A-level Chemistry/AQA/Module 2/Kinetics

Collision theory

For a reaction to take place between two particles they must collide and do so with enough energy to break bonds. The collision must take place between the parts of the particles that are going to react together, the orientation of the particles also affects the subsequent reaction. To get a lot of collisions you need a lot of particles in a small volume and for the particles to have enough energy to break bonds they need to be moving fast. So for a reaction rate which is fast you need plenty of particles moving rapidly in a small volume.

Most collisions between molecules or other particles do not lead to a reacion as they do not have enough energy or are orientated wrongly.

Factors that affect rate of chemical reactions are as followed.

• Increasing the temperature this increases the speed of the molecules which in turn increases both their energy and the number of collions.
• Increasing the concentration of a solution if there are more particles present in a given volume collisions are more likely therefore the reaction rate would be faster. However as the reaction proceeds the reactants are used up and their concentration falls resulting in most reaction rates to fall as the reaction continues.
• Increasing the pressure of a gas reaction this has the same effect as increasing the concentration of a sloution there are more molecules or atoms in a given volume so collisions are more likely
• Increasing the surface area of solid reactants the greater the total surface area of a solid the more of its particles are available to collide with molecules in the gas/liquid. This can be achieved by breaking up the solid or grinding down into a powder making more reaction sites available thus speeding up the rate of reaction
• Using a catalyst a catalyst is a substance that can change the rate of reactions without being used up throughout the reaction. It also reduces the activation energy needed to kick-start the reaction.