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Q. The equilibrium $2 Cu ^{1} \rightleftharpoons Cu ^{\circ}+ Cu ^{ ll }$ in aqueous medium at $25^{\circ} C$ shifts towards the left in the presence of

JEE AdvancedJEE Advanced 2011

Solution:

$Cu ^{2+}$ ions will react with $CN ^{-}$and $SCN ^{-}$forming $\left[ Cu ( CN )_{4}\right]^{3-}$ and $\left[ Cu ( SCN )_{4}\right]^{3-}$ leading the reaction in the backward direction.
$Cu ^{2+}+2 CN ^{-} \rightarrow Cu ( CN )_{2}$
$2 Cu ( CN )_{2} \rightarrow 2 CuCN +( CN )_{2}$
$CuCN +3 CN ^{-} \rightarrow\left[ Cu ( CN )_{4}\right]^{3-}$
$Cu ^{2+}+4 SCN ^{-} \rightarrow\left[ Cu ( SCN )_{4}\right]^{3-}$
$Cu ^{2+}$ also combines with $CuCl _{3-}$ which reacts with $Cu$ to produce $CuCl$ pushing the reaction in the backward direction.
$CuCl _{2}+ Cu \rightarrow 2 CuCl \downarrow$