Factors Affecting Rates of Reaction

Increasing temperature increases the rate of chemical reactions. This is observed during cooking when using a stove or oven, where one of the parameters that can be changed is the temperature. This often causes a change in how long it takes to cook. Generally, cooking at a higher temperature makes the chemical reactions involved in cooking occur faster.

There are two main reasons why an increased temperature increases the rate of reaction. The first is that particles have more kinetic energy, and are now moving faster. As a result, they have more frequent collisions. According to collision theory, this means there will be more successful overall collisions as the total number has increased. Secondly, a greater proportion of particles have energy equal to or greater than the activation energy, and therefore the proportion of collisions that are successful increases. Both of these factors increase the rate. We can see that in the diagram shown below. At T2, the temperature has been increased compared to T1, and more particles now have higher energy.

The greater the surface area of reactants, the greater the number of particles available to react. This leads to a larger number of collisions, which increases the rate of reaction. For example, grinding a solid into a powder increases the exposed surface area allowing for faster rates of reaction.

If the concentration of a reactant increases in a reaction, there will be a greater number of particles in a given space. This leads to a higher number of collisions between particles, which increases the rate of reaction.

Increasing the pressure in a gas system causes particles to be closer together, effectively increasing the concentration. This causes more frequent collisions, leading to an increased rate of reaction.

A catalyst is a molecule or chemical substance that can act to increase the rate of reaction. It is present in the reaction mixture and lowers the activation energy for the reaction to occur. Importantly, catalysts are not consumed by the chemical reaction.

A catalyst lowers the activation energy for a given reaction by providing a lower energy pathway for the reaction to proceed. Catalysts are widely used in the chemical manufacturing industry because of their significant ability to make reactions occur more quickly.

The energy profile diagram below illustrates the effect of adding a catalyst to a reaction mixture.

When a catalyst is added to a reaction mixture, the activation energy is lowered. This means a greater proportion of molecules can now react at a given temperature, as more of them have the required energy. This process is illustrated in the diagram below. This leads to an increased rate of reaction.

Typically, industrial chemists use a heterogeneous catalyst (a catalyst in a different state compared to the reaction mixture) to make it easier to separate the products from the catalyst. For solid catalysts, they use powdered or sponge-like catalysts. This increase in the surface area of the catalyst further increases the rate of reaction.