Chapter 16, Problem 16.1QP

Use Le Châtelier’s principle to explain how the common ion effect affects the pH of a solution.

Interpretation Introduction

Interpretation:

By using Le Chatelier’s principle, the changes of pH of a weak acid solution due to common ion effect has to be explained.

Concept introduction:

• Le Chatelier's principle states that if a system in equilibrium gets disturbed due to modification of concentration, temperature, volume, and pressure, then it reset to counteract the effect of disturbance.
• When the concentration of one of the ions of a chemical solution got higher, it reacts with counter charged ions and precipitated out as salt till the ion product equals solubility product is called common ion effect.
• Incomplete dissociation of an acid in aqueous solution is called weak acid.

Answer to Problem 16.1QP

$\begin{array}{l}{\text{CH}}_{\text{3}}{\text{CO}}_{\text{2}}{\text{H}}_{\left(\text{aq}\right)}\rightleftharpoons {\text{ H}}^{\text{+}}{}_{\left(\text{aq}\right)}\text{ + }C{H}_{3}C{O}_2{{}^{-}}_{\left(\text{aq}\right)}\\ {\text{CH}}_{\text{3}}{\text{CO}}_{\text{2}}{\text{Na}}_{\left(\text{s}\right)}\to {\text{ Na}}^{\text{+}}{}_{\left(\text{aq}\right)}\text{ + }C{H}_{3}C{O}_2{{}^{-}}_{\left(\text{aq}\right)}\\ C{H}_{3}C{O}_2{}^{-}\text{is}\text{common}\text{ion}\text{.}\end{array}$

The pH of weak acid (acetic acid) increases due to addition of acetate ions from sodium acetate which suppress ionization of acetic acid leads to decrease in percent ionization of acetic acid.  The equilibrium shifts towards left because of more acetate ion (common ion).

Explanation of Solution

To explain: The changes of pH of a weak acid solution due to common ion effect.

The equilibrium reaction of acetic acid and sodium acetate are as follows,

$\begin{array}{l}{\text{CH}}_{\text{3}}{\text{CO}}_{\text{2}}{\text{H}}_{\left(\text{aq}\right)}\rightleftharpoons {\text{ H}}^{\text{+}}{}_{\left(\text{aq}\right)}\text{ + }C{H}_{3}C{O}_2{{}^{-}}_{\left(\text{aq}\right)}\\ {\text{CH}}_{\text{3}}{\text{CO}}_{\text{2}}{\text{Na}}_{\left(\text{s}\right)}\to {\text{ Na}}^{\text{+}}{}_{\left(\text{aq}\right)}\text{ + }C{H}_{3}C{O}_2{{}^{-}}_{\left(\text{aq}\right)}\\ C{H}_{3}C{O}_2{}^{-}\text{is }\text{common}\text{ion}\text{.}\end{array}$

In an aqueous solution, dissolve acetic acid (weak acid) thoroughly which dissociates as acetate ions and hydronium ions.  Then add sodium acetate (strong electrolyte) and it dissociates completely to form sodium ions and acetate ions.  The common ion produced from both acetic acid and sodium acetate is acetate ion.  By following the principle of Le Chatelier Principle, the acetate ions from sodium acetate combine with hydronium ions and the equilibrium shifts towards left which reduce the ionization of acetic acid.  Thus lowers the percent dissociation of acetic acid and due to decrease in hydrogen ion concentration the pH of the solution will increase.

Conclusion

The change of pH of a weak acid solution due to common ion effect was explained by Le Chatelier's principle.