2.5.7Inorganic Chemistry
Transition Metal Catalysts — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions
5 min read 5 parts Lesson 24 of 27
Part 1 of 5Why Transition Metals Catalyse
Why Transition Metals Catalyse
Variable oxidation states
- A metal can gain and lose electrons easily, so it can form temporary intermediates during a reaction.
Vacant orbitals
- Partly filled d orbitals can form weak bonds to reactant molecules at a surface.
Lower activation energy
- The catalyst offers an alternative pathway with a lower activation energy, so more molecules react at a given temperature.
Not consumed
- The catalyst is regenerated at the end, though it may be poisoned over time.
Part 2 of 5Heterogeneous Catalysis
Definition
- The catalyst is in a different phase from the reactants, usually a solid catalyst with gaseous reactants.
Contact process
- Vanadium(V) oxide catalyses SO₂ + ½O₂ → SO₃.
Haber process
- Iron catalyses N₂ + 3H₂ ⇌ 2NH₃.
Hydrogenation
- Nickel catalyses the addition of hydrogen to alkenes to make alkanes.
Catalytic converters
- Platinum, palladium and rhodium convert CO and NOₓ to CO₂ and N₂.
| Process | Catalyst | Reaction |
|---|---|---|
| Contact | V₂O₅ | SO₂ + ½O₂ → SO₃ |
| Haber | Fe | N₂ + 3H₂ ⇌ 2NH₃ |
| Margarine | Ni | C=C + H₂ → C–C |
Part 3 of 5Mechanism and Poisoning
1
Reactant molecules are adsorbed onto active sites on the catalyst surface.
2
Bonds in the reactants weaken, and the molecules are held in a position where they can react.
3
Product molecules form and are desorbed from the surface, freeing the sites.
4
Impurities such as sulfur in the Haber process and lead in catalytic converters bind to the sites permanently and poison the catalyst.
5
A large surface area, obtained with a finely divided catalyst, increases the number of active sites.
Part 4 of 5Homogeneous Catalysis
Homogeneous Catalysis
Definition
- The catalyst is in the same phase as the reactants, usually all in aqueous solution.
Peroxodisulfate
- S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂ is slow because two negative ions repel; Fe²⁺ or Fe³⁺ provides a path.
Steps
- 12Fe²⁺ + S₂O₈²⁻22Fe³⁺ + 2SO₄²⁻ and then 2Fe³⁺ + 2I⁻32Fe²⁺ + I₂
Autocatalysis
- Mn²⁺ catalyses the reaction between MnO₄⁻ and C₂O₄²⁻, which is why the reaction starts slowly and then speeds up.
Part 5 of 5Exam Checklist & Tips
2.5.7 – Key Points for the Exam
- Heterogeneous catalyst A catalyst in a different phase from the reactants.
- Homogeneous catalyst A catalyst in the same phase as the reactants.
- Adsorption Attachment of molecules to a solid surface.
- Catalyst poison A substance that binds to catalyst sites and reduces activity.
Common Mistakes
- Saying a catalyst is used up in the reaction.
- Stating that the catalyst changes the position of equilibrium.
- Mixing up homogeneous and heterogeneous definitions.
- Forgetting to mention adsorption and desorption in heterogeneous catalysis.
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