Rates, concentration and pressure

The greater the frequency or rate 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 number of reactant particles in a given volume increases
  • the particles become more closely packed
  • the rate 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 rate of collisions increases (they happen more often), the rate of successful collisions also increases.

If the concentration of solution is doubled, the number of particles in a given volume is doubled and so too are successful collisions in a given time.

Rate of reaction is therefore proportional to the concentration.


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 horizontal line shows that no more product is being made - the reaction has finished
  • rate of reaction does not affect the total amount of product formed (but it is important that enough reactant is used, especially if 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
Always refer to the 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.