Measuring rates

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.

Reactants and products

There are two ways to measure the rate of a reaction:

1. measure the rate at which a reactant [reactant: substances present at the start of a chemical reaction ] is used up
2. measure the rate at which a product [product: A product is a substance formed in a chemical reaction. ] is formed

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.

Things to measure

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.

For example, if 24cm³ of hydrogen gas is produced in two minutes, the mean rate of reaction = 24 ÷ 2 = 12cm³ hydrogen / min.