Question

Complete the following:
Electrolysis of aqueous magnesium chloride (Pt electrodes)

${\mathrm{MgCl}}_2\left(\mathrm{aq}\right)\rightleftharpoons \dots \dots .\dots \dots +\dots$

${\mathrm{H}}_2\mathrm{O}\rightleftharpoons ..........+..........$

At cathode:…….$+1e^{-}⟶$…..

…….+……. $\to$ ${\mathrm{H}}_2$
At anode: $C{l}^{-}\to$…….+…….

…..+…. $\to {\mathrm{Cl}}_2$

Answer 1

• Different products are obtained when Magnesium chloride $\left({\mathrm{MgCl}}_2\right)$ is electrolysed in molten state and in aqueous state.
• Here, electrolysis is carried out in aqueous solution i.e. ${\mathrm{MgCl}}_2$ is dissolved in water and then an electric current is passed through it.
• ${\mathrm{MgCl}}_2$ in water dissociates into ${\mathrm{Mg}}^{2+}\mathrm{and}{\mathrm{Cl}}^{-}$ions.
• Also pure water dissociates to ${\mathrm{H}}^{+}\mathrm{and}{\mathrm{OH}}^{-}$ions to a small extent.
• So, in this solutions there are two cations$({\mathrm{Mg}}^{2+}\mathrm{and}{\mathrm{H}}^{+})$ and two anions $({\mathrm{Cl}}^{-}\mathrm{and}{\mathrm{OH}}^{-})$.
• In electrolytic cell there are two electrodes connected to a battery.
• The electrode connected to a negatively terminal of the battery is called cathode and attracts cations as it electron rich electrode and the electrode connected to the positively charged terminal is called anode and attracts anions towards it.
• At Cathode donation of electrons takes place i.e reduction and at anode electrons are lost from the species i.e. oxidation takes place.
• At cathode, both the cations are attracted but only ${\mathrm{H}}^{+}$ ions get discharged as it has high reducing tendency than ${\mathrm{Mg}}^{2+}$ ions.
• At anode, both the anions gets attracted but ${\mathrm{Cl}}^{-}$ ions get discharged as it is easily oxidised than ${\mathrm{OH}}^{-}$ ions.
• Therefore, hydrogen gas is liberated at cathode and chlorine gas is liberated at anode.
• So the equations for dissociation of ${\mathrm{MgCl}}_2\mathrm{and}{\mathrm{H}}_2\mathrm{O}$ are as follows: $$\begin{gathered} {\mathrm{MgCl}}_2\left(\mathrm{aq}\right)\rightleftharpoons {\mathbf{Mg}}^{\mathbf{2}\mathbf{+}}\mathbf{}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{}\mathbf{+}\mathbf{}\mathbf{2}\mathbf{}{\mathbf{Cl}}^{\mathbf{-}}\mathbf{}\mathbf{\left(}\mathbf{aq}\mathbf{\right)} \\ {\mathrm{H}}_2\mathrm{O}\rightleftharpoons \mathbf{}{\mathbf{H}}^{\mathbf{+}}\mathbf{}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}\mathbf{}\mathbf{+}\mathbf{}{\mathbf{OH}}^{\mathbf{-}}\mathbf{}\mathbf{\left(}\mathbf{aq}\mathbf{\right)} \end{gathered}$$
  • Reaction at Cathode is as: $$\begin{gathered} {\mathbf{H}}^{\mathbf{+}}\mathbf{\left(}\mathbf{aq}\mathbf{\right)}+1{\mathrm{e}}^{-}\to \mathbf{}\mathbf{H}\mathbf{}\mathbf{\left(}\mathbf{g}\mathbf{\right)} \\ \mathbf{H}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{}\mathbf{+}\mathbf{}\mathbf{H}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\to {\mathrm{H}}_2\left(\mathrm{g}\right) \end{gathered}$$
  • Reaction at Anode is as: $$\begin{gathered} {\mathrm{Cl}}^{-}\left(\mathrm{aq}\right)\to \mathbf{Cl}\mathbf{}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{}\mathbf{+}\mathbf{}\mathbf{1}{\mathbf{e}}^{\mathbf{-}} \\ \mathbf{Cl}\mathbf{}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{}\mathbf{+}\mathbf{}\mathbf{Cl}\mathbf{}\mathbf{\left(}\mathbf{g}\mathbf{\right)}\mathbf{}\to {\mathrm{Cl}}_2\left(\mathrm{g}\right) \end{gathered}$$