Factors Influencing Rate of a Reaction

We all know how to warm milk, right? When the gas stove is at low temperature, the milk heats very slowly but as soon as we increase the temperature, the milk starts heating up quickly, that is, the rate increases. In this case, temperature is the factor influencing the rate of reaction. Let’s learn about all the other factors that influence the rate of a reaction and how they affect the rate.

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Rate of Reaction

The study of the rate of reaction and factors affecting the rate of reaction is known as Chemical Kinetics. We can measure the rate of a chemical reaction by measuring either:

• how quickly a reactant is consumed
• how quickly a product is formed

Therefore, the rate of a reaction is the change in concentration of a reactant or a product over a certain period of time.

Factors influencing the Rate of Reaction

Chemical reactions only happen when the reactant molecules collide with one another. This is known as the Collision theory of Chemical Kinetics. The collision states that, in order for a reaction to happen, the reactants must collide with sufficient energy that is greater than the activation energy E_a.

But there are some factors which can influence the rate of reaction, that is, increase the fraction of molecules with energies more than activation energy E_a. There are four factors that influence the rate of a reaction. These are:

1) The Nature of the Reactants

Chemical reactions in an aqueous solution take place almost immediately. It is because the molecular bonds in the molecules of reactants are broken down. The attractive forces between the ions are broken and the ions become hydrated by the water molecules.

Additionally, most ions exert attractive forces equally in all directions. Usually, no covalent bonds need to be broken in these cases. In contrast, reactions between molecules that require covalent bonds to be broken tend to proceed very slowly. Therefore, certain structural characteristics of the reactant molecules, such as bond polarity, geometry, overall size and orientation influence the rate of reaction.

2) The Concentration of the Reactants

We know that for most reactions, the rate of reaction will increase as the concentration of the reactants increases. Increasing the concentration of a reactant means increasing the number of reactant molecules that are in that same volume.

For many reactions (not all), there is a direct relationship between the concentration and rate of reaction. Therefore, when the concentration doubles, the rate of reaction also doubles. We can explain this using the collision theory which says that if we double the number of reactant molecules, there will be twice as many collisions occurring at the same time.

3) The Temperature of the Reactants

In nearly all cases, the rate of a reaction increases with increasing temperature. Each time the temperature is increased by 10°, the rate of reaction doubles! This effect is extremely powerful. For instance, for a rise in temperature from 20° to 80°(which is six 10° increments), the rate of reaction will occur 2⁶ = 64 times faster. That is a great change in rate.

We can also explain this with the Collision theory: the average kinetic energy of all molecules is a direct function of temperature. The molecules collide with a greater energy, that is, their activation energy decreases.

We already know the dependence of the rate of reaction on temperature can be explained by Arrhenius equation which is, k = A e ^-E_a/RT where e ^-E_a/RT is the number of molecules with energies greater than the activation energy.

4) The Presence of a Catalyst

A Catalyst is a substance that increases the rate of reaction without actually participating in the reaction. Catalysts work by providing an alternate pathway for the reaction. It is the one which requires less energy for the reactants to convert into products. Some catalysts increase the rate of more than one type of reactions whereas catalyst like the enzymes in our bodies is specific to just one reaction or even one single type of reactant molecule. Therefore, a catalyst increases the rate of reaction.

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A Solved Question for You

Q: How does concentration affect the rate of reaction?

Solution: According to Collision theory, increasing the number of reactant molecules will increase the number of collisions taking place for a reaction to happen. In the same way, increasing the concentration, that is, increasing the number of reactant molecules in the solution will increase the number of collisions taking place in the solution. Thus, the rate of the reaction will increase.