The standard reduction potentials for $\mathrm{Zn}^{2+} / \mathrm{Zn}, \mathrm{Ni}^{2+} / \mathrm{Ni}$, and $\mathrm{Fe}^{2+} / \mathrm{Fe}$ are $-0.76,-0.23$ and -0.44 V respectively. The reaction $\mathrm{X}+\mathrm{Y}^{2+} \rightarrow \mathrm{X}^{2+}+\mathrm{Y}$ will be spontaneous when :

(D) $\mathrm{X}=\mathrm{Zn}, \mathrm{Y}=\mathrm{Ni}$

(A) $X=\mathrm{Ni}, Y=\mathrm{Fe}$

(B) $X=\mathrm{Ni}, Y=\mathrm{Zn}$

JEE Main · 2012 · 29 Apr 2012 · Offline · Redox Reactions and Electrochemistry

The standard reduction potentials for

Zn^{2 +} / Zn, Ni^{2 +} / Ni

, and

Fe^{2 +} / Fe

are

- 0.76, - 0.23

and -0.44 V respectively. The reaction

X + Y^{2 +} \to X^{2 +} + Y

will be spontaneous when :

Answer: (D)

The displacement

X + Y^{2 +} \to X^{2 +} + Y

runs forward only when the cell e.m.f.

E_{cathode}^{\circ} - E_{anode}^{\circ}

is positive, which needs

X

, the metal being oxidised, to have the more negative reduction potential and

Y

the less negative one. Zinc, at

- 0.76

V, is the most negative of the three, so with

X = Zn

and

Y = Ni

the e.m.f. is

- 0.23 - (- 0.76) = + 0.53

V and the reaction is spontaneous.

JEE Main · 2012 · 29 Apr 2012 · Offline · Single correct option