Rates of reaction

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.

Reactants and products

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

  1. Measure the rate at which a reactant is used up.
  2. Measure the rate at which a product 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.

\text{rate of reaction} = \frac{\text{amount of reactant used or amount of product formed}}{\text{time taken}}

The rate of a reaction increases if:

  1. The temperature is increased.
  2. The concentration of a dissolved reactant is increased.
  3. The pressure of a reacting gas is increased.
  4. Solid reactants are broken into smaller pieces.
  5. A catalyst is used.
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