Explain the following:
(i) Hydrolysis of ester (ethyl acetate) begins slowly but becomes fast after sometime.
(ii) The pH value of acetic acid increases on addition of a few drops of sodium acetate.

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Solution

(i) Hydrolysis of ester (ethyl acetate) begins slowly but becomes fast after sometime.
Initially catalytic amount of an acid is added to ethyl acetate. Hence, the hydrolysis reaction begins slowly. With passage of time, more and more acetic acid is formed during the hydrolysis. This acetic acid acts as a catalyst and increases the rate of the reaction.
$C H_{3} C O O C_{2} H_{5} + H_{2} O H^{+} - - \to C H_{3} C O O H + C_{2} H_{5} - O H$
(ii) The pH value of acetic acid increases on addition of a few drops of sodium acetate
Sodium acetate is a strong electrolyte and completely dissociates to provide acetate ions.
$C H_{3} - C O O N a \to C H_{3} - C O O^{-} + N a^{+}$
Acetic acid is weak electrolyte and partially dissociates.
$C H_{3} - C O O H ⇌ C H_{3} - C O O^{-} + H^{+}$
The dissociation of sodium acetate provides acetate ions that are common to the dissociation of acetic acid. Due to common ion effect, the dissociation of acetic acid is suppressed. This decreases the hydrogen ion concentration and increases the pH of the solution.

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