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Reaction Rates

01Reaction Rates

Measuring reaction rate

This section covers measuring reaction rate from changes in reactant or product concentration over time.

Rate as a change over time

During a reaction, reactants are consumed and products are formed. Reaction rate describes how quickly the concentration of a reactant or product changes.

rate of reaction=change in concentration of reactant or producttime\text{rate of reaction}=\frac{\text{change in concentration of reactant or product}}{\text{time}}

If concentration is measured in mol dm−3\mathrm{mol\,dm^{-3}} and time in seconds, the rate has units of mol dm−3 s−1\mathrm{mol\,dm^{-3}\,s^{-1}}.

Reaction rate is not normally constant because reactant concentrations decrease as the reaction proceeds.

Rate from a concentration–time graph

A concentration–time graph can be used to find the reaction rate at a particular instant.

  1. Locate the time at which the rate is required.
  2. Draw a tangent to the curve at that point.
  3. Choose two well-separated positions on the tangent.
  4. Calculate the gradient of the tangent.

For a reactant concentration curve, the tangent slopes downwards. The rate is taken from the magnitude of this concentration change per unit time.

time / minconcentration / mol dm−3chosen pointtangent

Exam Tip: If the rate at a particular time is requested, find the gradient of a tangent at that point rather than joining two points on the reaction curve.

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