Homogeneous and heterogeneous catalysts — practice questions

The uncatalysed reaction is slow because it involves the collision of two negatively charged ions (S₂O₈²⁻ and I⁻), which repel each other.

The Fe²⁺(aq) ion acts as a homogeneous catalyst by providing a two-step pathway where each step involves a collision between oppositely charged ions, which is more favourable.

Step 1: The Fe²⁺ ion is oxidised to Fe³⁺ by the peroxodisulfate(VI) ion. S₂O₈²⁻(aq) + 2Fe²⁺(aq) → 2SO₄²⁻(aq) + 2Fe³⁺(aq)

Step 2: The Fe³⁺ ion formed then oxidises the iodide ions to iodine, regenerating the original Fe²⁺ catalyst. 2Fe³⁺(aq) + 2I⁻(aq) → 2Fe²⁺(aq) + I₂(aq)

Conclusion: The Fe²⁺ ion is regenerated at the end of the reaction, fulfilling its role as a catalyst. It provides an alternative pathway with lower activation energy.

Vanadium(V) oxide acts as a heterogeneous catalyst because it is a solid, while the reactants (SO₂ and O₂) are gases.

The mechanism involves the V₂O₅ being reduced and then re-oxidised.

Step 1: Adsorption and Reaction Sulfur dioxide gas is adsorbed onto the surface of the V₂O₅ catalyst. It reacts with the catalyst, reducing the vanadium from the +5 oxidation state to the +4 oxidation state and forming sulfur trioxide. SO₂(g) + V₂O₅(s) → SO₃(g) + V₂O₄(s) Alternatively, this is often shown with ions: 2V⁵⁺ + O²⁻ + SO₂ → 2V⁴⁺ + SO₃

Step 2: Desorption and Regeneration The sulfur trioxide product desorbs from the surface. The reduced catalyst, V₂O₄, then reacts with oxygen from the air to regenerate the V₂O₅ catalyst. The vanadium is re-oxidised to its +5 state. V₂O₄(s) + ½O₂(g) → V₂O₅(s)

Overall: The V₂O₅ is not consumed. It provides a surface and an alternative pathway involving changes in its own oxidation state to facilitate the conversion of SO₂ to SO₃.