3.2.5.6 Catalysts (A-level only)

• • 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.
• The use of a support medium to maximise the surface area of a heterogeneous catalyst and minimise the cost.
• V2O5 acts as a heterogeneous catalyst in the Contact process.
• 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.
• • 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.
• Construct an overall equation for the reaction between S2O8 2– ions and I– ions.

  • S2O8 2– + 2I– 2SO4 2– + I2
• Give one reason why impurities in the reactants can cause problems in processes that use heterogeneous catalysts. [1 mark]

  • Impurities poison / deactivate the catalyst / block the active sites
  • Allow (adsorbs onto catalyst AND reduces surface area)
• Explain the meaning of the terms ligand and bidentate as applied to transition metal complexes.

  • A ligand is an electron pair / lone pair donor (Allow uses lone / electron pair to form a co-ordinate bond)
  • A bidentate ligand donates two electron pairs (to a transition metal ion) from different atoms / two atoms (on the same molecule / ion)
• The complex ion formed in aqueous solution between cobalt(II) ions and chloride ions is a different colour from the [Co(H2O)6 ] 2+ ion. Explain why these complex ions have different colours. [3 marks]

  • In different complexes the d orbitals / d electrons (of the cobalt) will have different energies / d orbital splitting will be different 1
  • Light / energy is absorbed causing an electron to be excited 1
  • Different frequency / wavelength / colour of light will be absorbed / transmitted / reflected
• State the type of acidity shown by FeCl3 (accepts electron) and by [Fe(H2O)6 ] 3+ in these reactions. Explain your answers.

  • FeCl3 accepts electron pairs from water 1
  • Hence acts as a Lewis acid 1
  • [Fe(H2O)6 ] 3+ donates protons 1
  • Hence acts as a Bronsted-Lowry acid
• In aqueous solution, Fe2+ ions act as a homogeneous catalyst in the reaction between I – and S2O8 2– ions. Give one reason why the reaction is slow in the absence of a catalyst.

  Write equations to show how Fe2+ ions act as a catalyst for this reaction.

  • Both ions are negative or ions repel
  • So activation energy is high!
  • 2Fe2+ + S2O8 2– → 2Fe3+ + 2SO4 2–
  • 2Fe3+ + 2I– → 2Fe2+ + I2
  • 2021 MISTAKE - FORGETTING TO BALANCE THE CHARGES ie add 2 in front of iron
• Explain the terms heterogeneous and active sites as applied to a catalyst.

  Give two reasons why a ceramic support is used for the catalyst in catalytic converters in cars.

  Explain how lead poisons this catalyst.

  • Heterogeneous: catalyst in a different phase or state to that of the reactants; 1
  • Active site: place where reactants adsorbed or attached or bond etc.; 1
  • Reaction occurs or an explanation of what happens; (allow absorbed) 1
  • reasons: large surface area
  • reduce cost or amount of catalyst
• Explain how lead poisons catalyst

  • catalyst poison: lead adsorbed; lead not desorbed or site blocked;
• Explain why a cobalt compound is able to act as a catalyst for this process.

  • Cobalt has variable oxidation states
  • It can act as an intermediate that lowers the activation energy
  • Allow alternative route with lowered activation energy
• • Explain, with the aid of equations, how Mn2+ ions autocatalyse the reaction between C2O4 2– and MnO4

  • The oxidation of C2O4 2− ions by MnO4 − ions in acidic solution is an example of a reaction that is autocatalysed.
  • Autocatalyse - The catalyst is a reaction product
  • 4Mn2+ + MnO4 − + 8H+ → 5Mn3+ + 4H2O
  • 2Mn3+ + C2O4 2− → 2Mn2+ + 2CO2
• Explain how properties of Mn2+ enable it to act as a catalyst

  • Mn (the ELEMENT, not an ion) exists in variable oxidation states
  • Activation energy is lowered because charged ions attract (Mn2+ and C2O42-)
• Alternative reaction pathway of how Mn2+ catalyse reaction between Fe2+ and MnO4-

  • Mn3+ (reduced) to Mn2+ by C2O4 2−
  • (oxidised (back)) to Mn3+ by MnO4 − equation

  1. Mn2+ -> Mn3+ + e-
  2. 8H+ + MnO4- + 5e- -> Mn2+ + 4H2O
  3. C2O4 -> 2CO2 + 2e-

  • Combine Half equation 1 and 3
  • Balance electrons
  • FLIP HALF EQUATION 1
  • COMBINE HALF EQUATION 1 and 2
  • Balance electrons