AQA A Level Chemistry

Revision Notes

6.2.7 Catalysis

Catalysis

Transition metals and their compounds can act as heterogeneous and homogeneous catalysts. A heterogeneous catalyst is in a different phase from the reactants and the reaction occurs at active sites on the surface. A homogeneous catalyst is in the same phase as the reactants. When catalysts and reactants are in the same phase, the reaction proceeds through an intermediate species. V₂O₅ acts as a heterogeneous catalyst in the Contact Process. Students should be able to: • explain the importance of variable oxidation states in catalysis

  • Transition elements and their compounds are often used as heterogeneous and homogeneous catalysts
  • A heterogenous catalyst is in a different physical state from the reactants
    • For example, vanadium(V)oxide, V2O5, is a solid catalyst used in the Contact process for making sulfuric acid
    • Another example is the use of solid iron in the Haber process for making ammonia
    • The reaction occurs at active sites on the surface of the catalyst

Reaction Kinetics - Iron Catalyst (1), downloadable AS & A Level Chemistry revision notes

Reaction Kinetics - Iron Catalyst (2), downloadable AS & A Level Chemistry revision notes

Heterogeneous catalysis takes place at active sites on the surface of the transition metal catalyst

  • A homogeneous catalyst is in the same physical state as the reactants
    • An example of a homogeneous catalyst is the role of iron(II) ions, Fe2+, in the reaction between iodide ions, I, and peroxydisulfate ions, S2O82-
  • Transition elements are often used as catalysts due to their ability to form ions with more than one stable oxidation state, and the fact that they contain vacant d orbitals

Oxidation states

  • Transition element ions can adopt more than one stable oxidation state
  • This means that they can accept and lose electrons easily to go from one oxidation state to another
  • They can therefore catalyse redox reactions, by acting as both oxidising agents and reducing agents
  • For example, iron (Fe) is often used as a catalyst due to its ability to form Fe(II) and Fe(III) ions, acting as an oxidising agent and a reducing agent
    • When Fe(II) acts as a reducing agent, it will reduce another species and become oxidised itself

Fe2+ → Fe3+ + e

  • The Fe3+ formed in the catalytic cycle, can then also act as an oxidising agent by oxidising another species and getting reduced itself to reform the Fe2+ ion

Fe3+ + e → Fe2+

  • Transition element ions with high oxidation states make powerful oxidising agents, because they will readily accept electrons
    • A common example of this is potassium permanganate (VII), where manganese has an oxidation state of +7

Vacant d-orbitals

  • When transition elements form ions, they have vacant d orbitals which are energetically accessible
    • The orbitals are not too high in energy
  • This means that dative bonds can be formed between the transition element ion and ligands
    • Each ligand provides the pair of electrons required for the formation of a bond between the ion and the ligand
    • This pair of electrons is donated into the ion’s vacant d orbital
  • The table below shows the electron configuration of the transition element atoms
  • When they form ions, empty d orbitals are obtained which can be filled by the pairs of electrons donated by the ligands

Electronic configuration of transition elements table

Chemistry of Transition Elements - Electronic configuration of transition elements table, downloadable AS & A Level Chemistry revision notes

Heterogeneous Catalysis

Heterogeneous Catalysis
V₂O₅ acts as a heterogeneous catalyst in the Contact Process. The use of a support medium to maximise the surface area of a heterogeneous catalyst and minimise the cost. Fe is used as a heterogeneous catalyst in the Haber Process. Heterogeneous catalysts can become poisoned by impurities that block the active sites and consequently have reduced efficiency; this has a cost implication. Students should be able to: • explain, with the aid of equations, how V₂O₅ acts as a catalyst in the Contact process

Homogeneous Catalysis

Homogeneous Catalysis

Autocatalysis

Autocatalysis

Students should be able to: • explain, with the aid of equations, how Mn²⁺ ions autocatalyse the reaction between C₂O₄²⁻ and MnO₄⁻ Students could investigate Mn²⁺ as the autocatalyst in the reaction between ethanedioic acid and acidified potassium manganate(VII).

Practical Skills – move to 8.2, but keep equations and theory here

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