6.9 Ionic Equilibrium in Solution

There are numerous equilibria that involve ions only. In the following sections, we will study the equilibria involving ions. It is well known that the aqueous solution of sugar does not conduct electricity. However, when common salt (sodium chloride) is added to water it conducts electricity. Also, the conductance of electricity increases with an increase in the concentration of common salt. Michael Faraday classified the substances into two categories based on their ability to conduct electricity. One category of substances conduct electricity in their aqueous solutions and are called electrolytes while the other do not and are thus, referred to as nonelectrolytes. Faraday further classified electrolytes into strong and weak electrolytes. Strong electrolytes on dissolution in water are ionized almost completely, while weak electrolytes are only partially dissociated.

For example, an aqueous solution of sodium chloride is comprised entirely of sodium ions and chloride ions, while that of acetic acid mainly contains unionized acetic acid molecules and only some acetate ions and hydronium ions. This is because there is almost \(100 \%\) ionization in case of sodium chloride as compared to less than \(5 \%\) ionization of acetic acid which is a weak electrolyte. It should be noted that in weak electrolytes, equilibrium is established between ions and the unionized molecules. This type of equilibrium involving ions in aqueous solution is called lonic equilibrium. Acids, bases and salts come under the category of electrolytes and may act as either strong or weak electrolytes. Another example of equilibrium involving ions is given below.

\(
\mathrm{Fe}^{3+}(\mathrm{aq})+\mathrm{SCN}^{-}(\mathrm{aq}) \rightleftharpoons[\mathrm{Fe}(\mathrm{SCN})]^{2+}(\mathrm{aq})
\)

Types of Electrolytes

• Strong Electrolytes: Compounds which completely ionise in aqueous solution and are good conductors of electricity e.g. \(\mathrm{HCl}, \mathrm{H}_2 \mathrm{SO}_4, \mathrm{HNO}_3, \mathrm{NaOH}, \mathrm{KOH}, \mathrm{CH}_3 \mathrm{COONa}, \mathrm{NH}_4 \mathrm{Cl}\), etc. \((\alpha=100 \%)\)
• Weak Electrolytes: Compounds which ionise partially in aqueous solution and are poor conductors of electricity e.g. \(\mathrm{CH}_3 \mathrm{COOH}, \mathrm{HCN}, \mathrm{NH}_4 \mathrm{OH}, \mathrm{Al}(\mathrm{OH})_3\), etc. \((\alpha<100 \%)\) In weak electrolytes, equilibrium is established between ions and unionized molecules. This type of equilibrium involving ions in aqueous solution is called ionic equilibrium.

Ionic Equilibrium Formulas

It becomes necessary to know what fraction of the initial amount of the reactants is converted into products at equilibrium. The fraction of the initial molecules that are converted at equilibrium is called the degree of dissociation/ionization.

Degree of dissociation or ionization \(=\alpha=\) (Number of reactant molecules dissociated \ionized at the start)/(Number of reactant molecules at the start).

The degree of dissociation in ionic equilibrium can be expressed in percentage.
\(\%\) Degree of dissociation or ionization \(=\alpha=\) (Number of reactant molecules dissociated or ionized at the start)/(Number of reactant molecules at the start) \(\times\) 100.

Degree of lonization

The degree of ionization depends on the following:

• Nature of the electrolyte: Strong, weak, insoluble.
• Nature of the solvent: High dielectric solvents increase ionization.
• Dilution: Larger the dilution higher the ionization.
• Temperature: Higher the temperature, the larger the ionization.
• The presence of common ions decreases the ionization of the weak electrolyte

Common Ion Effect on Degree of Dissociation

Weak electrolytes are poorly ionized in an aqueous solution. Their ionization may further be reduced if one of the ions is present from another source. This is called a common ion effect.

(i) Ammonium hydroxide is a weak base. When adding ammonium chloride (a salt), ammonium ions from it will make the ammonium ions combine with the hydroxide to form unionised ammonium hydroxide.
\(
\begin{aligned}
& \mathrm{NH}_4 \mathrm{Cl} \rightarrow \mathrm{NH}_4^{+}+\mathrm{Cl}^{-} \\
& \mathrm{NH}_4 \mathrm{OH} \rightleftharpoons \mathrm{NH}_4^{+}+\mathrm{OH}^{-}
\end{aligned}
\)

(ii) In the base hydrolysis of oil, the sodium salt of the fatty acid (soap) is in a dissolved state.

When sodium chloride salt is added, the concentration of \(\mathrm{Na}+\) ions increases considerably.
\(
\begin{aligned}
& \mathrm{C}_n \mathrm{H}_{2 \mathrm{n}+1}+\mathrm{COONa} \rightleftharpoons \mathrm{C}_n \mathrm{H}_{2 \mathrm{n}+1} \mathrm{COO}^{-}+\mathrm{Na}^{+} \\
& \mathrm{NaCl} \rightleftharpoons \mathrm{Na}^{+}+\mathrm{Cl}^{-}
\end{aligned}
\)

Hence, the ionic product \(\left[\mathrm{C}_n \mathrm{H}_{2 n+1} \mathrm{COO}^{-}\right]\left[\mathrm{Na}^{+}\right]\)exceeds the solubility product of soap and, therefore, soap precipitates out from the solution. This is called salting out of soap.

(iii) Manufacture of sodium bicarbonate (baking soda)

In Solvay’s soda process. The \(\mathrm{CO}_2\) gas is passed through ammonical brine to precipitate out \(\mathrm{NaHCO}_3\).
\(
\mathrm{NH}_4 \mathrm{OH}+\mathrm{CO}_2 \rightarrow \mathrm{NH}_4 \mathrm{HCO}_3
\)
\(
\mathrm{NH}_4 \mathrm{HCO}_3+\mathrm{NaCl} \rightarrow \mathrm{Na} \mathrm{HCO}_3+\mathrm{NH}_4 \mathrm{Cl}
\)
\(\mathrm{NaHCO}_3\) is precipitated first because of its lower solubility product as compared to those of \(\mathrm{NH}_4 \mathrm{Cl}, \mathrm{NH}_4 \mathrm{HCO}_3\) and \(\mathrm{NaCl}\).