Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Cu²⁺ in water is more stable than Cu⁺.
Reason (R) : Enthalpy of hydration for Cu²⁺ is much less than that of Cu⁺.
In the light of the above statements, choose the correct answer from the options given below :

Solution

Chemistry ma’am · 4 steps

Hydration enthalpy is what saves Cu²⁺, so check whether R gives it the right size.

1. Check the assertion. In water, Cu⁺ is unstable and disproportionates: 2Cu⁺ → Cu²⁺ + Cu. So Cu²⁺ is the stable ion in aqueous solution, and (A) is correct.
2. Why it is stable. Removing a second electron from Cu costs a lot of energy (second ionisation enthalpy about 1958 kJ mol⁻¹). The small, doubly charged Cu²⁺ ion attracts water molecules much more strongly than Cu⁺ does. Its hydration enthalpy is therefore far more negative, and that release of energy more than pays back the ionisation cost.

   Ion	Charge and size	Δ_(hyd)H (approx.)
   Cu⁺	+1, larger	-593 kJ mol⁻¹
   Cu²⁺	+2, smaller	-2100 kJ mol⁻¹, much more negative

3. Check the reason. R says the hydration enthalpy of Cu²⁺ is much less than that of Cu⁺. In fact it is much greater in magnitude, about 3.5 times more energy released. So (R) is not correct.
4. So. (A) is correct but (R) is not correct, which is option (a).

The idea to keep. A higher charge on a small ion means a much more negative hydration enthalpy. That extra hydration energy is why Cu²⁺ wins over Cu⁺ in water, despite the costly second ionisation.

Answer from the NTA final answer key. Solution by PrepWiser.