Chapter 18, Problem 18.115SP

Interpretation Introduction

Interpretation:

The $pH$ of the solution affects the formation of rust should be explained.

Concept introduction:

• $pH$: - It is defined as the negative logarithm of the hydrogen ion concentration in the aqueous solution.

$pH=-\log \left[H^{+}\right]$
• Rust is an example of corrosion which is the process of slowly eating away of metal due to atmospheric gases on metal surface which results in the creation of compounds such as oxides, sulphides etc.
• The cause of formation of rust is based on the concept of electrochemical theory of rusting as it explains the formation of rust based on the formation of electrochemical cells on the metals surface. The formation of rust takes place on the surface of iron metal due to attack of the atmospheric gases on the surface of the metal resulting into the formation of iron oxides called rust. It can be further explained by electrochemical theory of rusting where a small electrochemical cell is formed during rusting in which pure iron acts as anode, impure iron acts as cathode and moisture containing air acts as electrolytic solution.

The following reaction takes place:

Reaction at anode:

$Fe\to F{e}^{2+}+2e\to \left(eqn1\right)$

Reaction at cathode:

$2H^{+}+\frac{1}{2}{O}_2+2e^{-}\to H_{2}O\to \left(eqn2\right)$

The overall cell reaction

$Fe+2H^{+}+\frac{1}{2}{O}_2\to F{e}^{2+}+H_{2}O$

Now, ferrous iron $(F{e}^{2+})$ is oxidized by the atmospheric oxygen and water to form rust

$\begin{array}{l}F{e}^{2+}+O_2+H_{2}O\to F{e}_2{O}_3+H^{+}\\ F{e}_2{O}_3+x{H}_{2}O\to F{e}_2{O}_3.x{H}_{2}O\end{array}$

This $F{e}_2{O}_3.x{H}_{2}O$ is called rust