Transition Metals - Transition Metals as Homogeneous Catalysts (A-Level Chemistry)

Transition Metals as Homogeneous Catalysts

Homogeneous Catalysts

Homogeneous catalysts are in the same phase as the reactants.

The catalyst forms an intermediate with the reactant with a lower activation energy.

The intermediate often has a different oxidation state to the original transition metal. At the end of the reaction the original oxidation state will reoccur.

Transition metals are good homogeneous catalysts because they can change oxidation states.

Reaction Between Iodide (I⁻ ) and Persulfate (S₂O₈²⁻) Ions

The uncatalysed reaction between iodide ions and persulfate ions is very slow because it involves the reaction between two negative ions. Repulsion between the ions causes a high activation energy.

The reaction is catalysed by Fe²⁺ ions.

In the first stage the Fe²⁺ ions are oxidised to Fe³⁺ ions as they react with persulfate ions, which are reduced to sulfate ions.

Fe²⁺ (aq) + S₂O₈²⁻(aq) → Fe³⁺ (aq) + SO₄²⁻(aq)

Fe³⁺ ions are reduced by iodide ions, to reform Fe²⁺ ions. Iodide ions are oxidised to iodine.

Fe³⁺(aq) + 2I⁻ (aq) → Fe²⁺(aq) + I₂

The overall reaction is:

S₂O₈²⁻(aq) + 2I⁻ (aq) → SO₄²⁻(aq) + I₂

The Fe²⁺ ions are reformed to be used again.

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