AQA Chem5: Uses of Transition Metals

• Transition metals and their compounds can act as catalysts
• This is due to their variable oxidation states as they can transfer electrons
• Catalysts work by lowering the activation energy of a reaction by providing an alternative lower energy pathway
• Catalysts are not used up in reactions

• Heterogeneous catalysts are in a different phase to the reactants
• The reaction takes place on the surface of the catalyst so increasing surface area increase rate of reaction
• Things such as a ceramic honeycomb coated with a catalyst can be used to increase surface area
• The different stages are: the adsorption of reactants onto the active site of the catalyst; the reaction of a modified reactant takes place; the desorption of the product

Catalytic Efficiency:

• depends on the strength of adsorption
• too weak adsorption - modified reactant not sufficiently held on catalysts surface
• too strong adsorption - product unable to leave surface of catalyst

Examples:

• The contact process (production of H2SO4, V2O5 catalyst):                                                                                                                    V2O5 + SO2 -> V2O4 + SO3    V2O4 + 1/2O2 -> V2O5
• The Haber process (production of ammonia, Fe catalyst): N2 + 3H2 -> 2NH3
• The production of methanol (Cr2O3 catalyst): CO + 2H2 -> CH3OH

Poisoning of catalysts:

• a catalyst is poisoned when a substance blocks the active site or is not desorbed
• lead can poison catalytic converters
• sulphur can poison the Haber process

• The catalyst is in the same state (phase) as the reactants

Example: peroxodisulphate ions and iodide ion

• S2O82- + 2I-  -> 2SO42- + I2     both reactants are negatively charged so the reaction is slow
• Fe2+ or Fe3+ solutions can be used as a catalyst
• S2O82- + 2Fe2+ -> 2SO42- + 2Fe3+
• 2Fe3+  + 2I-  -> 2Fe2+  + I2

This is where the product of a reaction acts as the catalyst for the reaction. This means as the reaction progresses and the amount of product increases the rate of reaction increases.

An example of this is the reaction between C2O42- and MnO4- :

2MnO4- + 16H+  +  5C2O42-  -> 2Mn2+  +  8H2O + 10CO2

the Mn2+ acts as an autocatalyst...

• 4Mn2+  + MnO4- + 8H+  -> 5Mn3+  + 8H2O
• 2Mn3+  + C2O42-  -> 2CO2 + 2Mn2+

• Fe2+ in haemoglobin is used to carry oxygen around the body, this is disrupted by carbon monoxide which replaces the oxygen
• Cisplatin is an anti-cancer drug which is a complex of platinum, [Pt(Cl)2(NH3)2], it has a square planar shape and prevents cancer cells from dividing
• Tollens' Reagent is a solution of the complex [Ag(NH3)2]+, it has a linear shape and is used to distinguish between aldehydes and ketones