Effect of concentration and pressure on rate

The rate of a chemical reaction can be changed by altering the concentration of a reactant in solution, or the pressure of a gaseous reactant.

Diagram of a low and high concentration of a reactant or the low and high pressure of a gaseous reactant.

If the concentration or pressure is increased:

  • there are more reactant particles per unit volume (ie the reactant particles become more crowded)
  • there is a greater chance of the particles colliding, which leads to more successful collisions per second
  • the rate of reaction increases
Graph of total mass of product against time from start of reaction, for high and low concentration or pressure.

Compared to a reaction with a reactant at a low concentration (if a solution) or a low pressure (if a gas), the graph line for the same reaction but at a higher concentration or pressure:

  • has a steeper gradient at the start
  • becomes horizontal sooner

This shows that the rate of reaction was greater at the higher concentration or pressure.