Transition metals as catalystsEdexcel International A Level Chemistry: Revision notes

A catalyst increases the rate by providing an alternative route with a lower activation energy and is regenerated at the end of the reaction. Transition metals and their compounds are good catalysts because they can form bonds with reactants and have variable oxidation states.

A heterogeneous catalyst is in a different phase from the reactants, and the reaction takes place at its surface. A homogeneous catalyst is in the same phase as the reactants, and the reaction proceeds via an intermediate species.

In the Contact process solid V₂O₅ catalyses SO₂(g) + ½O₂(g) ⇌ SO₃(g), so this is heterogeneous catalysis. The mechanism uses the change of oxidation number of vanadium:

SO₂ + V₂O₅ → SO₃ + V₂O₄ (V reduced from +5 to +4)

V₂O₄ + ½O₂ → V₂O₅ (V oxidised from +4 to +5)

The catalyst is regenerated, so it appears in neither the overall equation nor the final products.

A catalytic converter uses platinum and rhodium spread thinly over a ceramic honeycomb, giving a large surface area from a small mass of metal. It converts CO and NO into harmless gases:

2CO + 2NO → 2CO₂ + N₂

1. CO and NO molecules are adsorbed onto the metal surface, which weakens the bonds within them.
2. The weakened molecules react on the surface.
3. CO₂ and N₂ are desorbed, freeing the surface for more reactant.

S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂ is slow because both reactants are negative ions that repel each other. Fe²⁺ or Fe³⁺ ions in the same aqueous phase give an alternative route in which each step is between oppositely charged ions:

S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺

2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂

The intermediate Fe³⁺ is formed in step 1 and used in step 2, and Fe²⁺ is regenerated. Iron works because it has two stable oxidation states.

2MnO₄⁻ + 16H⁺ + 5C₂O₄²⁻ → 2Mn²⁺ + 8H₂O + 10CO₂

The first drops of purple MnO₄⁻ decolourise slowly because MnO₄⁻ and C₂O₄²⁻ are both negative. The reaction makes Mn²⁺, and Mn²⁺ is a catalyst for the reaction, so it is autocatalytic: the rate increases as the reaction proceeds.

MnO₄⁻ + 4Mn²⁺ + 8H⁺ → 5Mn³⁺ + 4H₂O

2Mn³⁺ + C₂O₄²⁻ → 2Mn²⁺ + 2CO₂

Adding a little Mn²⁺ at the start removes the slow initial stage but does not change the volume needed in the titration.