Catalysts & Reaction Rate

Catalysis describes the process by which the rate of a reaction is increased by the addition of a substance known as a catalyst
- A catalyst is a substance that increases the rate of a reaction without being consumed by it
The effect of a catalyst is seen in its ability to reduce the activation energy required for a reaction

How Does a Catalyst Work?

Considering the general form of rate laws, rate = k[A]^x[B]^y, we can conclude that a catalyst affects the numerical value of k, the rate constant
According to the Arrhenius equation, k = Ae^-Ea/RT, the rate constant, k, is determined by the frequency factor, A, and the activation energy, E_a
Hence a catalyst may affect the rate of a reaction by altering the value of A or E_a
- A catalyst provides an alternative mechanism with a lower E_a compared to the uncatalysed reaction
- It could also help with the orientation of the reacting molecules, thereby increasing the frequency factor, A
The more dramatic effect of the catalyst involves the provision of an alternative reaction mechanism with lower activation energy

Effect of Catalyst on Activation Energy

effect-of-catalyst-on-activation-energy

The diagram shows that a catalyst allows the reaction to take place through a different mechanism, which has a lower activation energy than the original reaction

It is important to note that a catalyst has no effect on:
- The overall energy change of the reaction
- The kinetic energy of the reacting molecules
- The yield of the products

Worked example

The following graph shows two different reaction pathways, X and Y, for the same overall reaction at the same temperature.

energy-profiles-catalysis-worked-example-question

Identify which pathway is slower with reason(s)
How can there be two different reaction pathways for the same reaction at the same temperature?

Answers:

Answer a)

Pathway Y is slower because it has a higher activation energy

Answer b)

The difference in the reaction pathway indicates the presence of a catalyst
The catalyst speeds up the rate of the reaction by providing an alternative pathway X with lower activation energy

Catalysts & Rate-Determining Steps

Catalysts speed up reaction rates by providing an alternative mechanism with lower activation energy
In some instances, the catalyzed mechanism may include an additional reaction step
- Consider a simple, one-step, bimolecular reaction:

A + B → AB

A possible mechanism for the catalyzed reaction is shown below:

A + catalyst → A—catalyst (step 1)

A—catalyst + B → AB + catalyst (step 2)

Overall reaction: A + B → AB

From this proposed mechanism, we can see that:
- The chemical species A—catalyst is an intermediate
  - It is produced in step 1 and consumed in step 2
- The catalyst is regenerated in the reaction
  - It appears as a reactant in step 1 and as a product in step 2
Hence in a reaction mechanism, the overall concentration of the catalyst is constant
- The catalyst may change during the overall reaction
- It is typically involved in the rate-determining step
- It is regenerated unchanged at the end of the overall reaction
The effect of a catalyst can be shown on an energy profile diagram as follows:

Catalyst and Reaction Mechanisms

catalyst-and-reaction-mechanisms

A catalyst lowers the activation energy of a reaction by providing an alternative mechanism. A catalyst also increases the rate of the reverse reaction

Note that a catalyst decreases the activation energy for both the forward and the reverse reactions
- Hence, it speeds up the rate of both reactions and consequently allows the system to reach equilibrium quickly
Therefore, a catalyst has no effect on the position of equilibrium for a reaction

Catalysts & Reaction Rate (College Board AP Chemistry)

Revision Note