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

Oluwapelumi Kolawole

Written by: Oluwapelumi Kolawole

Reviewed by: Stewart Hird

Updated on

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

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
  1. Identify which pathway is slower with reason(s)

  2. 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

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Oluwapelumi Kolawole

Author: Oluwapelumi Kolawole

Expertise: Chemistry Content Creator

Oluwapelumi is a Pharmacist with over 15000+ hours of AP , IB, IGCSE, GCSE and A-Level chemistry tutoring experience. His love for chemistry education has seen him work with various Edtech platforms and schools across the world. He’s able to bring his communication skills as a healthcare professional in breaking down seemingly complex chemistry concepts into easily understood concepts for students.

Stewart Hird

Reviewer: Stewart Hird

Expertise: Chemistry Content Creator

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

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