Rates and temperature

The greater the frequency of successful collisions, the greater the rate of reaction. If the temperature of the reaction mixture is increased:

  • reactantparticles move more quickly
  • the energy of the particles increases
  • the frequency of successful collisions between reactant particles increases
  • therefore the rate of reaction increases

Note that the mean energy of the collisions changes when the temperature changes. When the temperature is increased, the mean energy of the particles also increases. The frequency of collisions increases, but so does the likelihood that the collision exceeds the activation energy. Both changes mean that the proportion of collisions that are successful increases.

Question

Explain what is meant by a 'successful' collision.

A collision between reactant particles with enough energy (the activation energy or more than the activation energy) to produce a reaction.

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 temperature.

The gradient of the line is equal to the rate of reaction. The faster reaction at the higher temperature:

  • 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 temperature reaches this mass or volume sooner.

curriculum-key-fact
The effect of temperature on the rate of reaction is due to two factors: frequency of collisions and energy of collisions. The increase in energy is usually the more important factor.