Question

The following reaction occurs at cathode during the electrolysis of aqueous sodium chloride solution.$Na_{aq}^ + + {e^ - } \to Na\left( s \right),{E^0} = - 2.71V$
$H_{aq}^ + + {e^ - } \to \dfrac{1}{2}H\left( g \right),{E^0} = 0.00V$
On the basis of their standard reduction potential values, which reaction is feasible at the cathode and why?
Why does the cell potential of mercury remain constant throughout its life?

Answer 1

Hint:
Electrode potential is the electromotive force of the cell that has two electrodes. Standard reduction potentials are calculated under standard temperature and pressures.During electrolysis there is a redox reaction which takes place. The reduction takes place at cathode and oxidation at anode.

Complete step by step answer:
The standard reduction potentials are defined as the potential of the half cell as the standard hydrogen potential.
It is denoted by the symbol ${E^0}$ .
Standard hydrogen electrode is a gas electrode which is used as a reference electrode for determining the standard electrode potential of elements and the other half cells.

$E_{N{a^ + }/Na}^0 = - 2.71V$
$E_{{H^ + }/{H_2}}^0 = 0.00V$
The standard reduction potential of $N{a^ + }$ is less than that of hydrogen.
From this we get to know that there will be reduction of hydrogen and oxidation of sodium.
So there will be a reduction of hydrogen at cathode as the reduction potential of hydrogen is greater than sodium.
Therefore the reduction reaction is given below as follows:
$H_{aq}^ + + {e^ - } \to \dfrac{1}{2}H\left( g \right)$
In a mercury cell there is a container made up of zinc.
In that container there is a cathode made up of carbon rod and there is a paste of mercuric oxide with $KOH$ as an electrolyte.
In mercury cell mercury oxide reacts with zinc in the presence of an alkali.
Thus in this cell there is no overall reduction taking place as it does not contain ions whose concentration will change.
As its potential does not change, the life span of mercury cells is greater than dry cells.
That is why the cell potential of mercury remains constant throughout its life.

Note: Standard electrode potentials are only applied to aqueous equilibrium only. From standard reduction potential we can only possibly predict the type of reaction and not the rate of reaction. In mercury cells $KOH$ is used as an electrolyte so there might be a chance of it spilling out of the cell due to which there is less ease of handling.