Question

Process of rusting of iron is:
(A)- oxidation
(B)- reduction
(C)- corrosion
(D)- polymerization

Answer 1

Hint: Due to the attack of atmospheric gases, the surface of the metal is slowly coated by the oxides, sulphides, sulphates or other salts of the metal. This process of conversion of metal surface into such compounds by the action of the gases in air is called corrosion.

Complete step by step answer:
Rusting of iron occurs in the presence of air and water to convert iron (Fe) into hydrated iron oxides, i.e. rust. Rust is chemically hydrated ferric oxide and has the formula$F{e}_2{O}_3.x{H}_{2}O$.

Let us discuss the mechanism of the formation of rust.
The surface of the metal contains $H^{+}$ ions resulting from the dissociation of carbonic acid which is formed due to the dissolution of carbon dioxide from the atmosphere into the water vapors on the metal surface.
     $$\begin{array}{rl}& H_{2}O+C{O}_2\to H_{2}C{O}_3\\ & H_{2}C{O}_3\rightleftarrows 2H^{+}+C{O}_3^{2-}\end{array}$$

The particular spot on the surface of iron which is in contact with the oxygen and $H^{+}$ ions, behaves as anode and undergoes oxidation. Fe loses two electrons and gets converted into $F{e}^{2+}$ ions. The oxidation reaction involved is given below:
     $$Fe(s)\to F{e}^{2+}+2e^{-};E_{(F{e}^{2+}/Fe)}^o=0.44V$$

Electrons from the anode move to another spot on the metal surface and reduce $O_2$ in presence of $H^{+}$ ions. Reduction of oxygen takes place at this spot on the metal surface, therefore, this spot acts as a cathode.
     $$O_2(g)+4H^{+}(aq)+4e^{-}\to 2H_{2}O(l);E^o=1.23V$$

The overall reaction can be written by adding the above two reactions as:
     $$2Fe(s)+O_2(g)+4H^{+}(aq)\to 2F{e}^{2+}(aq)+2H_{2}O(l);E^o=1.67V$$

Now further oxidation of $F{e}^{2+}$ ions occurs by oxygen in the air to form ferric oxide, i.e. $F{e}_2{O}_3$ followed by its hydration to form rust, i.e. $F{e}_2{O}_3.x{H}_{2}O$. The

chemical reactions involved are given below:
     $$\begin{array}{rl}& 4F{e}^{2+}+O_2+4H_{2}O\to 2F{e}_2{O}_3+8H^{+}(aq)\\ & F{e}_2{O}_3+x{H}_{2}O\to F{e}_2{O}_3.x{H}_{2}O\end{array}$$

We already know that corrosion is the slow conversion of metal surface into metal salts like oxides, sulphates etc. So, we can say that rusting of iron is corrosion as in the process of rusting of iron, iron metal is slowly being eaten away due to the attack of oxygen and water resulting in the conversion of iron into its oxide.
So, the correct answer is “Option C”.

Note: Do not get confused between the options. We may simply think that rusting of iron is an oxidation reaction as it involves conversion of iron into iron oxide in presence of oxygen and mark option (A), which is wrong. It is only partially correct as the process of rusting of iron also involves reduction.