Question

In the electrolytic process using Castner-Kellner cell, ${Hg}$ acts as an intermediate electrode by induction:
(A) True
(B) False

Answer 1

Hint: Castner-Kellner cells are used to produce sodium hydroxide from sodium chloride solution and use graphite, iron and mercury as electrodes.

Complete step by step answer:
The Castner-Kellner cell is used to produce sodium hydroxide by doing the electrolysis of aqueous sodium chloride solution (brine solution).
The cell consists of a rectangular tank that is divided into three compartments by two non-porous slate partitions that are dipped into a layer of mercury present at the bottom of the cell such that the mercury from one compartment can flow into another compartment that allows the conduction of the current between the three compartments along with the transportation of the sodium amalgam from the two peripheral compartments to the middle compartment without intermixing the solutions present in the individual compartments. The cell has a fulcrum on the left bottom and a rotating eccentric at the right bottom; they are used to set the mercury layer into motion.
The mercury layer acts as an intermediate electrode by induction since it acts as an anode in the middle compartment and as a cathode in the outer compartments.
The two outer compartments are filled with saturated brine solution and are provided with graphite anode. The middle compartment is filled with dilute sodium hydroxide solution and is provided with iron rods that act as cathode.
The circulation of electricity leads to the electrolysis of the brine solution in the outer compartments. Chlorine gas is liberated at the anodes while sodium ions are discharged at the mercury cathode forming sodium metal which combines with the mercury to produce the sodium amalgam. The discharge potential of sodium ions is lower than that of the proton in case of mercury electrode and therefore sodium metal is produced at the cathode rather than hydrogen gas.
$\begin{matrix} At\quad Anode(Carbon): \\ At\quad cathode(Hg): \end{matrix}\begin{matrix} 2{ Cl }^{ - }(aq)\longrightarrow { Cl }_{ 2 }(g)+2{ e }^{ \_ } \\ 2{ Na }^{ + }(aq)+2{ e }^{ - }\longrightarrow 2Na(Hg)(s) \end{matrix}$
$Net\quad reaction:\quad 2Nacl(aq)\longrightarrow { Cl }_{ 2 }(g)\quad +\quad Na(Hg)(s)$
The sodium amalgam formed is transferred to the middle compartment by rocking the cell with the use of the rotating eccentric which then acts as an anode and the iron rods act as the cathode. The sodium in the sodium amalgam gets oxidised to sodium ions while hydrogen is liberated at the cathode.
$\begin{matrix} At\quad anode(Hg): \\ At\quad cathode(Iron): \\ Net\quad reaction: \end{matrix}\begin{matrix} 2Na(Hg)(s)\longrightarrow 2{ Na }^{ + }(aq)+2{ e }^{ - } \\ 2{ H }_{ 2 }O+2{ e }^{ - }\longrightarrow 2{ OH }^{ - }(aq)+{ H }_{ 2 }(g) \\ 2Na(Hg)(s)+2{ H }_{ 2 }O\longrightarrow 2{ Na }OH+{ H }_{ 2 }(g) \end{matrix}$
The concentration of sodium hydroxide in the middle compartment increases while the concentration of sodium chloride in the outer compartments decreases with time.
Hence the statement that mercury acts as an intermediate electrode by induction in Castner-Kellner cell is A. True.
Note: It is because of the mercury electrode that the sodium ions are getting reduced to give sodium metal instead of water reducing to give hydrogen gas since the discharge potential of sodium ions in lower than that of protons in the case of a mercury electrode while it is usually the opposite for other electrodes.