Question

In rusting of iron, iron has been:
A.Oxidized
B.Reduced
C.Vaporized
D.Decomposed

Answer 1

Hint: Rusting of iron is the most common example of corrosion, which is the process of slowly eating away the metal resulting in the formation of compounds such as oxides, sulphides etc. During the rusting of iron, it loses electrons and is converted into ferrous ions. This loss of electrons takes place at sites which act as anodes.

Complete step by step answer:
The formation of rust can be explained on the basis of the electrochemical theory in the following steps.
-The water vapours present at the surface of the iron metal dissolve the carbon dioxide and oxygen from the air. Thus, the surface of metal is covered with the solution of carbon dioxide in water, i.e., carbonic acid.
${{\text{H}}_{\text{2}}}{\text{O + C}}{{\text{O}}_{\text{2}}} \to {{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}}$
-This acts as an electrolytic solution of the cell and the carbonic acid dissociates into hydrogen ions.
${{\text{H}}_{\text{2}}}{\text{C}}{{\text{O}}_{\text{3}}} \rightleftharpoons 2{{\text{H}}^ + } + {\text{C}}{{\text{O}}_{\text{3}}}^{2 - }$
-The iron metal in contact with the dissolved oxygen and the presence of hydrogen ions undergoes oxidation as follows:
${\text{Fe}} \to {\text{F}}{{\text{e}}^{2 + }} + 2{{\text{e}}^{\text{ - }}}$
-Thus, the sites where the above reaction occurs act as anodes.
-The electrons lost by iron migrate through the iron metal to another site on the surface and reduce the dissolved oxygen in the presence of hydrogen ions.
${{\text{O}}_{\text{2}}}{\text{ + 4}}{{\text{H}}^{\text{ + }}}{\text{ + 4}}{{\text{e}}^{\text{ - }}} \to 2{{\text{H}}_{\text{2}}}{\text{O}}$
The sites where the above reaction takes place act as cathodes.
-Thus, the overall reaction obtained by combining the oxidation and reduction half reactions will be
${\text{2Fe}} + {{\text{O}}_{\text{2}}}{\text{ + 4}}{{\text{H}}^{\text{ + }}} \to 2{\text{F}}{{\text{e}}^{2 + }} + 2{{\text{H}}_{\text{2}}}{\text{O}}$
The ferrous ions react with dissolved oxygen or oxygen from air to form ferric oxide.
$4{\text{F}}{{\text{e}}^{2 + }} + {{\text{O}}_{\text{2}}}{\text{ + 4}}{{\text{H}}_{\text{2}}}{\text{O}} \to 2{\text{F}}{{\text{e}}_2}{{\text{O}}_3} + 8{{\text{H}}^{\text{ + }}}$
The ferric oxide then undergoes hydration to form hydrated ferric oxide which is called rust.
${\text{F}}{{\text{e}}_2}{{\text{O}}_3}{\text{ + x}}{{\text{H}}_{\text{2}}}{\text{O}} \to {\text{F}}{{\text{e}}_2}{{\text{O}}_3}.{\text{x}}{{\text{H}}_{\text{2}}}{\text{O}}$
Hence, iron is oxidized in the rusting of iron.

So, the correct option is A.

Note:
Rust is a non-sticking compound which means that rust does not stick to the surface. Instead, rust peels off exposing fresh iron surface for further rusting. The rust is often formed at the sites acting as cathodes since cathode is generally the area having the largest amount of dissolved oxygen. Rust is generally formed at a site other than the site where pitting has occurred.