Different reactions can happen at different rates. Reactions that happen slowly have a low rate of reaction. Reactions that happen quickly have a high rate of reaction. For example, the chemical weathering of rocks is a very slow reaction: it has a low rate of reaction. Explosions are very fast reactions: they have a high rate of reaction.
Rate of reaction is an example of a compound measure. This is something that is made up from two or more other measurements. Speed is an example of this because it is made from distance and time. Rate of reaction is a compound measure because it is made from amount of reactant used or product formed (see below) and time.
There are two ways to measure the rate of a reaction:
The method chosen depends on the reaction being studied. Sometimes it is easier to measure the change in the amount of a reactant that has been used up. Sometimes it is easier to measure the change in the amount of product that has been produced.
The measurement itself depends on the nature of the reactant or product:.
The mass of a substance - solid, liquid or gas - is measured with a balance.
The volume of a gas is usually measured with a gas syringe, or sometimes an upside down measuring cylinder or burette.
It is usual to record the mass or total volume at regular intervals and plot a graph. The readings go on the vertical axis, and the time goes on the horizontal axis.
The rate of a reaction increases if: