There are 4 methods by which you can increase the rate of a reaction:
1. Increase the concentration of a reactant.
2. Increase the temperature of the reactants.
3. Increase the surface area of a reactant.
4. Add a catalyst to the reaction.
Before, we discover the reasons for the above causing an increase in rate, we must first look at what is needed to cause a reaction to occur!
If we take the reaction between magnesium and hydrochloric acid, in order for them to react together:
1. They must collide with each other
2. The collision must be with sufficient energy.
The rate of a reaction depends on how many successful collisions there are in a given unit of time.
The Effect of Concentration
If the concentration of acid (a reactant) is increased, the reaction proceeds at a quicker rate.
In dilute acid there are less acid particles. This means there is less chance of an acid particle hitting a magnesium particle as compared with acid of a higher concentration.
In concentrated acid there are more acid particles, therefore there is a greater chance of an acid particle hitting a magnesium particle.
Remember: the more successful collisions there are the faster the reaction.
The graph below shows results from two experiments. Experiment A was with concentrated acid and experiment B used dilute acid.
As you can see, the greater the concentration of the acid used in a reaction the steeper the curve and the shorter the reaction time. Hence, these results show that an increase in concentration increases the rate of a reaction.
The Effect Of Temperature
At low temperatures the reacting particles have less energy. When particles are heated they gain energy. The gaining of energy enables the particles to move around quicker, this increases their chance of colliding but also, the increase in energy increases the possibility of a collision occurring with sufficient energy. Therefore rate of reaction increases with increasing temperature.
The Effect of Surface Area
The rate of reaction between magnesium and hydrochloric acid increases as you increase the surface area of the magnesium.
For example: powdered metal (greater surface area) reacts quicker with acid than strips of metal (lower surface area).
The greater the surface area of the metal the more of its particles are exposed to the acid. This increase in exposure increases the frequency of successful collisions.
The Effect of a Catalyst
Some reactions may be speeded up by using a catalyst. A catalyst reduces the energy required for the reactants to successfully collide. The result is more collisions become successful, hence the rate of a reaction increases.