Rates, concentration and pressure

Collision theory

For a chemical reaction to happen:

  • reactantparticles must collide with each other
  • the particles must have enough energy for them to react

A collision that produces a reaction is called a successful collision. The activation energy is the minimum amount of energy needed for a collision to be successful. It is different for different reactions.

Two pairs of particles move towards each other.

Colliding particles

1. Two pairs of particles move towards each other

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 become more crowded
  • the frequency of collisions between reactant particles increases
  • the rate of reaction increases

Note that the mean energy of the particles does not change. However, since the frequency of collisions increases, the frequency of successful collisions also increases.


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.

The curved red and green lines are the reaction data. The straight dashed lines at the start of the two reactions are called tangents. They follow the curve at the beginning of the reaction. The steeper the tangent, the faster the reaction.

Tangents can also be drawn at other points on the graph. As the reaction carries on, the steepness of the tangent to the curve decreases (eg the blue dashed line) so the reaction is slowing down.

The rate of reaction can be analysed by plotting a graph of mass or volume of product formed against time. The graph shows this for two reactions.

Calculating the rate of reaction from the gradient of a tangent - Higher

To calculate the rate at the start of a reaction, you need to find the mathematical gradient (steepness) of the gradient. Use this equation:

rate~of~reaction = \frac{change~in~mass~or~volume~of~product}{change~in~time}

For example, using the green tangent below:

Line graph of two curved lines, one for high concentration/pressure which reaches its peak sooner than the other, which is low concentration/pressure.The green tangent shows lower pressure

Rate\ of\ reaction\ =\ \frac{60\ cm^{3}}{5\ s}

Rate of reaction = 12 cm3/s