Question

The chemical process involved in the corrosion of iron is _______.

• A. redox reaction
• B. decomposition reaction
• C. displacement reaction
• D. double displacement reaction

Answer 1

The correct option is A

redox reaction

A

• The slow deterioration of the surface of any metal by its reaction with air, moisture or any chemical substance is termed as corrosion.
• Corrosion in the case of Iron $\left(\mathrm{Fe}\right)$ is termed as rusting.
• Iron $\left(\mathrm{Fe}\right)$ undergoes reaction with Oxygen gas $\left({\mathrm{O}}_2\right)$ in air and Water $\left({\mathrm{H}}_2\mathrm{O}\right)$ to form hydrated Iron(III) oxide $({\mathrm{Fe}}_2{\mathrm{O}}_3·{\mathrm{H}}_2\mathrm{O})$. This brown colored hydrated Iron(III) oxide is termed as rust.

$4\mathrm{Fe}\left(\mathrm{s}\right)+3{\mathrm{O}}_2\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)\to 2{\mathrm{Fe}}_2{\mathrm{O}}_3·{\mathrm{H}}_2\mathrm{O}\left(\mathrm{s}\right)$

• The reaction corresponding to the gain of Oxygen (O) or loss of Hydrogen (H) by a species is called an oxidation reaction. On the other hand, the reaction corresponding to the loss of Oxygen or gain of Hydrogen by a species is called a reduction reaction.
• Oxidation is accompanied by increase in the oxidation number while reduction is accompanied by decrease in the oxidation number.
• The reaction in which oxidation and reduction reactions take place simultaneously are known as redox reactions.
• In the corrosion of iron, the oxidation state of Fe changes from zero in Fe to +3 in ${\mathrm{Fe}}_2{\mathrm{O}}_3·{\mathrm{H}}_2\mathrm{O}$, hence it is undergoing oxidation. Whereas, the oxidation state of O changes from 0 in ${\mathrm{O}}_2$ to -2 in ${\mathrm{Fe}}_2{\mathrm{O}}_3·{\mathrm{H}}_2\mathrm{O}$, hence it is undergoing reduction. Since reduction and oxidation takes place simultaneously, the given reaction is a redox reaction.

B

• The reaction in which either two or more than two products are formed from a single reactant molecule is termed as decomposition reaction. Example:

$\underset{\mathrm{Calcium}\mathrm{carbonate}\mathrm{or}\mathrm{Limestone}}{{\mathrm{CaCO}}_3\left(\mathrm{s}\right)}\stackrel{∆}{\to }\underset{\mathrm{Calcium}\mathrm{oxide}\mathrm{or}\mathrm{Quicklime}}{\mathrm{CaO}\left(\mathrm{s}\right)}+\underset{\mathrm{Carbon}\mathrm{dioxide}}{{\mathrm{CO}}_2}$

• Due to the presence of three reactant molecules $\left(\mathrm{Fe},{\mathrm{O}}_2\mathrm{and}{\mathrm{H}}_2\mathrm{O}\right)$, corrosion of iron is not classified as decomposition reaction.

C

• In a displacement reaction or displacement reaction, an element from its compound is substituted by another element to form two product species. Example:

$\underset{\mathrm{Copper}\mathrm{sulfate}}{{\mathrm{CuSO}}_4}\left(\mathrm{aq}\right)+\underset{\mathrm{Zinc}}{\mathrm{Zn}\left(\mathrm{s}\right)}\to \underset{\mathrm{Zinc}\mathrm{sulfate}}{{\mathrm{ZnSO}}_4\left(\mathrm{aq}\right)}+\underset{\mathrm{Copper}}{\mathrm{Cu}\left(\mathrm{s}\right)}$

• Due to presence of only product molecule $\left({\mathrm{Fe}}_2{\mathrm{O}}_3·{\mathrm{H}}_2\mathrm{O}\right)$, corrosion of iron is not classified as a displacement reaction.

D

• In double displacement reaction, either cations or anions of two reactant compounds are interchanged with each other to form the two new compounds.

$\underset{\mathrm{Hydrochloric}\mathrm{acid}}{\mathrm{HCl}}\left(\mathrm{aq}\right)+\underset{\mathrm{Sodium}\mathrm{hydroxide}\mathrm{or}\mathrm{Caustic}\mathrm{soda}}{\mathrm{NaOH}}\left(\mathrm{aq}\right)\to \underset{\mathrm{Sodium}\mathrm{chloride}}{\mathrm{NaCl}}\left(\mathrm{aq}\right)+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}}\left(\mathrm{l}\right)$

• Due to presence of only product molecule $\left({\mathrm{Fe}}_2{\mathrm{O}}_3·{\mathrm{H}}_2\mathrm{O}\right)$, corrosion of iron is not classified as double displacement reaction.

Hence, option A is correct. Corrosion of iron is redox reaction.