If there is more particles in the same unit volume the frequency of collision will increase, this increase in rate of collision will increase the rate of reaction. When we are explaining rate of reaction we must always indicate the aspect of time either by explicitly mentioning more collisions in the same time or by using the word frequency.

2.      Increasing surface area of a solid

By increasing the surface area of a solid there is going to be a greater amount of collisions with the surface of the solid per unit time, which will lead to an increase in the rate of reaction.

Particle size is inherently linked with the surface area, for the same mass of a solid the surface area is inversely proportional to the particle size. The greater the particle size the smaller the surface area. This leads to very small particles, nanoparticles, being explosive due to such a large surface area. A particle with sides of 2cm will have a surface area of 24cm^2, if the same cube is split in to 8 pieces it would have a surface area of 48cm^2.

3.     Increasing Temperature

 Increasing the temperate will increase the internal kinetic energy of a substance, which means that the individual particles are traveling at a greater speed. The particles traveling at a greater speed are more likely to collide with each other which therefore has the affect of more collisions per second and therefore a greater rate of reaction.

 Not all collisions will lead to a reaction, as mentioned above they must enough energy to react, this is known as the activation energy.

Activation Energy – The minimum amount of energy required for the reaction to take place.

By increasing the temperature a higher percentage off the particles have the activation energy, this means that a higher percentage of collisions will lead to a reaction, which would lead to a greater rate of reaction.Therefore increasing temperature has two increasing affects on the rate of reaction.

 4. Introduction of a catalyst

 A Catalyst – a substance which reduces the activation energy of a reaction by providing an alternative reaction pathway while not being consumed in the process will also increase the rate of reaction. As the activation energy is decreased and the amount of energy per particle stays the same a higher percentage of the particles will have the activation energy so will react when colliding. This will increase the rate of reaction.

 Measuring rates of reaction experimentally

The disappearing cross is a practical where the turbidity of a solution is used as an indication of reaction occurring. A reaction between sodium thiosulfate and hydrochloric acid is set up where the independent variable is the temperature of the reaction mixture is varied and the time taken for the cross to be obscured is measured.

As expected when the reaction is done at increasing temperatures the reaction is complete as a decreasing duration of time.

Rate of reaction is often recorded as mols^-1, in this experiment the end of the reaction is when a certain amount of mols is produced so we can remove this from our equation and have the unit for rate of reaction as s^-1, so to calculate our rate we divide 1 by the time.

Maxwell-Boltzmann distribution

If we could record the energy of each particle as plot them on the above axis we would discover that not all of the particles within a solution have a uniform, all the same, amount of energy nor is it a standard distribution.  

The distribution of particles is a shape known as the Maxwell-Boltzmann distribution which is the shape below. The peak of the line is the most probable energy, the modal value, it is not the average energy. Due to the a symmetrical shape of the distribution the average energy of particles lies slightly to the right of the peak/ most probably energy. The area under the graph is the total number of particles.

Note that we do start at the origin, no particles have zero energy, and as we increase energy the line does not meet the axis.

If we add a line to indicate the activation energy of a reaction we can calculate the amount of particles with he activation energy as we can calculate the area under the graph to the right of the activation energy.

If we increase the temperature of the particles the shape of the distribution curve will change, the peak will be lower and it will be further to the right, the total area under the line will remain constant, but the amount of particles which have the activation energy will have increased, the opposite is true if we decrease the temperature, the peak moves up and to the left.

By adding a catalyst we reduce the activation energy, on the distribution curve if we add two lines, one with and one without a catalyst we can see that when the activation energy is lower there is a greater amount of particles with the activation energy, this is what leads to the increased rate of reaction.

The final way we could change the distribution is by increasing the total amount of particles, the most probably energy and average energy would not be affected, only the total area under the graph will change.

Catalysis

Catalysts speed up a chemical reaction by reducing activation energy and are not used up in the process.

If we remind ourselves of the energy profile diagrams from thermodynamics we had one of the diagrams including a catalyst.

On the energy profile diagram with the catalyst the peak is smaller, this indicates that less energy is required for the reaction to reach the “transition state” which is a point between the reactants and products. The amount of energy for the reactants to reach this transition state is the activation energy.

Heterogenous catalysts

A heterogenous catalyst is in a different phase to the reactants, in the below example is a catalytic convertor found in cars. A catalytic convertor converts unburnt hydrocarbons to carbon dioxide and water, reduces oxides of nitrogen to nitrogen and oxides carbon monoxide to carbon dioxide. The process of this catalysis is in 3 steps

1.       The reactants adsorp on to the surface of the catalyst, this weakens the bonds within the molecules

2.     The reactants with weakened bonds have their activation energy reduced and the reaction takes place.

3.       The products now desorp from the surface of the catalyst