Ncert Exercises & Solutions

6.25 What is the role of graphite rod in the electrometallurgy of aluminium?

In the electrometallurgy of aluminium, a fused mixture of purified alumina (

A l_{2} O_{3}

), cryolite (

N a_{3} A I F_{6}

) and fluorspar (

C a F_{2}

) is electrolysed. In this electrolysis, graphite is used as the anode and graphite-lined iron is used as the cathode. During the electrolysis, Al is liberated at the cathode, while CO and

C O_{2}

are liberated at the anode, according to the following equation.

$C a t h o d e : A {l^{3 +}}_{(m e l t)} + 3 e^{-} \to A l_{(l)}$

$A n o d e : C_{(s)} + {O^{2 -}}_{(m e l t)} \to C O_{(g)} + 2 e^{-}$

$C_{(s)} + 2 {O^{2 -}}_{(m e l t)} \to C O_{2 (g)} + 4_{e^{-}}$

If a metal is used instead of graphite as the anode, then $O_{2}$ will be liberated. This will not only oxidise the metal of the electrode, but also convert some of the Al liberated at the cathode back into $A l_{2} O_{3}$ . Hence, graphite is used for preventing the formation of $O_{2}$ at the anode. Moreover, graphite is cheaper than other metals.

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