Transition Metal Catalysis

Transition metals and their compounds are among the most important catalysts in chemistry. This lesson covers homogeneous and heterogeneous catalysis, key examples from the AQA specification (3.2.5), and autocatalysis.

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Why Transition Metals Are Good Catalysts

Transition metals can act as catalysts because:

1. Variable oxidation states: They can accept and donate electrons, cycling between oxidation states during a reaction. This provides an alternative reaction pathway with a lower activation energy.
2. Ability to form complexes: Reactant molecules can bond to the metal surface or to the metal ion, bringing reactants into close proximity and the correct orientation.
3. Availability of 3d and 4s electrons: These electrons can be used to form temporary bonds with reactant molecules.

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Homogeneous Catalysis

Key Definition: A homogeneous catalyst is in the same phase (state) as the reactants. In A-Level chemistry, this typically means all reactants and the catalyst are in aqueous solution.

Mechanism

The catalyst reacts with one of the reactants, forming an intermediate. It then reacts with the second reactant, regenerating the original catalyst. The catalyst provides a two-step pathway, each with a lower activation energy than the uncatalysed single-step reaction.

Key Example: Fe²⁺/Fe³⁺ Catalysis of S₂O₈²⁻ with I⁻

The reaction between peroxodisulfate ions and iodide ions is very slow because both reactants are negatively charged (electrostatic repulsion):

Overall reaction: S₂O₈²⁻(aq) + 2I⁻(aq) → 2SO₄²⁻(aq) + I₂(aq)

This is slow because both ions are negative — they repel each other and collisions with enough energy in the correct orientation are rare.

Catalysed mechanism (using Fe²⁺):

Step 1: Fe²⁺ is oxidised by S₂O₈²⁻: S₂O₈²⁻(aq) + 2Fe²⁺(aq) → 2SO₄²⁻(aq) + 2Fe³⁺(aq)

• Fe²⁺ is oxidised to Fe³⁺ (iron changes from +2 to +3).
• The S₂O₈²⁻ reacts with a positive ion instead of a negative one → lower activation energy.

Step 2: Fe³⁺ is reduced by I⁻: 2Fe³⁺(aq) + 2I⁻(aq) → 2Fe²⁺(aq) + I₂(aq)

• Fe³⁺ is reduced back to Fe²⁺ (iron changes from +3 to +2).
• The catalyst is regenerated.
• I⁻ reacts with a positive ion instead of a negative one → lower activation energy.

Overall: Adding Steps 1 and 2 gives the original overall equation. Fe²⁺ acts as a catalyst because it is regenerated at the end and provides an alternative pathway.

Exam Tip: When explaining homogeneous catalysis, you MUST show both steps and demonstrate that the catalyst is regenerated. Always state that the alternative pathway has a lower activation energy.

Energy Profile

The uncatalysed reaction has one large activation energy barrier. The catalysed reaction has two smaller barriers (one for each step), with an intermediate state between them. Both pathways have the same overall enthalpy change.

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Heterogeneous Catalysis

Key Definition: A heterogeneous catalyst is in a different phase from the reactants. Typically, the catalyst is a solid and the reactants are gases or liquids.

Mechanism

1. Adsorption: Reactant molecules diffuse to the catalyst surface and are adsorbed. They form temporary bonds with the surface atoms (using d electrons).
2. Weakening of bonds: The bonds within the reactant molecules are weakened because electron density is drawn towards the catalyst surface.
3. Reaction: Reactant molecules are held in close proximity and in the correct orientation on the surface, allowing reaction to occur with lower activation energy.
4. Desorption: Product molecules leave the catalyst surface, freeing active sites for more reactant molecules.

Key Term: Adsorption (not absorption) means molecules bonding to the surface. Absorption means molecules entering the bulk of a material.

Key Example 1: V₂O₅ in the Contact Process

The Contact process manufactures sulfuric acid. The key catalysed step is:

2SO₂(g) + O₂(g) ⇌ 2SO₃(g) ΔH = −197 kJ mol⁻¹

Catalyst: Vanadium(V) oxide, V₂O₅, at 450 °C.

Mechanism (showing variable oxidation states):

Step 1: V₂O₅ oxidises SO₂: SO₂(g) + V₂O₅(s) → SO₃(g) + V₂O₄(s)

• Vanadium is reduced from +5 to +4.

Step 2: V₂O₄ is re-oxidised by O₂: V₂O₄(s) + ½O₂(g) → V₂O₅(s)

• Vanadium is oxidised from +4 back to +5.
• The catalyst is regenerated.

Note: Although V₂O₅ is a heterogeneous catalyst (solid with gaseous reactants), its mechanism involves changes in oxidation state similar to homogeneous catalysis. Some textbooks refer to this as having features of both types.

Key Example 2: Iron in the Haber Process

N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ mol⁻¹

Catalyst: Finely divided iron at 450 °C and 200 atm.