24.4 Relative Stability of +2 and +4 Oxidation States

The stability of +2 and +4 oxidation states

• 1. +4 compounds at the top of the Group is more stable than at the bottom.
  2. +2 compounds at the bottom of the Group is more stable than at the top.

• For carbon, carbon dioxide(+4) is more stable than carbon monoxide(+2). Therefore, carbon monoxide is readily oxidised to carbon dioxide, it is a good reducing agent. For example, it reduces iron(III) ions to iron
  Fe2O3 + 3CO → 2Fe + 3CO2

• For lead, lead(II) oxide(+2) is more stable than lead(IV) oxide(+4). Therefore, lead(IV) oxide is readily reduced to lead(II) oxide, it is a good oxidising agent. For example, lead(II) oxide oxidises hydrochloric acid to chlorine
  PbO2 + 4HCl → PbCl2 + Cl2 + 2H2O

• Between these extremes, there is a gradual change in stability of the +2 compared with the +4
  1. The +4 state of germanium compounds is more stable than the +2 Therefore, germanium(II) compounds are good reducing agents.
  2. The +4 state of tin compounds is slightly more stable than the +2 Therefore, tin(II) compounds are weak reducing agents.

• This can be interpreted from the E° values of the ions of Group IV elements in aqueous solutions
• 1. The more positive the value of E°, the easier it is to reduce the species on the left of the half-equation.
  2. The less positive the value of E°, the easier it is to oxidise the species on the right of the half-equation.