During a chemical reaction the reactants collide with each other, however these collisions must be successful. For a successful collision two things must occur:
- The molecules must collide with the correct orientation
- The molecules must collide with enough energy to overcome the activation energy
If the collisions don’t meet BOTH of these criteria the reaction won’t happen.
The rate of a reaction is determined by the rate of successful collisions. The greater the frequency of successful collisions, the faster the rate of the reaction. There are 5 factors that can influence the rate of a reaction: concentration, temperature, surface area, pressure and the addition of a catalyst.
Concentration
Increasing the concentration increases the rate of a reaction.
An increase in concentration increases the number of molecules within the same volume. This means that within a given time there is a greater chance of the molecules successfully colliding. As the frequency of successful collisions is higher the rate of the reaction will be faster.
Temperature
Increasing the temperature increases the rate of a reaction.
As temperature increases the molecules gain kinetic energy and so move around more. The greater the energy of the molecules the more particles will have the activation energy or higher. This increases the chance that the molecules will collide successfully (in the correct orientation) and so the rate of the reaction will increase.
Surface Area
Increasing the surface area increases the rate of a reaction.
By reducing the size of the reactants, the surface are of the reactants increases. This increases the frequency of successful collisions and so the rate of the reaction increases.
Pressure
Increasing the pressure increases the rate of a reaction.
When the pressure is increased the volume occupied by the molecules is reduced. In a given time there will be a higher frequency of successful collisions (molecules colliding with at least the activation energy and at the correct orientation). This will therefore increase the rate of the reaction.
Catalyst
Adding a catalyst to a reaction increases the rate of a reaction.
Catalysts provide the reaction with an alternate route that has a lower activation energy. The lower the activation energy, the more particles there are that will have the activation energy or above. This increases the frequency of successful collisions and so the rate of the reaction increases.
IMPORTANT – The catalyst is not consumed by the overall reaction (it will be unchanged at the end of the reaction).
In the diagram below Ea represents a reaction with a catalyst and Ea represents a reaction without a catalyst.
Key Points
- Reactants must collide successfully for a collision to take place.
- For a successful collision reactants must collide in the correct orientation and with at least the activation energy.
- The rate of a reaction can be increased by increasing the frequency of successful collisions.
- The rate of a reaction can be altered by the following factors: concentration, temperature, surface area, pressure and the addition of a catalyst.
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