JEE Challenger
More from d and f Block Elements
ChemHardAdv 2011 • Paper 2

Shift of Copper Disproportionation Equilibrium in Aqueous Medium

The equilibrium

2 CuI⇌Cu0+CuII2\text{ Cu}^{\text{I}} \rightleftharpoons \text{Cu}^0 + \text{Cu}^{\text{II}}

in aqueous medium at 25 ∘C25\text{ }^{\circ}\text{C} shifts towards the left in the presence of

Options

A

NO3−\text{NO}_3^-

B

Cl−\text{Cl}^-

Correct
C

SCN−\text{SCN}^-

Correct
D

CN−\text{CN}^-

Correct

Topics & Concepts

d and f Block ElementsChemical EquilibriumLe Chatelier's Principled-Block Elements

Step-by-Step Solution

To determine which species cause the disproportionation equilibrium:

2CuI(aq)⇌Cu0(s)+CuII(aq)2\text{Cu}^{\text{I}}(\text{aq}) \rightleftharpoons \text{Cu}^0(\text{s}) + \text{Cu}^{\text{II}}(\text{aq})

to shift towards the left (i.e., favoring the formation/stabilization of CuI\text{Cu}^{\text{I}}), we apply Le Chatelier's principle and thermodynamic stability concepts:

  1. Role of Cl−\text{Cl}^-:
    Chloride ions precipitate copper(I) as insoluble cuprous chloride (CuCl\text{CuCl}), which has a very low solubility product (KspK_{\text{sp}}), or form stable chloro-complexes such as [CuCl2]−[\text{CuCl}_2]^- in concentrated solutions:
    Cu0(s)+Cu2+(aq)+2Cl−(aq)⇌2CuCl(s)\text{Cu}^0(\text{s}) + \text{Cu}^{2+}(\text{aq}) + 2\text{Cl}^-(\text{aq}) \rightleftharpoons 2\text{CuCl}(\text{s})
    Removal of Cu+\text{Cu}^+ as an insoluble precipitate or stable complex significantly stabilizes the +1+1 oxidation state relative to the +2+2 state in aqueous solution, thereby shifting the equilibrium to the left.

  2. Role of SCN−\text{SCN}^-:
    Thiocyanate ions react with copper(I) to form a highly insoluble white precipitate of copper(I) thiocyanate (CuSCN\text{CuSCN}):
    Cu0(s)+Cu2+(aq)+2SCN−(aq)⇌2CuSCN(s)\text{Cu}^0(\text{s}) + \text{Cu}^{2+}(\text{aq}) + 2\text{SCN}^-(\text{aq}) \rightleftharpoons 2\text{CuSCN}(\text{s})
    This precipitation heavily stabilizes the CuI\text{Cu}^{\text{I}} state, shifting the equilibrium to the left.

  3. Role of CN−\text{CN}^-:
    Cyanide ions are strong reducing agents and strong π\pi-acid ligands that form extremely stable cyano-complexes with CuI\text{Cu}^{\text{I}}, such as [Cu(CN)4]3−[\text{Cu(CN)}_4]^{3-} and [Cu(CN)2]−[\text{Cu(CN)}_2]^-. The stability constant of the CuI\text{Cu}^{\text{I}}-cyano complex is exceptionally high, which makes CuI\text{Cu}^{\text{I}} far more stable than CuII\text{Cu}^{\text{II}} in the presence of CN−\text{CN}^-, thereby shifting the equilibrium to the left.

  4. Role of NO3−\text{NO}_3^-:
    Nitrate ion (NO3−\text{NO}_3^-) is a poor coordinating ligand and does not form insoluble salts or stable complexes with CuI\text{Cu}^{\text{I}} to stabilize it over CuII\text{Cu}^{\text{II}}. Therefore, it does not shift the equilibrium towards the left.

Hence, the equilibrium shifts towards the left in the presence of Cl−\text{Cl}^-, SCN−\text{SCN}^-, and CN−\text{CN}^-.

Correct Options: (B), (C), (D)

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