Homogeneous and heterogeneous catalysts

Chemistry A2 · Reaction kinetics. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A catalyst speeds up a reaction by giving a different route with a lower activation energy. It is not used up.

A heterogeneous catalyst is in a different phase from the reactants, usually a solid with gases. The reactant molecules stick to its surface (adsorption), their bonds are weakened, they react, and the products leave the surface (desorption).

A homogeneous catalyst is in the same phase as the reactants. It reacts in one step to form an intermediate and is made again in a later step. Transition metal ions do this well because they can change oxidation state easily.

Rules to remember

Heterogeneous sequence: adsorption of reactants → bonds weakened → reaction → desorption of products.
Haber process: solid iron with N₂(g) and H₂(g). Adsorption weakens the N≡N and H−H bonds.
Catalytic converter: Pt, Pd and Rh on a ceramic honeycomb; 2NO + 2CO → N₂ + 2CO₂.
Adsorption must be strong enough to weaken bonds but weak enough for the products to leave the surface.
Homogeneous, in the atmosphere: SO₂ + NO₂ → SO₃ + NO, then NO + ½O₂ → NO₂.
Homogeneous, in solution: S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺, then 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂.
The uncatalysed S₂O₈²⁻/I⁻ reaction is slow because two negative ions repel each other.

Common mistake

Students write absorption instead of adsorption. Adsorption means the molecules stick to the surface of the catalyst; they do not go inside it.

Worked example

Fe³⁺(aq) is added, instead of Fe²⁺(aq), to a mixture of S₂O₈²⁻(aq) and I⁻(aq). Show that it still acts as a catalyst.

Fe³⁺ first reacts with the oppositely charged iodide ion: 2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂.
The Fe²⁺ formed then reacts with peroxodisulfate: S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺.
Adding the two steps cancels the iron ions: S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂.
Fe³⁺ is used in the first step and re-formed in the second, and all species are aqueous.

Answer: Fe³⁺ is a homogeneous catalyst: the same two steps happen in the opposite order and Fe³⁺ is regenerated.

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