Some chemical reactions are fast e.g. combustion and explosions.
Some chemical reactions are slow e.g. rusting and fermentation.
The rate and yield of a chemical reaction can be altered by changing the physical conditions.
It is thought that for a chemical reaction to occur the atoms or molecules of the
reactants must come together or collide.
This is known as the ‘collision theory’.
There are three important parts to the collision theory.
The reacting substances must:
- Collide
- Collide with enough energy
- Collide with the correct orientation.
The collision theory is used to predict the rates (or speed) of chemical reactions.
The greater the frequency of successful collisions between reactant particles the
greater the reaction rate.
The activation energy is the minimum amount of energy needed by particles for a collision to be successful.
So by...
- Lowering the activation energy, OR
- Giving more energy to the moving particles, OR
- Increasing the number or particles
…the rate of the chemical reaction can be increased.
The above can be achieved by:
- Increasing the temperature of the reaction mixture. This will increase the
energy of the reactant particles allowing them to move faster and therefore
raising the frequency and proportion of successful collisions.
The result: the rate of reaction increases. - Increasing the concentration (or pressure [as in gases]) of the reaction
mixture. This means the reactant particles become more crowded therefore
increasing the frequency of successful collisions between the reacting
particles.
The result: the rate of reaction increases. - Increasing the Surface Area: Volume ratio or a reactant. By grinding ‘lumps’
of reactants into powders the area of exposed surface increases and so the
SA: Vol. ratio increases. This results in a greater frequency of successful
collisions between the reactant particles.
The result: the rate of reaction increases. - Using a catalyst. A catalyst is a substance that can speed up a chemical
reaction but does not interfere or get used up in the reaction. Only a small
amount of catalyst is needed to increase the rate of the reaction. It works by
lowering the activation energy of the reaction. This means less energy is
needed to ‘get the reaction going’.
The result: the rate of reaction increases.