Transition metals as catalystsAQA A-Level Chemistry: Revision notes
Section 1
Why transition metals make good catalysts
A catalyst speeds up a reaction by providing an alternative route with a lower activation energy, and is chemically unchanged at the end. Transition metals and their compounds are effective catalysts because they have variable oxidation states. They can gain and lose electrons easily, so they form intermediate species and are then regenerated. Catalysts can be heterogeneous or homogeneous.
A catalyst does not change the position of equilibrium or the enthalpy change.
Section 2
Heterogeneous catalysts
A heterogeneous catalyst is in a different phase from the reactants, usually a solid with gaseous or liquid reactants. The reaction occurs at active sites on the surface: reactants are adsorbed, react, and the products are released. A support medium such as silica or an inert ceramic is used to spread out the catalyst. This maximises the surface area and minimises the cost, because less of the expensive metal is needed.
Examples: Fe in the Haber process, N₂ + 3H₂ ⇌ 2NH₃, and V₂O₅ in the Contact process.
Section 3
Catalyst poisoning
Impurities in the feed can bind to the active sites and block them. This is catalyst poisoning. Fewer reactant molecules can be adsorbed, so the efficiency of the catalyst falls and the rate of reaction drops. An example is sulfur compounds poisoning iron in the Haber process. The cost implication is that the feed gases must be purified, or the catalyst must be regenerated or replaced.
Poisoning does not change the equilibrium position. It lowers the rate by reducing the number of available active sites.
Section 4
V₂O₅ in the Contact process
In the Contact process, SO₂ is oxidised to SO₃ using V₂O₅. The mechanism has two steps, and the vanadium changes oxidation state:
SO₂ + V₂O₅ → SO₃ + V₂O₄ (V reduced from +5 to +4)
2V₂O₄ + O₂ → 2V₂O₅ (V oxidised from +4 to +5)
The overall reaction is SO₂ + ½O₂ → SO₃, and V₂O₅ is regenerated. Variable oxidation states make this possible.
Write both steps and show V₂O₅ regenerated at the end.
Section 5
Homogeneous catalysts
A homogeneous catalyst is in the same phase as the reactants, usually aqueous ions. The reaction goes through an intermediate species.
Fe²⁺ catalysing S₂O₈²⁻ + 2I⁻ → 2SO₄²⁻ + I₂: the uncatalysed reaction is slow because both ions are negative and repel each other. The catalysed route is:
S₂O₈²⁻ + 2Fe²⁺ → 2SO₄²⁻ + 2Fe³⁺
2Fe³⁺ + 2I⁻ → 2Fe²⁺ + I₂
Each step involves oppositely charged ions, so there is no repulsion and the activation energy is lower. Fe²⁺ is regenerated.
Section 6
Autocatalysis: Mn²⁺
In the reaction 2MnO₄⁻ + 16H⁺ + 5C₂O₄²⁻ → 2Mn²⁺ + 8H₂O + 10CO₂, MnO₄⁻ and C₂O₄²⁻ are both negative, so the reaction is slow at first. The product Mn²⁺ catalyses the reaction, so it is autocatalytic. The steps are:
4Mn²⁺ + MnO₄⁻ + 8H⁺ → 5Mn³⁺ + 4H₂O
2Mn³⁺ + C₂O₄²⁻ → 2Mn²⁺ + 2CO₂
As Mn²⁺ builds up, the rate increases. In a titration, the first drops decolourise slowly, then very quickly.
The reaction starts slowly because there is no Mn²⁺ yet, not because of a faulty burette.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Transition metals as catalysts
- An ammonia plant makes ammonia by the Haber process. A gaseous mixture of nitrogen and hydrogen at 450 °C and 200 atm is passed over a bed of solid iron. The plant manager reports that, whenever the feed gas is contaminated with traces of sulfur compounds, the output of ammonia falls.Explain why catalyst poisoning has a cost implication for the plant.2 marks
- Sulfuric acid is manufactured by the Contact process. Sulfur dioxide and oxygen are passed at 450 °C over vanadium(V) oxide, V₂O₅, which is spread thinly on the surface of inert silica pellets. The reaction is: SO₂(g) + ½O₂(g) ⇌ SO₃(g).Write two equations to show how V₂O₅ acts as a catalyst in this reaction.2 marks
- In aqueous solution, iodide ions react with peroxodisulfate ions: S₂O₈²⁻(aq) + 2I⁻(aq) → 2SO₄²⁻(aq) + I₂(aq). A technician finds that the reaction is very slow when the two solutions are mixed, but speeds up greatly when a few drops of iron(II) sulfate solution are added.Explain, with reference to the ions involved, why the uncatalysed reaction is slow.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).