In aqueous solution stability depend on Hydration energy (HE)

is more stable than .
Stability depends on the hydration energy (enthalpy) of the ions when they bond to the water molecules. The ion has a greater charge density than ion and thus forms much stronger bonds releasing more energy. The extra energy needed for the second ionization of the copper is more than compensated by the hydration energy. So that the ion loses an electron to become which can then release this hydration energy. In aqueous solution disproportionate to and Cu