Question

The standard electrode potential of two half cells is given below-

${\mathrm{Ni}}^{2+}+2{\mathrm{e}}^{-}\to \mathrm{Ni}$ ; $E_1^0=-0.25V$

${\mathrm{Zn}}^{2+}+2{\mathrm{e}}^{-}\to \mathrm{Zn}$ ; $E_2^0=-0.77V$

The voltage cell formed by combining the two half cells would be:

• A. $+0.52V$
• B. $-0.52V$
• C. $+1.02V$
• D. $-1.02V$

Answer 1

The correct option is A

$+0.52V$

Explanation of Correct option:

(A) $+0.52V$

Standard Electrode Potential:

• A measurement of the potential for equilibrium is the standard electrode potential.
• The electrode's potential is the difference in potential between the electrode and the electrolyte.
• The electrode potential is referred to as the standard electrode potential when unity represents the concentrations of all the species involved in a semi-cell.

Given Reactions:

${\mathrm{Ni}}^{2+}+2{\mathrm{e}}^{-}\to \mathrm{Ni}$ ; $E_1^0=-0.25V$

${\mathrm{Zn}}^{2+}+2{\mathrm{e}}^{-}\to \mathrm{Zn}$ ; $E_2^0=-0.77V$

• Hence for oxidation to take place, we reverse both the reactions,

$\mathrm{Ni}\to {\mathrm{Ni}}^{2+}+2{\mathrm{e}}^{-}$; $E_{O.P.}^0=+0.25V$

$\mathrm{Zn}\to {\mathrm{Zn}}^{2+}+2{\mathrm{e}}^{-}$ ; $E_{O.P.}^0=+0.77V$

• The sign of electrode potential also reverses.
• Hence the oxidation potential of Zinc is more so we will take it as Anode and Nickel as Cathode.
• The reaction can be represented as,

$\underset{\mathrm{Anode}}{\underbrace{\mathrm{Zn}|{\mathrm{Zn}}^{2+}}}\left|\right|\underset{\mathrm{Cathode}}{\underbrace{{\mathrm{Ni}}^{2+}|\mathrm{Ni}}}$

• Hence there will be oxidation on the anode and reduction at the cathode.
• Now the standard electrode potential of the cell is written as,

$E_{cell}^0=E_{Zn|Z{n}^{2+}}^0+E_{N{i}^{2+}|Ni}^0$

$$\begin{gathered} =+0.77V-0.25V \\ =+0.52V \end{gathered}$$

Explanation of incorrect option:

(B) $-0.52V$

• If the oxidation potential of Zinc would have been negative i.e. $E_2^0=-0.77V$, and that of Nickel is also negative i.e. $E_{O.P.}^0=-0.25V$ then standard electrode potential of the cell will be $-0.52V$.

(C) $+1.02V$

• If the oxidation potential of Nickel would have been negative i.e. $E_1^0=-0.25V$, then standard electrode potential of the cell will be $+1.02V$.

(D) $-1.02V$

• If the oxidation potential of Zinc would have been negative i.e. $E_1^0=-0.77V$, and that of Nickel is positive i.e. $E_{O.P.}^0=+0.25V$ then standard electrode potential of the cell will be $-1.02V$.

Hence correct option is (A)