Homogeneous and Heterogeneous Catalysts

· Catalyst = a substance that increases the rate of reaction and is chemically unchanged at the end.
· Catalysis = the process where a catalyst speeds up a reaction.
· A catalyst provides a different reaction mechanism with a lower activation energy, Eₐ.
· The catalyst does not change the reactants, products, or overall enthalpy change, ΔH.
· A catalyst allows the reaction to happen faster because more particles have enough energy for effective collisions.

How catalysts lower activation energy

· Without a catalyst, particles must overcome a higher activation energy barrier.
· With a catalyst, the reaction follows an alternative pathway with lower Eₐ.
· Lower Eₐ means a greater proportion of collisions are effective at the same temperature.
· The catalyst is involved in the reaction mechanism but is regenerated by the end.
· In exam answers, always say: “a catalyst provides an alternative route with lower activation energy.”

The diagram shows that the catalysed pathway has a lower activation energy than the uncatalysed pathway. The reactants and products are at the same energy levels, so the catalyst changes the rate, not the overall energy change. Source

Catalysts and the Boltzmann distribution

· A Boltzmann distribution shows the spread of molecular energies in a sample.
· Only particles with energy equal to or greater than Eₐ can react successfully.
· A catalyst lowers Eₐ, so the line for activation energy moves left on the distribution.
· This increases the area under the curve to the right of Eₐ.
· Therefore, a larger fraction of particles have enough energy for effective collisions.

The diagram shows that a catalyst lowers the activation energy, increasing the number of particles with sufficient energy to react. The important exam idea is the larger area beyond Eₐ, which means more effective collisions per unit time. Source

Reaction pathway diagrams with and without a catalyst

· A reaction pathway diagram plots energy against progress of reaction.
· The peak represents the activation energy barrier.
· The catalysed reaction has a lower peak than the uncatalysed reaction.
· The starting and finishing energy levels are unchanged, so ΔH is unchanged.
· A catalysed pathway may have more than one step, but the highest barrier is lower.

Homogeneous vs heterogeneous catalysts

· Homogeneous catalyst = catalyst in the same phase as the reactants.
· Heterogeneous catalyst = catalyst in a different phase from the reactants.
· Both types still work by providing an alternative reaction pathway with lower Eₐ.
· Do not confuse lower activation energy with higher particle energy: the catalyst lowers the barrier; it does not heat the particles.
· In 8.3 questions, the key focus is usually the effect on mechanism, Eₐ, Boltzmann distribution, and pathway diagrams.

This diagram illustrates a heterogeneous catalyst, where reactants interact with a solid catalyst surface. It helps students visualise how reactants can follow a different pathway at the surface, leading to a lower activation energy. Source

Common exam wording

· “A catalyst increases rate by providing an alternative reaction pathway with lower activation energy.”
· “More particles have energy greater than or equal to Eₐ, so there are more effective collisions.”
· “The catalyst is chemically unchanged at the end of the reaction.”
· “The catalysed and uncatalysed reactions have the same reactants and products.”
· “The value of ΔH is unchanged because the initial and final energy levels are unchanged.”

Common mistakes to avoid

· Do not say a catalyst increases the energy of particles.
· Do not say a catalyst is used up.
· Do not say a catalyst changes ΔH or the products formed.
· Do not draw the catalysed pathway starting or ending at different energy levels.
· Do not forget to link lower Eₐ to more effective collisions.