Question

Which of the following cuprous compounds is not stable?

• A. CuCl₂
• B. Cu₂(CNS)₂
• C. Cu₂Cl₂
• D. Cu₂SO₄

Answer 1

Answer: (D)

Cu₂SO₄

Answer 2

Cuprous compounds (Cu⁺) are generally unstable in aqueous solutions due to disproportionation into Cu(0) and Cu(II). This is favored by the standard electrode potentials: $E^\circ(\text{Cu}^+/\text{Cu}) = +0.52 \text{ V}$ and $E^\circ(\text{Cu}^{2+}/\text{Cu}^+) = +0.15 \text{ V}$. For the disproportionation $2\text{Cu}^+ \rightarrow \text{Cu} + \text{Cu}^{2+}$, the overall cell potential is positive ($+0.37 \text{ V}$), indicating spontaneity. Therefore, soluble cuprous salts are unstable. Insoluble cuprous salts like CuCl (Cu₂Cl₂) and CuSCN (Cu₂(CNS)₂) are stable because the concentration of Cu⁺ ions in solution is very low, preventing significant disproportionation. Cu₂SO₄ would be a soluble cuprous salt, hence it is unstable and disproportionates. CuCl₂ is a stable cupric compound, not cuprous.