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