Why is Cu(II) more stable than Cu(I)?
The stability of an ion in an aqueous medium is determined by the tendency of attraction between the ion and the water molecules termed as hydration enthalpy. It, in turn, depends on the charge on the ion and its size.
Smaller the ion greater the hydration power of the ion. It is because when the ion is small the distance between its centre and the molecule that solvates it is less hence providing a stronger attraction, thereby releasing more energy attributing to stability. The example of this fact is provided by Cu2+ which has a smaller cationic radius due to a stronger hold of the nucleus on the remaining 27 electrons after the loss of two as compared to Cu+ where only one electron is lost and the hold of the nucleus is less. The stability gained by energy loss in hydration is more stabilising than the destabilising effect of the removal of 2 electrons.
Also, as mentioned earlier, the greater the charge on the ion greater is its tendency to attract and keep the water molecules solvated to it. Cu2+ having a greater charge has a greater tendency of hydration than Cu+.