Why Cu²⁺ Is More Stable Than Cu⁺ in Aqueous Solution
Chemistry · D And F Block Elements · NEET
In water, Cu⁺ is not stable and turns into Cu²⁺ and Cu metal. This happens because Cu²⁺ is smaller and has a +2 charge, so it grabs water molecules very strongly and releases a huge hydration energy. This large energy more than pays back the extra second ionisation enthalpy needed to make Cu²⁺. Memory hook: "Cu²⁺ hugs water harder" — the strong water-hug (hydration energy) wins.
Cu²⁺ needs a high second ionisation enthalpy (IE₂) to form, but because it is small with a +2 charge it releases a very large hydration energy in water. This hydration energy overpays the IE₂ cost, so Cu⁺ is unstable in water and disproportionates: 2Cu⁺ → Cu²⁺ + Cu.
Your doubts, answered
Cu⁺ has a full 3d¹⁰ configuration. Isn't a full d-subshell supposed to be extra stable? Then why is Cu⁺ still unstable in water?
Yes, Cu⁺ is 3d¹⁰ (fully filled) and Cu²⁺ is 3d⁹, so on paper Cu⁺ looks more stable. This full-shell stability is real in the GAS phase and in solids like CuCl or Cu₂O. But stability in WATER is decided by energy in solution, not just the configuration. When Cu²⁺ forms in water, it releases a very large hydration energy that outweighs the d¹⁰ advantage of Cu⁺. So in aqueous solution Cu²⁺ wins.
Making Cu²⁺ needs a high second ionisation enthalpy. Doesn't that make Cu²⁺ harder to form and less stable?
The second ionisation enthalpy of copper (Cu⁺ → Cu²⁺) is indeed high because you remove an electron from a stable 3d¹⁰ shell. This step costs energy. But it is only half the story. Once Cu²⁺ is in water, it is hydrated so strongly that the hydration energy released is even larger than that cost. Net energy change favours Cu²⁺. So the high second ionisation enthalpy is overpaid by the big hydration energy.
What exactly is hydration energy and why is it bigger for Cu²⁺?
Hydration energy (hydration enthalpy) is the energy released when a gas-phase ion is surrounded by water molecules. It is larger when the ion is SMALLER and has a HIGHER charge, because the ion pulls water dipoles more strongly. Cu²⁺ has charge +2 and is smaller than Cu⁺ (charge +1). So Cu²⁺ has a much more negative hydration enthalpy — it 'hugs' water far harder. This extra release of energy is the main reason Cu²⁺ is favoured in water.
What actually happens to Cu⁺ ions if I put them in water?
They disproportionate. The reaction is 2Cu⁺ → Cu²⁺ + Cu. One Cu⁺ is oxidised to Cu²⁺ and another Cu⁺ is reduced to Cu metal (Cu⁰). Both come from the same +1 oxidation state, so this is a disproportionation reaction. That is why simple Cu⁺ salts like Cu₂SO₄ do not survive in water — they break into Cu²⁺ salt plus copper metal.
So does this mean Cu⁺ never exists at all?
No. Cu⁺ can exist as stable SOLID compounds and when it is not in water — for example CuCl, CuI, Cu₂O, and copper(I) complexes. It is only in AQUEOUS SOLUTION that Cu⁺ is unstable and disproportionates. The whole rule 'Cu²⁺ more stable than Cu⁺' is specifically about water, because hydration energy is what tips the balance.
Which single factor should I write in a NEET one-word answer?
Write HYDRATION ENERGY (or hydration enthalpy). NEET 2023 asked exactly this and the answer was hydration energy — not first ionisation enthalpy, not enthalpy of atomisation, not second ionisation enthalpy. The large hydration energy of Cu²⁺ is the deciding factor.
⚠️ The NEET trap ✗ Cu²⁺ is more stable than Cu⁺ because of the second ionisation enthalpy of copper. ✓ Cu²⁺ is more stable than Cu⁺ in water because of the large HYDRATION ENERGY of Cu²⁺. The high second ionisation enthalpy is a cost, not the reason — it is the hydration energy that overpays this cost. 🧠 The second ionisation enthalpy is the OBSTACLE, not the reason. What makes Cu²⁺ win is what happens AFTER — the huge hydration energy released in water. Pick hydration energy.
Real NEET questions
NEET 2023 Phase 1
The stability of Cu²⁺ salts is more than Cu⁺ salts in aqueous solution due to
A · First ionisation enthalpy
B · Enthalpy of atomization
C · Hydration energy ✓
D · Second ionisation enthalpy
Solution: Making Cu²⁺ needs a high second ionisation enthalpy, so that step costs energy. But this cost is more than paid back by the very large hydration energy released when Cu²⁺ forms in water. Cu²⁺ is smaller and carries a higher (+2) charge than Cu⁺, so it is hydrated far more strongly. This large negative hydration enthalpy outweighs the high second ionisation enthalpy and makes Cu²⁺ the more stable species in aqueous solution. So the correct factor is hydration energy.
NEET 2019
Which of the following reactions are disproportionation reactions?
(a) 2Cu⁺ → Cu²⁺ + Cu
(b) 3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O
(c) 2KMnO₄ → K₂MnO₄ + MnO₂ + O₂
(d) 2MnO₄⁻ + 3Mn²⁺ + 2H₂O → 5MnO₂ + 4H⁺
A · (a) and (b) only ✓
B · (a), (b) and (c)
C · (a), (c) and (d)
D · (a) and (d) only
Solution: In a disproportionation reaction the SAME element in ONE oxidation state is both oxidised and reduced.
(a) 2Cu⁺ → Cu²⁺ + Cu: Cu(+1) goes to Cu(+2) and to Cu(0). Disproportionation. This is exactly why Cu⁺ is unstable in water.
(b) 3MnO₄²⁻ → 2MnO₄⁻ + MnO₂: Mn(+6) goes to Mn(+7) and Mn(+4). Disproportionation.
(c) here Mn(+7) is reduced but oxygen(−2) is oxidised to O₂(0) — two different elements change, so NOT disproportionation.
(d) is the reverse (comproportionation), NOT disproportionation.
So only (a) and (b) are disproportionation reactions. Answer: (a) and (b) only.
Solved D And F Block Elements NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Because Cu²⁺ releases a much larger hydration energy, so Cu⁺ disproportionates into Cu²⁺ and Cu (2Cu⁺ → Cu²⁺ + Cu).
Is Cu⁺ or Cu²⁺ more stable overall?
It depends on the state. In aqueous solution Cu²⁺ is more stable. But as dry solids, Cu⁺ compounds like CuCl and CuI are stable. NEET usually asks about the aqueous case, where Cu²⁺ wins.
What is the main reason NEET wants for Cu²⁺ stability?
Hydration energy (hydration enthalpy). Cu²⁺ is smaller with +2 charge, so its hydration energy is very large and beats the high second ionisation enthalpy.
Does Cu⁺ have a more stable electronic configuration than Cu²⁺?
Yes, Cu⁺ is 3d¹⁰ (fully filled) and Cu²⁺ is 3d⁹. But in water this d¹⁰ advantage is overcome by the much larger hydration energy of Cu²⁺.
What kind of reaction is 2Cu⁺ → Cu²⁺ + Cu?
It is a disproportionation reaction, because copper in the same +1 state is at once oxidised to +2 and reduced to 0. This was tested in NEET 2019.
