Question

\[C{u^{2 + }}\;\] ions will be reduced to \[C{u^ + }\;\] ions by the addition of an aqueous solution of_________.
(A) $KI$
(B) $KCl$
(C) $KSCN$
(D) Both (a) and (c)

Answer 1

Hint: Substance that oxidises itself to reduce others is called a reducing agent. In other words, it is an agent used for reduction and itself gets oxidised. Thus, reduction is carried out by a reducing agent. A reducing agent must be in a lower oxidation state so that it can get oxidised and convert to a higher oxidation state.

Complete Step by Step Solution:
reduction of copper can be represented by the following reduction half reaction involving electron change.
$C{u^{ + 2}} + {e^ - } \to C{u^ + }$

Gain of electrons is termed as reduction whereas loss of electron is termed as oxidation.
Thus, from the given options the anion that loses electrons to convert to its higher oxidation state can reduce cupric to cuprous.
${I^ - } \to {I_2} + e - $
$C{l^ - } \to C{l_2} + e - $
$SC{N^ - } \to SC{N_2} + e - $

Iodide and thiocyanate ions can reduce copper and oxidise itself to iodine and thiocyanate respectively but chlorine cannot because the reaction of copper ions with potassium chloride is not a redox reaction.
\[KCl + Cu{\left( {NO₃} \right)_2} \to KN{O_3} + CuCl\]is not a feasible reaction.
Rather, the actual product formation is \[KCl + Cu{\left( {NO₃} \right)_2} \to KN{O_3} + CuC{l_2}\]

The reactions observed with iodide and thiocyanate is as follows
$ C{u^{2 + }} + {I^ - } \to C{u_2}{I_{2}} \downarrow + {I_3} - \\$
$ C{u^{2 + }} + SC{N^ - } \to Cu{(SCN)_{2}} \downarrow \\ $

The precipitate \[Cu{(SCN)_{2}} \downarrow \]undergoes slow decomposition to form white copper (I) thiocyanate and thiocyanogen by the following reaction.
\[Cu{\left( {SCN} \right)_2} \to CuSCN + {\left( {SCN} \right)_{2}}\]
Thus, the correct option is D.

Note: \[CuC{l_{2}}\]is observed and not \[Cu{I_{2}}\]because iodide is a stronger reducing agent than chloride and it reduces cupric ($C{u^{ + 2}}$) to cuprous ($C{u^ + }$) very rapidly. Hence, \[CuI\]exists and not\[Cu{I_{2}}\]. \[C{u_2}{I_{2}}\]and \[C{u_2}C{l_{2}}\]exist because of formation of $C{u_2}^{2 + }$due to strong metal and metal ion interaction between $Cu$and $C{u^{ + 2}}$.