18.104.22.168 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.
Students should be able to: • explain the importance of variable oxidation states in catalysis • explain, with the aid of equations, how V2 O5 acts as a catalyst in the Contact process • explain, with the aid of equations, how Fe2+ ions catalyse the reaction between I− and S2 O8 2– • explain, with the aid of equations, how Mn2+ ions autocatalyse the reaction between C2 O4 2– and MnO4 –