collision theory

Higher and foundation tier

Temperature and rates of reaction

graph to show how rate of reaction changes with temperature When the temperature of the reactants particles is raised they gain kinetic energy, this causes them to move faster and collide more often. Not only do the particles collide more but more of the collisions will be successful. The more successful collisions there are the faster will be the rate of reaction. Temperature has a large effect on reaction rates. As a rough rule of thumb if you raise the temperature by 100C then the rate of reaction doubles. As the temperature increases a larger proportion of the particles will have energies above the activation energy, this means they will be able to collide successfully.

The graph opposite shows a set of results collected by a student. The experiment released a gas and the student repeated the experiment at a low, medium and high temperature. You can see that the red line, for high temperature reaction is the steepest, showing that the largest volume of gas was released in the shortest time- that is the rate was the fastest for the high temperature reaction. The low temperature reaction is shown by the purple curve, you can clearly see that this curve is the shallowest, showing that the reaction was the slowest. The yellow line for medium temperature is located between the high and low temperature curves, showing that the rate of reaction is is also between the high and low temperature rates.

Concentration and rates of reaction

Concentration is the number of particles in a given volume. If you increase the concentration then you increase the number of particles in a given volume, this means that there is more of a chance that the particles will collide successfully (see image below). If there are more successful collisions then the rate of reaction will increase.

rates of reaction and concentration

Concentration and gases

The particles in a gas are spread out, indeed gases are mostly empty space with large gaps between the particles. This means that the chances of the reacting particles colliding successfully are not high. To increase the chances of the particles colliding we can increase the pressure. Increasing the pressure causes the particles be squashed closer together, this is the same as increasing the concentration in solutions. This is shown in the image below:

pressure and rates of reaction

Key Points

Practice questions

Check your understanding - Questions temperature and rates

Check your understanding - Questions concentration and rates