Rates, concentration and pressure

The greater the frequency of successful collisions, the greater the rate of reaction. If the concentration of a reacting solution or the pressure of a reacting gas is increased:

  • the reactant particles are closer together
  • the frequency of collisions between reactant particles increases
  • therefore the rate of reaction increases

Note that the mean energy of the particles does not change. However, since the frequency of collisions increases (they happen more often), the frequency of successful collisions also increases.

Graphs

The rates of two or more reactions can be compared using a graph of mass or volume of product formed against time. The graph shows this for two reactions.

Graph of total mass of product against time from start of reaction, for high and low concentration.

The gradient of the line is equal to the rate of reaction. The faster reaction at the higher concentration or pressure:

  • gives a steeper line
  • finishes sooner

Note that the amount of product does not change when the reaction rate changes. The same amount of product is formed, but in a shorter time. Therefore, both lines on the graph finish in the same value of mass or volume, but the reaction at the higher concentration or pressure reaches this mass or volume sooner.

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Make sure you answer questions in terms of frequency or rate of successful collisions, rather than the number of successful collisions. This is because, given enough time, even a slow reaction will have a large number of collisions.