Question

The addition of sodium acetate to 0.1 M acetic acid will cause

• A. increase in its pH value
• B. decrease in its pH value
• C. no change in pH value
• D. change in pH which cannot be predicted

Answer 1

The correct option is A

increase in its pH value

Common ion effect:
The degree of ionisation of an electrolyte decreases in the presence of a strong electrolyte having a common ion. For example, ionisation of $C{H}_{3}COOH$ is suppressed in presence of HCl due to common $H^{+}$ ions.
In this case, adding acetate ion into the solution will drive the reaction backward.
$C{H}_{3}COOH$(aq) + $H_{2}0$ (l) $\leftrightharpoons$ ${C{H}_{3}COO}^{-}$(aq) + ${H_{3}O}^{+}$ (aq)
Adding more sodium acetate will yield more of the common acetate ion, hence suppressing the ionization of the weak acid. Concentration of ${H_{3}O}^{+}$decreases and hence pH increases.
A more formal an detailed definition of the common ion effect is :
To the solution of a weak electrolyte, if a solution of strong electrolyte is added which furnishes an ion common to that furnished by the weak electrolyte, the ionization of the weak electrolyte is suppressed.' For example, $N{H}_{4}OH$ is a weak electrolyte and ionizes to a small extent.
$N{H}_{4}OH$ $\leftrightharpoons$ ${N{H}_4}^{+}$ + ${OH}^{-}$

When a strong electrolyte furnishing ${N{H}_4}^{+}$ or ${OH}^{-}$ion is added (e.g., $N{H}_{4}Cl$ , NaOH, etc.) to $N{H}_{4}OH$solution, the above equilibrium will be shifted to the left because of high concentration of the common ion and thus the ionisation of $N{H}_{4}OH$is further suppressed. Similarly, the ionisation of $C{H}_{3}COOH$ is suppressed by the addition of $C{H}_{3}COONa$ or HCl. Thus in short; the suppression of ionization of a weak acid or a weak base by adding one of its own ions is known as common ion effect. Since the dissociation constant of an acid (or base) remains constant at a given temperature, the presence of a common ion decreases the degree of dissociation of an acid (or a base).