GCSE Chemistry – Rates of reactions

Rate of reaction

It is the change in concentration in a given time

The rate of reaction is how fast a reaction happens

We can measure this by: the mass of a mixture, the volume of gas given off, measure the light transmitted.

Important in chemical industry – must make as much of a product as possible as cheaply as possible therefore need to be made quickly and safely.

Collision Theory

Affected by the temperature, concentration, surface area, pressure and whether there is a catalyst.

Reacting particles don’t just bump into each other. They must collide with enough energy otherwise they will not react.

Activation energy – minimum energy needed for a chemical reaction to take place.

The factors that affect the rate


Higher concentration means more particles in the same volume so the reaction is faster because there are more collisions per second Remember: increasing concentration or pressure does not increase the energy with which the particles collide. It does increase the frequency of the collisions, however.

Surface Area

Smaller pieces of materials have an increased surface area for a reaction to take place so more collisions per second.


At higher temperatures particles move faster so there are more collisions per second. The collisions are more energetic.


Sometimes we need to change the rate of a reaction; we can speed up the rate of a reaction by adding a catalyst. A catalyst is something which increases the rate of a reaction but it is not affected chemically itself at the end of a reaction. Catalysts are not used up in the reaction so they can be used over and over again. Catalysts are often very expensive as they are made of precious metals. But, it is usually cheaper to pay for a catalyst for all the energy needed for the high temperature or high pressure. Some catalysts work by providing a surface for the reacting particles to come together. They lower the activation energy for the particles to react. Catalysts often come in the form of powders, pellets or fine gauzes, this provides the largest possible surface area for them to work.