8.3 Homogeneous and heterogeneous catalysts
- Syllabus
- 9701–2028–2029
- Topic
- 8.3
- Level
- AS
Learning objectives
A catalyst changes the mechanism and lowers the activation-energy threshold
A catalyst increases the rate of a reaction without being chemically changed overall. Catalysis occurs because the catalyst provides a different reaction mechanism with a lower activation energy, Eₐ.
| Catalyst type | Phase relationship | Typical mechanism location |
|---|---|---|
| homogeneous | catalyst and reactants are in the same phase | intermediate species form within the shared phase |
| heterogeneous | catalyst is in a different phase from the reactants | reaction occurs at active sites on the catalyst surface |
At the same temperature, the Boltzmann distribution is unchanged. Draw the catalysed Eₐ line to the left of the uncatalysed Eₐ line: the area to the right of the lower threshold is larger, so a greater fraction of collisions has sufficient energy and effective collisions occur more frequently.
| Reaction-pathway feature | Uncatalysed | Catalysed |
|---|---|---|
| reactant enthalpy level | same | same |
| product enthalpy level | same | same |
| peak / forward Eₐ | higher | lower for the alternative mechanism |
| ΔH from reactants to products | same | same |
On one enthalpy-versus-reaction-progress diagram, use common reactant and product levels and draw two labelled pathways. Measure each forward Eₐ from the reactant level to its own peak; the catalysed peak must be lower, while the product–reactant difference remains unchanged.
A catalyst does not raise particle energies, shift the Boltzmann distribution, change ΔH, or alter the equilibrium constant or equilibrium composition. It is regenerated overall even though it may form intermediates during the mechanism.