Transition Metals as Homogeneous Catalysts

Transition metals can function as both homogeneous and heterogeneous catalysts.

Let's focus on homogeneous catalysis in this video created by Chemistry Guru, Singapore's leading JC Chemistry tuition centre.

In homogeneous catalysis, the catalyst is in the same physical state as reactants.

Transition metals can act as homogeneous catalysts due to their variable oxidation states.

This allows the transition metal to act as a medium to transfer electrons between reactants.

Our example is the reaction between S₂O₈^2- and I^- via the following equation:

S₂O₈^2-(aq) + 2I^-(aq) → 2SO₄^2-(aq) + I₂(aq)

Both reactants are negatively charged and they repel each other.

Activation energy for the reaction is high hence reaction is slow or kinetically less feasible.

This reaction can be catalysed by either Fe²⁺(aq) or Fe³⁺(aq).

To deduce the mechanism for catalysed reaction, it's useful to list down the half equations and E values for the reactants and catalyst.

Notice the 2 points that are important to chose a suitable catalyst:

1. Species on both sides of half equation for catalyst are cations (usually M³⁺ and M²⁺) so they can attract the negative reactants.

2. E value for catalyst is between E values for reactants. In this case E(Fe³⁺/Fe²⁺) = +0.77V is between E(S₂O₈^2-/SO₄^2-) = +2.01V and E(I₂/I^-) = +0.54V.

In step 1, catalyst Fe²⁺ is oxidised to Fe³⁺ and we pair this up with reduction of S₂O₈^2-.

We can balance the equation and show E_cell for step 1 is positive hence feasible.

In step 2, Fe³⁺ is reduced, Fe²⁺ is regenerated and we pair this up with oxidation of I^-.

Again we can show that E_cell for step 2 is positive and feasible.

The Fe²⁺ changes the mechanism since it interferes with the reaction directly.

Hence the number of steps in the mechanism will increase but activation energy is lowered and reaction is faster.

Topic: Transition Elements, Inorganic Chemistry, A Level Chemistry, Singapore

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