Why will iron displace copper from aqueous solutions of copper salts?

Answer 1

Because iron is more reactive than copper. If iron displaces copper, that releases energy (enthalpy). If copper were to displace iron, that would require energy to be used. This is less favourable and , averaged over the huge number of molecules, atoms and ions in the solution, the more energy producing reaction is vastly preferred. Hence, iron put into copper sulphate solution gets coated in copper and the solution slowly loses its blue colour. But if you put copper metal in iron sulphate solution, nothing noticable occurs.

Answer 2

Iron is more reactive than copper, so when iron is added to a solution of copper salt, iron will displace copper by reducing it from its cationic form (Cu2+) to its elemental form (Cu). This is due to the difference in the reactivity series of metals, with iron being higher in the series than copper.