Question

State whether entropy change is positive or negative in the following examples : (i) Melting of ice (ii) Vaporisation of a liquid Explain ‘common ion effect’ with example.

Hint:

Entropy generally increases as a substance goes from solid to liquid to gas. The common ion effect is a direct application of Le Chatelier's principle to ionic equilibria, crucial for understanding buffer solutions and solubility.

Answer 1

Entropy Change
Entropy is a measure of the randomness or disorder of a system.


[(i)] Melting of ice: This is a phase transition from a highly ordered solid state to a less ordered liquid state. The disorder increases, so the entropy change (\(\Delta S\)) is positive.
[(ii)] Vaporisation of a liquid: This is a phase transition from a less ordered liquid state to a highly disordered gaseous state. The disorder increases significantly, so the entropy change (\(\Delta S\)) is positive.
Common Ion Effect
The common ion effect is the suppression of the dissociation of a weak electrolyte upon the addition of a strong electrolyte that contains an ion in common with the weak electrolyte.
Example: Consider the dissociation of the weak acid, acetic acid (CH\(_3\)COOH):
\[ \text{CH}_3\text{COOH(aq)} \rightleftharpoons \text{H}^+\text{(aq)} + \text{CH}_3\text{COO}^-\text{(aq)} \]
If we add sodium acetate (CH\(_3\)COONa), a strong electrolyte, it dissociates completely into Na\(^+\) and CH\(_3\)COO\(^-\) ions. This increases the concentration of the common ion, acetate (CH\(_3\)COO\(^-\)). According to Le Chatelier's principle, the equilibrium of the acetic acid dissociation will shift to the left to counteract this increase, thus reducing the ionization of acetic acid.