Question

Which of the following reaction will provide maximum voltage?
Given:
$Z{n}^{2+}\left(aq\right)+2e^{-}\to Zn\left(s\right);E^0=-0.76V$
$C{u}^{2+}\left(aq\right)+2e^{-}\to Cu\left(s\right);E^0=0.34V$
$A{g}^{+}\left(aq\right)+e^{-}\to Ag\left(s\right);E^0=0.80V$
$2H^{+}\left(aq\right)+2e^{-}\to H_2\left(g\right);E^0=0V$
$N{i}^{2+}\left(aq\right)+2e^{-}\to Ni\left(s\right);E^0=-0.25V$

• A. $Zn\left(s\right)+C{u}^{2+}\left(aq\right)\to Cu\left(s\right)+Z{n}^{2+}\left(aq\right)$
• B. $Zn\left(s\right)+2A{g}^{+}\left(aq\right)\to 2Ag\left(s\right)+Z{n}^{2+}\left(aq\right)$
• C. $H_2\left(g\right)+C{u}^{2+}\left(aq\right)\to 2H^{+}\left(aq\right)+Cu\left(s\right)$
• D. $H_2\left(g\right)+N{i}^{2+}\left(aq\right)\to 2H^{+}\left(aq\right)+Ni\left(s\right)$

Answer 1

The correct option is B $Zn\left(s\right)+2A{g}^{+}\left(aq\right)\to 2Ag\left(s\right)+Z{n}^{2+}\left(aq\right)$

Standard reduction potentials (SRP) of the half cell electrodes are calculated and arranged in a series of increasing order which is known as electrochemical series.
The standard electrode potential of $H^{+}/H_2$ couple is zero.

Here, a negative $E^0$ signifies that it has high tendency to get oxidised and the redox couple is a stronger reducing agent than the $H^{+}/H_2$ couple.

A positive $E^0$ signifies that it has high tendency to get reduced and the redox couple is a stronger oxidising agent than the $H^{+}/H_2$ couple.

Electrochemical series helps in the selection of electrode assemblies to construct the galvanic cells of desired EMFs.

For a galvanic cell to provide high cell voltage, the half cell having higher reduction potential and lower reduction potential should be connected.

In the given half cell reactions,
$A{g}^{+}\left(aq\right)+e^{-}\to Ag\left(s\right);\left(\text{Highest value}\right)$
$Z{n}^{2+}\left(aq\right)+2e^{-}\to Zn\left(s\right);\left(\text{Lowest value}\right)$

Hence, cell reaction providing maximum voltage will be
$Zn\left(s\right)+2A{g}^{+}\left(aq\right)\to 2Ag\left(s\right)+Z{n}^{2+}\left(aq\right)$