Homogeneous and Heterogeneous Catalysts

Specification Reference Physical Chemistry, Kinetics 26.2

Quick Notes

Catalysts increase the rate of a reaction by lowering the activation energy, providing an alternative reaction pathway.
Heterogeneous catalysts are in a different phase to the reactants (usually solid with gaseous or liquid reactants).
Homogeneous catalysts are in the same phase as the reactants (often all aqueous).
In heterogeneous catalysis, the reaction occurs on the catalyst’s surface through adsorption, reaction, and desorption.
In homogeneous catalysis, the catalyst forms an intermediate and is regenerated in a later step.
Examples:
- Heterogeneous: Fe in the Haber process, Pt/Pd/Rh in catalytic converters.
- Homogeneous: NO/NO₂ in SO₂ oxidation; Fe²⁺ in the I⁻/S₂O₈²⁻ reaction.

Full Notes

Types of catalyst have been outlined in more detail here and here.
This page is just what you need to know for CIE A-level Chemistry :)

Catalysts are substances that increase the rate of a reaction without being used up in the process. They achieve this by offering an alternative pathway with lower activation energy, allowing more reactant particles to successfully collide and react.

There are two types of catalyst - heterogeneous and homogeneous.

Heterogeneous Catalysts

Heterogeneous catalysts are in a different phase to the reactants, typically solid catalysts used in gas-phase or liquid-phase reactions.

How They Work:

Reactants are adsorbed onto the surface of the catalyst.
Bonds within the reactant molecules are weakened, making them easier to break.
New bonds form between reactants.
Products are desorbed (released) from the surface, freeing it for further reactions.

Examples:

Iron in the Haber Process:
N₂ + 3H₂ ⇌ 2NH₃
Iron provides a surface where nitrogen and hydrogen molecules are adsorbed and can react more easily.

Platinum/Palladium/Rhodium in catalytic converters:
Convert NO_x and CO in car exhausts into N₂ and CO₂. These reactions occur on the surface of the metal catalysts.

Catalyst Poisoning: Substances like lead or sulfur can block active sites on the catalyst, reducing its efficiency. Once poisoned, the catalyst becomes less effective and may need to be replaced.

Homogeneous Catalysts

Homogeneous catalysts are in the same phase as the reactants, usually aqueous solutions.

CIE A-Level Chemistry diagram of homogeneous catalyst forming intermediates in solution.

How They Work:

The catalyst forms an intermediate with one or more reactants.
The intermediate reacts further to produce the products.
The catalyst is regenerated by the end of the process.

Examples:

Fe²⁺ in the reaction between I⁻ and S₂O₈²⁻:
Without a catalyst, the reaction is slow due to repulsion between negative ions. Fe²⁺ facilitates the reaction by providing a two-step route involving Fe³⁺.

CIE A-Level Chemistry mechanism of Fe2+ catalysing the reaction between iodide and peroxodisulfate ions.

NO and NO₂ in the oxidation of SO₂ to SO₃:
In atmospheric chemistry, NO acts as a catalyst:
- NO + ½O₂ → NO₂
- NO₂ + SO₂ → SO₃ + NO
NO is regenerated, making it a homogeneous catalyst.

Summary

Type of Catalyst	Phase	How It Works	Examples
Heterogeneous	Different phase to reactants (usually solid with gases/liquids)	Adsorption, Reaction, Desorption	Fe in Haber process and Pt/Pd/Rh in catalytic converters
Homogeneous	Same phase as reactants (often aqueous)	Forms intermediate then Intermediate reacts then catalyst regenerated	Fe²⁺ in I⁻/S₂O₈²⁻ reaction and NO/NO₂ in SO₂ oxidation