Question

If you want to electrolyse concentrated $\mathrm{HCl}$, which of the following will you choose for making the anode?

• A. Graphite
• B. Aluminium
• C. Iron
• D. Copper

Answer 1

The correct option is A

Graphite

Explanation for the correct options

(a) Graphite is used for the electrolysis of concentrated $\mathrm{HCl}$.

Electrolysis is the decomposition of compound by passing electric current in its aqueous or molten state.

The electrolysis of concentrated $\mathrm{HCl}$ provides high concentration of ${\mathrm{H}}^{+}$ ions and ${\mathrm{Cl}}^{-}$ ions in the solution.

$$\begin{gathered} \mathrm{HCl}\left(\mathrm{aq}\right)\rightleftharpoons {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{aq}\right) \\ \mathrm{hydrogen}\mathrm{hydrogen}\mathrm{chloride} \\ \mathrm{chloride}\mathrm{ion}\mathrm{ion} \end{gathered}$$

Water contains covalent ${\mathrm{H}}_2\mathrm{O}$ molecules which undergo self-ionization.

$$\begin{gathered} {\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\rightleftharpoons {\mathrm{H}}^{+}\left(\mathrm{aq}\right)+{\mathrm{OH}}^{-}\left(\mathrm{aq}\right) \\ \mathrm{water}\mathrm{hydroxide}\mathrm{ion} \end{gathered}$$

1. Metals are very reactive towards an acid. So, we need an inert electrode which do not react with concentrated $\mathrm{HCl}$.

2. Graphite is an inert electrode used in electrolysis.

Explanation for the incorrect options

Reactivity series is the arrangement different metals in order of their reactivity.

Some of them are : $\mathrm{Al}>\mathrm{Zn}>\mathrm{Fe}>\mathrm{H}>\mathrm{Cu}>\mathrm{Ag}$

(b) Aluminium is a highly reactive metal.

(c) Iron is a less reactive than aluminum but react in the electrolytic solution of concentrated $\mathrm{HCl}$.

(d) Copper is present at the lower position in reactivity series but it also react in the solution.

Hence, option (A) is correct. Graphite will be choose for making anode in electrolyse concentrated $\mathrm{HCl}$.