Question

In the rusting of Iron, which of the following cell reactions occurs at the anode?

• A. $Fe|F{e}^{2+}$
• B. $Fe|F{e}^{3+}$
• C. $O_2|H_{2}O$
• D. $F{e}^{3+}|F{e}^{2+}$

Answer 1

The correct option is A $Fe|F{e}^{2+}$

The theory of corrosion can be explained by taking an example of the rusting of iron. The theory is called electrochemical theory because it explains the formation of rust on the basis of the formation of electrochemical cells on the surface of the metal.
The formation of rust on the basis of this theory may be explained in the following steps:
a. The water vapours on the surface of the metal dissolve $C{O}_{2}and{O}_2$ from the air. Thus, the surface of metal is covered with the solution of $C{O}_2$ in water, i.e., carbonic acid $\left(H_{2}C{O}_3\right)$. $H_{2}O+C{O}_2\to H_{2}C{O}_3$ ....(i)
This acts as an electrolytic solution of the cell. The carbonic acid and water dissociate to a small extent as follows:
$H_{2}C{O}_3\rightleftharpoons 2H^{\oplus }+C{O}_3^{2-}{H}_{2}O\rightleftharpoons H^{\oplus }+O{H}^{\ominus }\cdots \left(ii\right)$
b. Iron in contact with dissolved CO2 and oxygen undergoes oxidation as follows: $Fe\left(s\right)\to F{e}^{2+}+2e^{-}(E_{ox}^{\ominus }=0.44V)\cdots \left(iii\right)$
Thus, the sites where the above reaction takes place act as anodes. As a result of the above reaction, iron is converted into ferrous $\left(F{e}^{2+}\right)$ ions.