Measuring Rates

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 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.

The rate of reaction is equal to the amount of reactant used divided by the time taken. Or it can expressed as the amount of product formed divided by the time taken

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Rates of reaction

Factors affecting the rate

You will be expected to remember the factors that affect the rate of reactions, and to plot or interpret graphs from rate experiments.

How to increase the rate of a reaction

The rate of a reaction increases if:

  • The temperature is increased
  • The concentration of a dissolved reactant is increased
  • The pressure of a reacting gas is increased
  • Solid reactants are broken into smaller pieces
  • A catalyst is used

Rate of reaction and changing conditions

The graph above summarises the differences in the rate of reaction at different temperatures, concentrations and size of pieces. The steeper the line, the greater the rate of reaction. Reactions are usually fastest at the beginning, when the concentration of reactants is greatest. When the line becomes horizontal, the reaction has stopped.

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