Why is the $Cu^{2+}$ ion more stable in an aqueous solution than the $Cu^+$ ion, despite $Cu^+$ having a stable, fully-filled $3d^{10}$ configuration?

Why is the Cu²⁺ ion more stable in an aqueous solution than the Cu⁺ ion, despite Cu⁺ having a stable, fully-filled 3d¹⁰ configuration? +

Correct Answer

Cu²⁺ is more stable in aqueous solution mainly because it has a much larger hydration enthalpy than Cu⁺.

Detailed Explanation

The electronic configurations are:

Cu: [Ar] 3d¹⁰4s¹

Cu⁺: [Ar] 3d¹⁰
Cu²⁺: [Ar] 3d⁹

At first sight, Cu⁺ appears more stable because it possesses the completely filled 3d¹⁰ subshell. However, in an aqueous solution, electronic configuration alone does not determine the overall stability of an ion. The important factor is the balance between:

Ionisation enthalpy + Hydration enthalpy

Cu²⁺ has a charge of +2 and a relatively small ionic radius. Therefore, it attracts water molecules much more strongly than Cu⁺. Hence:

|Hydration enthalpy of Cu²⁺| >> |Hydration enthalpy of Cu⁺|

The large negative hydration enthalpy releases a considerable amount of energy and stabilises Cu²⁺ in water. Although the formation of Cu²⁺ requires removal of another electron from Cu⁺, the energy released during hydration of Cu²⁺ more than compensates for this unfavourable ionisation step.

Why Cu⁺ is unstable in Water

Cu⁺ tends to undergo disproportionation in aqueous solution:

2Cu⁺ → Cu²⁺ + Cu

Thus, Cu⁺ simultaneously undergoes oxidation and reduction:

• One Cu⁺ is oxidised to Cu²⁺.
• Another Cu⁺ is reduced to metallic Cu.

This reaction is thermodynamically favourable in aqueous medium. Therefore, Cu⁺ is comparatively unstable in water, while Cu²⁺ is much better stabilised by hydration.

JEE-Level Concept

The key point is:

Stable electronic configuration ≠ maximum stability in every medium

For aqueous ions, the total stability depends on several energetic factors, especially:

• Ionisation enthalpy
• Hydration enthalpy
• Crystal/lattice effects where applicable
• Overall thermodynamic feasibility

For Cu:

Cu⁺ → Cu²⁺

The second ionisation is energetically costly, but Cu²⁺ receives a much greater hydration stabilisation because of its higher charge density.

Cu²⁺ + H₂O → strongly hydrated Cu²⁺

This makes Cu²⁺ thermodynamically favoured in aqueous solution.

