Question

For rusting of iron to take place, ________ are required.

• A. H${}_2$O and O${}_2$
• B. CO${}_2$ and O${}_2$
• C. H${}_2$ and H${}_2$O
• D. N${}_2$ and H${}_2$O

Answer 1

The correct option is A H${}_2$O and O${}_2$

The rusting of Iron requires iron, water, and oxygen.
First step: Oxidation of solid iron
Dissolution of solid iron into solution
$Fe\left(s\right)\to F{e}^{2+}\left(aq\right)+2e^{-}$

$4e^{-}+4H^{+}\left(aq\right)+O_2\left(aq\right)\to 2H_{2}O\left(l\right)$
Second step: Formation of hydrated iron oxide(rust)
iron dissolves a predominance of hydroxide ions in the water to form green rust.
$F{e}^{2+}\left(aq\right)+2O{H}^{-}\left(aq\right)\to Fe\left(OH{)}_2\right(s)$

$4F{e}^{2+}\left(aq\right)+4H^{+}\left(aq\right)+O_2\left(aq\right)\to 4F{e}^{3+}\left(aq\right)+2H_{2}O\left(l\right)$
The overall reaction is written as:
$4Fe+3O_2+6H_{2}O\to 4Fe(OH{)}_3$

This compound dehydrates to become $F{e}_2{O}_3.H_{2}O$ (chemical formula for rust).