Homogeneous and Heterogeneous Catalysts

Full Notes

Types of catalyst have been outlined in more detail at Heterogeneous Catalysis and Homogeneous Catalysis.
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What Is a Catalyst?

A catalyst is a substance that speeds up a chemical reaction by providing an alternative reaction pathway with a lower activation energy.

This means more collisions are effective, even at the same temperature.

Catalysts are not used up in the reaction (they may change during a reaction, however will be reformed by the end of the reaction).

Activation Energy and Boltzmann Distribution

As a catalyst lowers the activation energy (E_a), on a Boltzmann distribution graph, this shifts the E_a line to the left.

• More particles now have energy ≥ E_a.
• So the rate of reaction increases, without needing to raise temperature.
• The shape of the Boltzmann curve stays the same — only the E_a threshold moves.

Reaction Pathway Diagrams

In a reaction profile diagram:

• The catalysed pathway has a lower peak (activation energy).
• The enthalpy change (ΔH) is the same with or without the catalyst.
• The catalyst does not affect equilibrium position — only how fast it is reached.

Types of Catalysis

Heterogeneous Catalysis

Catalyst and reactants are in different phases.

Reaction happens on the surface of the catalyst.

Examples:

• Iron (Fe) in the Haber process:
  N₂ + 3H₂ ⇌ 2NH₃
  Fe provides a surface where N₂ and H₂ adsorb and react.


• Vanadium(V) oxide (V₂O₅) in the Contact process:
  SO₂ + ½O₂ → SO₃
  V₂O₅ is reduced and then re-oxidised in a cycle.

Catalyst poisoning:
Impurities (e.g. sulfur or lead) can block active sites and reduce catalyst efficiency.

Homogeneous Catalysis

Catalyst and reactants are in the same phase, usually aqueous.

Reaction proceeds via an intermediate formed with the catalyst.

Example:

• Fe²⁺ catalyses the reaction between I⁻ and S₂O₈²⁻:
S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺
2Fe³⁺ + 2I⁻ → I₂ + 2Fe²⁺
• Fe²⁺ is regenerated, so it remains a catalyst.

Summary

• Catalysts speed up reactions by lowering activation energy (E_a).
• On a Boltzmann distribution, a catalyst moves the E_a line left → more particles exceed it.
• Reaction profile: lower peak for catalysed pathway; ΔH unchanged; equilibrium position unchanged.
• Heterogeneous catalysis: catalyst in different phase to reactants and reaction occurs at catalyst surface; examples include Fe in Haber process and V₂O₅ in Contact process; can suffer catalyst poisoning.
• Homogeneous catalysis: catalyst in same phase as reactants and reaction involves intermediates; example Fe²⁺ in I⁻/S₂O₈²⁻ reaction.
• Catalysts are reformed at the end and not chemically changed overall.