collision theory

Higher and foundation tiers

Temperature and rates of reaction

graph to show how rate of reaction changes with temperature When the temperature of the reactant particles is raised they gain kinetic energy; this will allow the particles to move faster and collide more often with each other. Not only do the particles collide more frequently 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 and ultimately form the products quicker.

The graph opposite shows a set of results collected by a student. The experiment released a gas and the student repeated the experiment at low, medium and high temperatures. You can see that the red line for the high temperature reaction has the steepest gradient or slope showing that the largest volume of gas was released in the shortest time- that is the rate was the fastest for this high temperature reaction. The low temperature reaction is shown by the blue curve. You can clearly see that this curve has the shallowest gradient or slope 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 this reaction is somewhere 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 present in any 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

As a simple example of how concentration affects the rate of a reaction consider the reaction of hydrochloric acid with marble chips (calcium carbonate); equations for this reaction are shown below:

calcium carbonate(s) + hydrochloric acid(aq) calcium chloride(aq) + carbon dioxide(g) + water(l)
CaCO3(s) + 2HCl(aq) CaCl2(aq) + CO2(g) + H20(l)

How quickly the bubbles of carbon dioxide are released in this reaction gives an excellent indication of how fast the reaction is going. If equal volumes of hydrochloric acid with different concentrations are added to two identical conical flasks containing the same sized and an equal mass of marble chips then the rate or speed of the reaction can be quickly measured just by observing how quickly the bubbles of CO2 are released, this is shown below:

2M hydrochloric acid is more concentrated than 0.5M hydrochloric acid; this means it will react much faster with marble chips than the less concentrated 0.5M acid.

Concentration and gases

The particles in a gas are spread out with large areas of empty space between them meaning that there are large gaps between them. This means that the chances of the particles in two different gases colliding and reacting with each other are quite low. To increase the chances of the particles colliding with each other we can increase the pressure. Increasing the pressure causes the particles to 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 on temperature and rates

Check your understanding - Additional questions on temperature and rates

Check your understanding - Questions on concentration and rates