Question

In a cell of normal hydrogen electrode and calomel electrode, the electrode reaction taking place at calomel electrode will be

• A. $Hg _{2} Cl _{2(s)} \rightarrow 2 Hg _{(l)}+2 Cl ^{-}+2 e^{-}$
• B. $2 Hg _{(l)}+2 Cl ^{-} \rightarrow Hg _{2} Cl _{2(s)}+2 e^{-}$
• C. $Hg _{2} Cl _{2(s)}+2 e^{-} \rightarrow 2 Hg _{(l)}+2 Cl ^{-}$
• D. none of these.

Answer 1

Answer: (C)

A calomel electrode consists of mercury at the bottom over which a paste of mercury-mercurous chloride is placed. A solution of potassium chloride is then placed over the paste. A platinum wire sealed in a glass tube helps in making the electrical contact. The electrode is connected with the help of the side tube, on the left through the salt bridge with the other electrode to make complete cell. When a calomel electrode is used with a normal hydrogen electrode then the electrode reaction taking place when the electrode acts as cathode is

$\frac{1}{2} Hg _{2} Cl _{2}+e^{-} \rightleftharpoons Hg + Cl ^{-} $

or $ Hg _{2} Cl _{2}+2 e^{-} \rightleftharpoons 2 Hg _{(l)}+2 Cl ^{-}$