GCSE Science/Rates of Reactions

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Faster reaction rates are achieved by increasing the probability of a successful collision. Particles require a certain level of energy, called activation energy or EA before a successful collision can occur.

Ways to measure rate of reaction:
- measure volume of gas evolved at regular time intervals
- measure rate of decrease in mass of reactant
- measure time taken for a precipitate to appear

Rates of reaction are based on five factors:

1. Concentration of substance
2. Pressure
3. Temperature
4. Catalyst
5. Surface area (per volume ratio)

1. Concentration of reactant
Increased concentration of reactants = Increased rate of reaction
As the concentration of reactants increases, the number of reactant particles per unit area increases. This in turn increases the chances of successful collisions, increasing the rate of reaction.

2. Pressure
Increased pressure = increased rate of reaction
This generally applies for gases only.
When there is an increase in pressure, the volume containing the same number of particles is decreased and this increases the chances of successful collisions. Thus the rate of reaction is increased.

3. Temperature
Increased temperature = increased rate of reaction
The molecules gain kinetic energy and move at a faster speed. Increased chances of successful collisions leading to a faster rate of reaction.
More molecules may also attain activation energy and the number of successful collisions also increases. Thus increased temperature leads to a faster rate of reaction.

4. Catalyst
Use of catalyst increases rate of reaction.
A catalyst lowers the activation energy and so more particles can attain activation energy, thus a faster rate of reaction.
Only a small amount of catalyst is required. Increasing the amount of catalyst used will not increase the rates of reaction beyond a cerain point.

5. Surface area
Increased surface area = increased rate of reaction
A larger surface area per volume ratio increases the areas of contact between reactant particles. This increases the chances of successful collisions. Thus the rate of reaction is increased.