Catalysts & Reaction Rate (College Board AP® Chemistry)
Study Guide
Written by: Oluwapelumi Kolawole
Reviewed by: Stewart Hird
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, Ea
Hence a catalyst may affect the rate of a reaction by altering the value of A or Ea
A catalyst provides an alternative mechanism with a lower Ea 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
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.
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
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
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