Chemical Kinetics - Factors affecting rate of reaction.

Understanding Collision Theory is helpful in deciding on the different factors that affects the rate of a reaction. The basic tenet of the Collision Theory is this: Reactant particles must collide in order for reaction to take place.

(a) Concentration
Of course, there are other conditions. But, this basic rules results that when concentration is increased the rate of reaction increases. This is because that the probability of collision between reactant particles increases (see illustration below).

However, be mindful that usually there is a limit to how much increasing concentration of reactants can result in the reaction to be faster. This is because most chemical reactions have an activation energy (E_a), which is the minimum energy which the particles must have for the reaction to proceed.

Reactions which have small activation energy, tend to be diffusion affected, hence concentration has a far greater effect on these reactions.

(b) Pressure
Increasing pressure is applicable for gases since gases can be compressed. This effect causes the reactant particles to be closer together, hence higher probability for effective collisions.

Transition state theory is developed to understand why reactant particles requires to have a minimum energy before a reaction can take place. In this theory, a transition state must be obtained from the reactants before the latter can form the products.

The transition state exist for a very infinite amount of time. It is highly energetically unstable, because the reactant bonds are starting to break and the bonds for the products are merely starting to be produced. It is therefore theoretical, geometry is an unknown. Most importantly, the energy to taken in to reach the transition state is called the activation energy, E_a

The concept of activation energy is important because not all the reactant particles undergo reaction at the same time. This is because, there is an energy distribution of the particles. At a particular temperature, the particles do not possess the same energy.

(c) Temperature
Hence, at a particular temperature, only a portion of the reactant molecules have the minimum energy for reaction to take place. Therefore, when we increase the temperature, more molecules are able to move faster, hence more of them will have the minimum energy.

(d) Catalyst
The final factor that affects the rate of reaction is the use of a catalyst. The calayst is involved in the chemical reaction so that energy barrier for the reaction is lowered.

Since, the energy barrier for the reaction is reduced. The distribution curves of the reactant particles show that more molecules have the minimum energy to participate in the chemical reaction.

It is the use of the catalyst, which enables the chemical reaction to occur at a lower temperature and hence the reducing the need to use dangerous conditions just to speed the rate of reaction.

Knowing all the different factors in increasing rate of reaction is important. This is because not one factor is omnipotent. Many occasions, it is the a consideration of cost, feasibility and practicality which dictates which factor is more relevant. However, more often than not, all these factors are taken into account.

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Article written by Kwok YL 2009.
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