas particles closer together, so collisions are more frequent and the rate increases.

Surface area of solids

Surface area of solids Breaking a solid into smaller pieces exposes more particles to the other reactant, so more collisions happen each second.

Temperature

Temperature Particles move faster, so they collide more often and, more importantly, a bigger fraction collide with enough energy to react.

1Cut the solid up

2Greater surface area for the same mass

Greater surface area for the same mass More of the solid's particles are on the outside where they can be hit.

3More frequent collisions

More frequent collisions The other reactant meets exposed particles more often.

4Faster reaction

Faster reaction The rate increases, and a powder can even react explosively, as with flour dust.

Catalysts

Catalysts A catalyst speeds up a reaction without being used up, by providing an alternative pathway with lower activation energy.

Explaining answers

Explaining answers Always link the factor to collisions: more frequent collisions, or more collisions with sufficient energy.
Do not say that the particles 'collide more' without saying 'per second'.

Term	Meaning
Rate of reaction	How fast reactants are used up or products are made.
Concentration	The amount of solute dissolved in a given volume of solution.
Surface area to volume ratio	The total outside area of a solid compared with its volume; it rises as the pieces get smaller.
Collision frequency	The number of collisions between reacting particles each second.
Catalyst	A substance that increases the rate of a reaction but is not used up.
Tangent	A straight line touching a curve at one point, used to find the rate at that instant.

Saying that a higher temperature gives particles more energy 'so they react' without mentioning collisions that are more frequent and more energetic.

Thinking a larger lump has a larger surface area to volume ratio; breaking it up increases the ratio.

Stating that a catalyst is a reactant or is used up, when it is unchanged at the end.

Writing 'the reaction stops' for a flat line without adding that a reactant has been used up completely.

Last updated: 20 October 2026