Chemistry - Complete d-block Question with Solution | TestHub

Rusting of iron is an electrochemical phenomenon in which one spot of iron acts as anode.

$\mathrm{F}\mathrm{e}\left(\mathrm{s}\right)\to \mathrm{F}{\mathrm{e}}^{2+}\left(\mathrm{a}\mathrm{q}\right)+2{\mathrm{e}}^{-}$

These electrons move through the metal and are taken up by the oxygen already dissolved in water in presence of${\mathrm{H}}^{+}$furnished by${\mathrm{H}}_2\mathrm{C}{\mathrm{O}}_3$, which is formed by the dissolution of$\mathrm{C}{\mathrm{O}}_2\left(\mathrm{g}\right)$in water. The concentration of${\mathrm{H}}^{+}$depends upon the concentration of$\mathrm{C}{\mathrm{O}}_2$in water.

Cathode${\mathrm{O}}_2\left(\mathrm{a}\mathrm{q}\right)+4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}\to 2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$

Since increase in the concentration of${\mathrm{H}}^{+}$results in the increase in the rate of the reaction, therefore, rusting of iron occurs quicker, when the concentration of dissolved$\mathrm{C}{\mathrm{O}}_2$in water increases.

$\mathrm{F}{\mathrm{e}}^{2+}$so-obtained are further oxidized to$\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3$by atmospheric oxygen.$\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3$is then hydrated to form rust.

$\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+\mathrm{x}{\mathrm{H}}_2\mathrm{O}\to \underset{\left(\mathrm{R}\mathrm{u}\mathrm{s}\mathrm{t}\right)}{\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3.\mathrm{x}{\mathrm{H}}_2\mathrm{O}}$