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$\mathrm{Cu}^+$ ion is unstable in aqueous solution because it:
AIs rapidly oxidised by atmospheric oxygen
BUndergoes disproportionation to give $\mathrm{Cu}^{2+}$ and $\mathrm{Cu}$, driven by the large hydration enthalpy of $\mathrm{Cu}^{2+}$
CForms an insoluble hydroxide immediately
DHas a $d^9$ configuration which is intrinsically unstable
Answer & Solution
Correct answer: B. Undergoes disproportionation to give $\mathrm{Cu}^{2+}$ and $\mathrm{Cu}$, driven by the large hydration enthalpy of $\mathrm{Cu}^{2+}$
$\mathrm{2\,Cu^+(aq) \to Cu^{2+}(aq) + Cu(s)}$. The disproportionation is spontaneous because the hydration enthalpy released on forming $\mathrm{Cu^{2+}(aq)}$ more than compensates for the ionisation energy needed to take $\mathrm{Cu^+}$ to $\mathrm{Cu^{2+}}$. As a result $\mathrm{Cu^+}$ is only stable in low-dielectric environments such as in $\mathrm{Cu_2O}$ or with bulky ligands.
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