The Effect of a Catalyst
Chemistry: IB Diploma Higher Level, topic 5.2.3
Catalysis
A catalyst is a substance that can increase the reaction rate without being consumed in the reaction.
Catalysts
- Catalysts increase the rate of reactions without being used up in the reaction themselves.
- Catalysts can work by binding to reactants or covalently bonding to reactants.
- Catalysts do this by increasing the number of successful collisions between reactants and/or providing an alternate route for the reaction to follow.
- The alternate route will have a lower activation energy than the original route.
- This can be demonstrated on energy profiles and Maxwell-Boltzmann distributions.
Reaction energy profile
Enthalpy profile explained
- Here you can see both reaction pathways.
- The catalytic pathway has a lower hump.
- The hump represents the activation energy.
- So the catalytic pathway has a lower activation energy.
Regeneration
- A lot of catalysts work by changing oxidation state.
- In one step of the reaction, a catalyst may be reduced, only to be oxidized and regenerated in a subsequent step.
- The net concentration of a catalyst remains constant throught the reaction.
- Catalysts are typically consumed during on the rate-determining (slowest) step of a reaction mechanism and regenerated thereafter.
Maxwell-Boltzmann graph
Maxwell-Boltzmann graph explained
- Here you can see the different activation energies for each pathway.
- Some molecules can use the old pathway.
- Some molecules can use the new pathway but not the old pathway.
- Some molecules can't use any.
- Overall, the catalytic pathway can be used by more molecules.
- More molecules have enough energy to react via the catalyzed pathway.
Test yourself on The Effect of a Catalyst
How is the hump in the catalyzed energy profile different to the uncatalysed energy profile?
In the Maxwell-Boltzmann distribution, where will the catalyzed activation energy line lie relative to the uncatalysed activation energy line?
Which is true for the catalyzed reaction?
Which of the following is NOT true for catalyzed reactions?