Why is Cu²⁺ ion more stable than Cu⁺ ion in aqueous solution?

 

Many Cu⁺ compounds are not stable in aqueous solution. These compounds undergo disproportionation reactions to form metallic copper and Cu²⁺ compounds.

 

That is, Cu²⁺ ion is more stable than Cu⁺ ion in aqueous solution. This can be explained on the basis of their electronic configuration.

 

The electronic configuration of Cu⁺ and Cu²⁺ ions show that, the 3d subshell of Cu⁺ ion is completely filled with electrons, which is very stable electronic configuration.

 

Naturally, the stability of Cu⁺ ion should be higher than that of Cu²⁺ ion. But in reality it is seen that the stability of Cu²⁺ ion is more than Cu⁺ ion in aqueous solution.

 

This is because the hydration enthalpy (-2121 kJ/mol) of Cu²⁺ ion in aqueous solution is so high that it exceeds the second ionization enthalpy (1958 kJ/mol) of copper.

 

Why Cu²⁺ salts are colored while Cu⁺ salts are colorless?

 

Inorganic compounds appear colored mainly due to three reasons that is, d-d transition, charge transferand polarization.

 

Copper metal is a transition element. We know that, most of the transition elements form colored compound due to d-d transition of electrons.

 

Now the question is why Cu⁺ salts are colorless but Cu²⁺ salts are colored? This can be explained by their electronic configuration.

 

The outer electronic configuration of Cu⁺ and Cu²⁺ are 3d¹⁰4s⁰ and 3d⁹4s⁰ respectively.

From the above electronic configuration, it can be seen that the 3d subshell of Cu⁺ ion is completely filled with electrons. In this case there is no unpaired electron.

 

Due to this no d-d transition occurs for Cu+ ions. But the 3d subshell of Cu²⁺ ion is partially filled with electrons. In this case, an unpaired electron is present. So d-d transition occurs in case of Cu²⁺ ion.

 

Since the color of transition elements is due to the presence of unpaired electrons and hence Cu²⁺ salts are colored while Cu⁺ salts are colorless.

 

•  Why is Cu2+ ion more stable than Cu+ ion in aqueous solution?
• Why Cu2+ salts are colored while Cu+ salts are colorless?