Chapter 18.4, Problem 45LC

Interpretation Introduction

Interpretation: The explanation for how the common ion effect illustrates Le Chatelier’s principle is to be stated.

Concept Introduction: Le Chatelier’s principle states that if stress is applied to a system that is in dynamic equilibrium then the system changes in a way that relieves the stress.

Answer to Problem 45LC

If a common ion is added to a weak acid or weak base then it suppresses the ionization of a weak acid that is the product of the equilibrium. In this way, the addition of a common ion affects the equilibrium.

Explanation of Solution

The chemical equation for the reaction of ammonia with water to give ammonium ion and hydroxide ion is given as;

  ${\text{NH}}_{\text{3}}+{\text{H}}_{\text{2}}\text{O}\rightleftharpoons {\text{NH}}_4^{+}+{\text{OH}}^{\text{-}}$

If ammonium chloride is added to the above reaction, the concentration of common ammonium ions increases to a greater extent as ammonium chloride is highly ionizable.

According to Le Chatelier’s principle, the equilibrium will first shift to the left and thus, the ionization of ammonia suppresses and it becomes a weaker base.