Ionic Equilibrium In Solution

INTRODUCTION

Most of the chemical reactions occur in solutions. When a given substance is dissolved in the solvents like water. In an ionic solution the substance splits up into ions whereas in molecular solution, the substance remains as s1uch. Both the solutions can be represented as

$XY(s)+aqXY(aq)X^{+}(aq)+Y^{-}(aq)$ (for ionic solution)

$XY(s)+aqXY(aq)$ (for molecular solution)

The substances are classified regarding this concept is;

1. Electrolytes and

2. Non-electrolytes

1. Electrolyte (conducting)

Those substance whose aqueous solution or molten form can conduct electricity.

They are further classified into

(a) Strong electrolyte: Those substance whose aqueous solution or molten form conduct electricity to a greater extent. They almost completely ionised in water.

e.g. NaCl, H₂SO₄, HCl, NaOH, NH₄Cl

Since strong electrolyte completely ionised in aqueous solution so their ionisation is represented as

$HCl+aq\to H^{+}(aq)+C{l}^{-}(aq)$

(b) Weak electrolyte: Those substance whose aqueous solution or molten form conducts electricity to a lesser extent. They do not completely ionised in water i.e. partly ionised. They behaves as poor conductor of electricity.

e.g. when CH₃COOH is dissolved in water, it is ionised partly and an equilibrium is setup between the ions and the unionised electrolyte.

$C{H}_{3}COOH+aqC{H}_{3}CO{O}^{-}(aq)+H^{+}(aq)$

2. Non-electrolyte (non-conducting)

Those substance whose aqueous solution or molten form does not conduct electricity to any extent. They are bad conductor of electricity.

e.g. aqueous solution of sugar, urea, etc. do not conduct electricity.

DEGREE OF IONISATION '$\alpha$'

It may be defined as "fraction of total number of molecules which dissociates into ion". It is represented by $\alpha$.

$\alpha =\frac{\text{Number}\text{of}\text{molecules}\text{dissociated}\text{as}\text{ions}}{\text{Total}\text{number}\text{of}\text{molecules}\text{dissolved}}$

$=\frac{\text{Number}\text{of}\text{moles}\text{dissociated}}{\text{Total}\text{number}\text{of}\text{moles}\text{dissolved}}$

Ostwald dilution law (Ionisation of weak electrolytes)

It is the law of mass action as applied to weak electrolytes like CH₃COOH, NH₄OH, HCN, etc.

Consider a binary electrolyte AB which dissociates into its ions, there exist a dynamic equilibrium between ions and unionised molecule of the electrolyte as

$\begin{align} & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,AB+aq \rightleftharpoons {{A}^{+}}(aq)+{{B}^{-}}(aq)\,\,\,\,\,\,\,\,...........\,(1) \\ & \text{Initial}\,\,\text{conc}\text{.} \\ & \text{when}\,\,\text{t}=0\,\,\,\,\,\,\,\,\,\,\,\,C\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,0\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,0 \\ & \,\,\text{Conc}\text{.}\,\,\text{at}\, \\ & \text{equilibrium}\,\,\,\,\,\,\,\,C-C\alpha \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,C\alpha \,\,\,\,\,\,\,\,\,\,\,\,C\alpha \\ \end{align}$Apply the law of mass action to equilibrium (1)

$K=\frac{[A^{+}(aq)][B^{-}(aq)]}{[AB]}$

$=\frac{C\alpha \times C\alpha }{[C-C\alpha ]}$

$=\frac{C{\alpha }^2}{1-\alpha }$

For weak electrolyte, $\alpha <<<1,\Rightarrow 1-\alpha \approx 1$i.e. $\alpha$ < 5%

$K=C{\alpha }^2$

or $\alpha =\sqrt{\frac{K}{C}}$ ……… (2)

If one mole of electrolyte is dissolved in V litre of solution i.e. $C=\frac{1}{V}$

$\alpha =\sqrt{KV}$………… (3)

Thus "degree of dissociation of a weak electrolyte is proportional to the square root of dilution". This is called Ostwald law.

Note:

If $\alpha$ > 5% then we can not neglect $\alpha$ from denominator.

$K=\frac{C{\alpha }^2}{(1-\alpha )}$

$\alpha$ can be calculate by quadratic equation (ax² + bx + c = 0)

$C{\alpha }^2-(1-\alpha )K=0$

$\alpha =\frac{-b\pm \sqrt{b^2-4ac}}{2a}$

Here $b=-(1-\alpha )$

a = C and c = 0

COMMON ION EFFECT

Considering the dissociation of weak electrolyte AB with a salt containing a common ion [A⁺] or [B^–].

Salt AC is completely ionised, as a result concentration of A⁺ ions in equilibrium increases. By Le-chateliers principle, the addition of A⁺ ion to an equilibrium, having common ion makes the equilibrium in backward direction i.e. dissociation of AB electrolyte is suppressed. This is called common ion effect.

In other words "The degree of dissociation of weak electrolyte is suppressed by the addition of strong electrolyte containing a common ion is called common ion effect".

Example: To the solution of weak acid CH₃COOH if its salt CH₃COONa is added. The dissociation of CH₃COOH is suppressed.

Illustration 1. Give reason that acetic acid is less acidic in sodium acetate solution than in sodium chloride solution.

Solution: Ionisation of acetic acid is suppressed in sodium acetate due to common ion effect. While in sodium chloride, no such effect is noticed.