Question

The standard reduction potentials for $\text{Z}{{\text{n}}^{\text{2+}}}\text{ }\!\!|\!\!\text{ Zn}$, $\text{N}{{\text{i}}^{\text{2+}}}\text{ }\!\!|\!\!\text{ Ni}$ and $\text{F}{{\text{e}}^{\text{2+}}}\text{ }\!\!|\!\!\text{ Fe}$ are -0.76, -0.23 and -0.44V respectively. The reaction $\text{X + }{{\text{Y}}^{\text{2+}}}\text{ }\to \text{ }\ {{\text{X}}^{\text{2+}}}\text{ + Y}$ will be spontaneous when:
(A) X = Ni, Y = Fe
(B) X = Ni, Y = Zn
(C) X = Fe, Y = Zn
(D) X = Zn, Y = Ni

Answer 1

Hint: The electrode potentials of metals given above help us to understand the ease of a metal to get oxidized or reduced. With this we can select the anode and cathode electrodes. Oxidation takes place at anode and reduction at cathode, so choose the electrodes that satisfy the requirements of the respective electrodes.

Complete step-by-step answer:
Electrochemistry is the branch of physical chemistry that mainly deals with the relationship between electricity and identifiable chemical change. Electrochemical reaction is a chemical reaction in which current is externally supplied or produced through a spontaneous chemical reaction. Chemical reactions where electrons are directly transferred between the constituent molecules or atoms are called oxidation-reduction or rather redox reactions.
In general, we can say that a reaction is spontaneous when the emf of the cell is positive. It indicates that the cell reaction is feasible and thus occurs. In the electrochemical series the order of elements is Zn > Fe > Ni in terms of ease of oxidation. So, the cell is feasible when Zn is anode and either Fe or Ni is cathode. Based on our explanation and the options provided, we can conclude that the cell reaction, $\text{X + }{{\text{Y}}^{\text{2+}}}\text{ }\ to \text{ }\ {{\text{X}}^{\text{2+}}}\text{ + Y}$ is feasible when, X = Zn and Y = Ni.

Therefore, the correct answer is option (D).

Note: It is important to understand that although the positive polarity of the cell denotes a reaction to be spontaneous, the accurate measure is given by the Gibbs free energy. The Gibbs-Helmholtz equation provides an insight on the relation between the electrochemical factor of the cell and the thermodynamic factor. So, when the change in Gibbs free energy is negative, the reaction is considered to be feasible and spontaneous in nature.