Catalysts & Reaction Rate (College Board AP® Chemistry): Study Guide

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 with the uncatalyzed 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

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

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

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