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JEE Main levelSingle correct optionDifficulty 2 of 3df018

In aqueous solution $Cu X^{+}$ disproportionates to $Cu X^{2 +}$ and Cu mainly because

Athe very high hydration enthalpy of $Cu X^{2 +}$ more than compensates for the second ionisation enthalpy of Cu
B $Cu X^{+}$ ( $3 d^{10}$ ) is paramagnetic and therefore unstable
Cthe second ionisation enthalpy of Cu is lower than the first
D $Cu X^{2 +}$ has a half-filled d subshell

Answer: A

Forming

Cu X^{2 +}

costs a large second ionisation enthalpy, but its small size and 2+ charge give a very negative hydration enthalpy that outweighs this in water. Hence

2 Cu X^{+} Cu X^{2 +} + Cu

is favourable.

Cu X^{+}

is diamagnetic and

Cu X^{2 +}

is

d^{9}

, so the other reasons are wrong.

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