26.2 Homogeneous and Heterogeneous Catalysts

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Explain that catalysts can be homogeneous or heterogeneous.
  2. Describe how a heterogeneous catalyst works — including adsorption of reactants, bond weakening, and desorption of products — using the examples of:
    • (a) Iron in the Haber process
    • (b) Palladium, platinum, and rhodium in the catalytic removal of nitrogen oxides from car exhaust gases
  3. Describe how a homogeneous catalyst works — being used in one step and reformed in a later step — using the examples of:
    • (a) Atmospheric oxides of nitrogen in the oxidation of atmospheric sulfur dioxide
    • (b) Fe²⁺ or Fe³⁺ ions in the I⁻/S₂O₈²⁻ reaction

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1. What Is a Catalyst?

A catalyst is a substance that speeds up a chemical reaction without being permanently used up. It does this by providing an alternative reaction pathway — think of it like a shortcut through a mountain. The shortcut has a lower hill to climb, which means the reaction needs less energy to get started. This "energy needed to start the reaction" is called the activation energy.

Because the catalyst provides a pathway with a lower activation energy, more reactant particles can successfully collide and react, so the reaction goes faster.

2. Two Types of Catalyst: Homogeneous and Heterogeneous

Catalysts are divided into two groups depending on whether they are in the same state as the reactants or a different state.

State (also called phase) refers to whether a substance is solid, liquid, or gas, or dissolved in water (aqueous). For example, a dissolved substance and water are both in the "aqueous" phase.

2.1 Homogeneous Catalysts

A homogeneous catalyst is one that is in the same phase (state) as the reactants and products.

  • For example, if the reactants are dissolved in water (aqueous), a homogeneous catalyst is also dissolved in water.
  • Because everything is mixed together in the same phase, the catalyst can interact directly with the reactant molecules throughout the solution.

Simple example to picture it: Imagine stirring sugar into a glass of water — both the sugar and the water are in the same liquid phase. A homogeneous catalyst works in a similar "mixed together" way.

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