Quick Revision

Cu⁺: 3d¹⁰ but relatively poor hydration
Cu²⁺: 3d⁹ but much stronger hydration

Therefore, in aqueous medium: Cu²⁺ > Cu⁺ in stability

30 Related MCQs with Solutions

1. Electronic configuration of Cu⁺ is:

A. 3d⁹

B. 3d¹⁰

C. 3d⁸4s¹

D. 3d¹⁰4s¹

Answer: B. 3d¹⁰

2. Electronic configuration of Cu²⁺ is:

A. 3d¹⁰

B. 3d⁹

C. 3d⁸

D. 3d¹⁰4s¹

Answer: B. 3d⁹

3. Which ion is more stable in aqueous solution?

A. Cu⁺

B. Cu²⁺

C. Both equally

D. Neither

Answer: B. Cu²⁺

4. The major factor stabilising Cu²⁺ in aqueous solution is:

A. 3d¹⁰ configuration

B. Large hydration enthalpy

C. Low electronegativity

D. Large size

Answer: B. Large hydration enthalpy

5. Which ion has greater charge density?

A. Cu⁺

B. Cu²⁺

C. Both same

D. Cu atom

Answer: B. Cu²⁺

6. Hydration enthalpy of Cu²⁺ is generally:

A. Less negative than Cu⁺

B. More negative than Cu⁺

C. Zero

D. Positive

Answer: B. More negative than Cu⁺

7. Cu⁺ in aqueous solution tends to undergo:

A. Hydrolysis only

B. Disproportionation

C. Polymerisation

D. Sublimation

Answer: B. Disproportionation

8. The disproportionation reaction of Cu⁺ is:

A. Cu⁺ → Cu

B. 2Cu⁺ → Cu²⁺ + Cu

C. Cu²⁺ → Cu⁺

D. Cu → Cu²⁺

Answer: B

9. Cu⁺ has a:

A. Half-filled d-subshell

B. Completely filled d-subshell

C. Empty d-subshell

D. Half-filled p-subshell

Answer: B. Completely filled 3d¹⁰ subshell

10. Cu²⁺ has how many d-electrons?

A. 7

B. 8

C. 9

D. 10

Answer: C. 9

11. The hydration process involves:

A. Removal of electrons by water

B. Solvation of an ion by water molecules

C. Formation of a metal

D. Removal of water

Answer: B

12. Higher ionic charge generally results in:

A. Weaker hydration

B. Stronger hydration

C. No hydration

D. Lower charge density

Answer: B. Stronger hydration

13. Cu²⁺ is better hydrated than Cu⁺ because:

A. It has lower charge

B. It has higher charge density

C. It has no d-electrons

D. It is neutral

Answer: B

14. Which factor cannot be ignored when comparing ionic stability in water?

A. Hydration enthalpy

B. Only electronic configuration

C. Atomic number only

D. Colour only

Answer: A

15. Cu⁺ disproportionation produces:

A. Cu and Cu²⁺

B. Only Cu²⁺

C. Only Cu

D. Cu³⁺

Answer: A

16. The filled 3d¹⁰ configuration of Cu⁺ makes it:

A. Electronically favourable

B. Always the most stable ion in water

C. Impossible to oxidise

D. A noble gas

Answer: A
It provides electronic stability, but total aqueous stability also depends strongly on hydration and redox energetics.

17. Which ion experiences stronger electrostatic attraction toward water molecules?

A. Cu⁺

B. Cu²⁺

C. Cu

D. Both Cu⁺ and Cu²⁺ equally

Answer: B. Cu²⁺

18. Which statement is correct?

A. Electronic configuration alone determines ionic stability

B. Hydration can alter the relative stability of ions in water

C. Cu⁺ cannot exist

D. Cu²⁺ has 3d¹⁰

Answer: B

19. The oxidation state of Cu in CuSO₄ is:

A. 0

B. +1

C. +2

D. +3

Answer: C. +2

20. CuCl contains copper mainly in:

A. +1 state

B. +2 state

C. +3 state

D. 0 state

Answer: A. +1

21. Which process releases substantial energy for a highly charged small ion?

A. Hydration

B. Sublimation

C. Ionisation only

D. Reduction only

Answer: A. Hydration

22. The stability of an ion in solution is best understood using:

A. Only Hund's rule

B. Only Aufbau principle

C. Overall thermodynamics

D. Only Pauli principle

Answer: C. Overall thermodynamics

23. Cu⁺ is particularly susceptible to disproportionation in:

A. Aqueous solution

B. Vacuum

C. Solid copper metal only

D. Dry nitrogen only

Answer: A. Aqueous solution

24. Which configuration belongs to neutral copper?

A. [Ar]3d⁹4s²

B. [Ar]3d¹⁰4s¹

C. [Ar]3d⁸4s²

D. [Ar]3d¹⁰

Answer: B

25. Which statement about Cu⁺ is false?

A. It has 3d¹⁰ configuration

B. It can undergo disproportionation

C. It is strongly stabilised by hydration compared with Cu²⁺

D. It can form CuCl

Answer: C
Cu²⁺ is much more strongly hydrated.

26. Which ion has the greater hydration enthalpy magnitude?

A. Cu⁺

B. Cu²⁺

C. Both equal

D. Cu atom

Answer: B. Cu²⁺

27. The main reason Cu²⁺ is favoured in water is:

A. It has a completely filled d-subshell

B. Strong hydration compensates for ionisation energy

C. It is neutral

D. It has no unpaired electrons

Answer: B

28. In the reaction 2Cu⁺ → Cu²⁺ + Cu, Cu⁺ acts as:

A. Only oxidising agent

B. Only reducing agent

C. Both oxidising and reducing species

D. Catalyst

Answer: C

29. Which concept explains why Cu²⁺ can be more stable than the electronically attractive Cu⁺ in water?

A. Hydration enthalpy

B. Boyle's law

C. Raoult's law

D. Le Chatelier's principle only

Answer: A. Hydration enthalpy

30. Which is the best conclusion?

A. Cu⁺ is always more stable because of 3d¹⁰

B. Cu²⁺ is favoured in aqueous medium because of strong hydration and overall thermodynamics

C. Cu²⁺ cannot exist in water

D. Cu⁺ has a 3d⁹ configuration

Answer: B

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