Excess KCN added to Cu^2+ reduces it to copper(I) with evolution of cyanogen: 2Cu^2+ + 8CN- -> 2[Cu(CN)3]^2- + (CN)2. How many moles of KCN are consumed per mole of Cu^2+?

JEE Advanced levelSingle correct optionDifficulty 3 of 3qc160

Excess $KCN$ added to $Cu X^{2 +}$ reduces it to copper(I) with evolution of cyanogen: $2 Cu X^{2 +} + 8 CN X^{-} 2 [Cu (CN) X_{3}] X^{2 -} + (CN) X_{2}$ . How many moles of $KCN$ are consumed per mole of $Cu X^{2 +}$ ?

The equation uses 8

CN X^{-}

for 2

Cu X^{2 +}

, i.e. 4 per copper: 3 end up in the complex and 1 is oxidised to cyanogen. Charge balance:

4 - 8 = - 4

on the left and

2 (- 2) = - 4

on the right. Counting only the ligands in the complex (3) neglects the cyanide consumed as reductant.