Question

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

Answer 1

Hint: i. Hydrolysis is the reaction, where a compound is chemically breakdown using water (${H_2}O$) molecules, and hydrolysis of ester yields 2 compounds, one of them has a carbonyl group(-c=o).
ii. The pH of a solution indicates its acidic and alkaline property of a given solution; it is calculated by the ant-log concentration of hydrogen ions present in the solution.

Complete Step by step answer:i). Ester is hydrolyzed under normal conditions and yields, alcohol and a carboxylic acid, we can write the general reaction as;
${\text{R}} - \mathop {\text{C}}\limits^{\mathop {{\text{||}}}\limits^{\text{O}} } - {\text{OR' + }}{{\text{H}}_{\text{2}}}{\text{O }} \rightleftharpoons {\text{ R}} - \mathop {\text{C}}\limits^{\mathop {{\text{||}}}\limits^{\text{O}} } - {\text{OH}} + {\text{ROH}}$
Due to the absence of any catalyst, the reaction is slow at first, but after some time as the reaction moves towards the right side, carboxylic acid acts as a catalyst and fastens ups the reaction, hence the rate of the reaction increases.

ii. Acetic acid is a weak acid otherwise known as vinegar, and it is only slightly ionized
${\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH}} \to {\text{C}}{{\text{H}}_3}{\text{CO}}{{\text{O}}^ - } + {\text{ }}{{\text{H}}_3}{{\text{O}}^ + }$
Sodium acetate is a strong strong electrolyte and dissociates into carboxylate and sodium ions;

${\text{C}}{{\text{H}}_{\text{3}}}{\text{COONa}} \to {\text{C}}{{\text{H}}_{\text{3}}}{\text{CO}}{{\text{O}}^ - }{\text{ + N}}{{\text{a}}^ + }$

And when it is added to acetic acid, the concentration of carboxylate ion increases in the product side, this results in the equilibrium to shift at the reactant side, consequently, the concentration of H ion decreases in the solution, and the overall pH of the solution increases, making it weaker acid.

Note: i. Usually, due to the slow pace of hydrolysis with water, we do not use water for ester hydrolysis, instead, a dilute acid is used. E.g: Propanoate, ethyl – ethanoate.
ii. The increase in the concentration of ${{\text{H}}^ + }$ ion results in the pH of the solution to be acidic and the increase in ${\text{O}}{{\text{H}}^ - }$ ion results in an alkaline solution.