Chapter 18, Problem 78A

Use Le Chátelier's principle to explain what happens to the equilibrium ${\text{H}}_2\text{O}(\text{l})\rightleftharpoons {\text{H}}^{+}(\text{aq})+{\text{OH}}^{-}(\text{aq})$ when a few drops of HCl are added to pure water.

Interpretation Introduction

Interpretation:

The effect on equilibrium of pure water is to be discussed when a few drops of $HCl$ are added to it.

Concept introduction:

Le Chatelier’s principle states that whenever there is a change in temperature, pressure or concentration of a reaction, equilibrium shifts in a direction to nullify the effect.

Answer to Problem 78A

Equilibrium will shift towards left side.

Explanation of Solution

Water dissociates as:

$H_{2}O\rightleftharpoons H^{+}+O{H}^{-}$

When $HCl$ is added to it, it dissociates in water as:

$HCl\to H^{+}+C{l}^{-}$

Thus on adding $HCl$ concentration of hydrogen ions is increased. Therefore, according to Le Chatelier’s principle, equilibrium shifts in backward direction.

Conclusion

Thus, equilibrium will shift towards left side on adding $HCl$ to pure water.