Chapter 16, Problem 16.39QP

Interpretation Introduction

Interpretation:

The reason why do we normally not quote $K_a$ values for strong acids such as $\text{HCl}$ and ${\text{HNO}}_{\text{3}}$, and it is necessary to specify temperature when giving $K_a$ values has to be explained

Concept Information:

Acids ionize in water.  Strong acids ionize completely whereas weak acids ionize to some limited extent.

The degree to which a weak acid ionizes depends on the concentration of the acid and the equilibrium constant for the ionization.

The ionization of a weak acid $\text{HA}$ can be given as follows,

  ${\text{HA}}_{(aq)}\rightleftarrows {\text{H}}^{+}{}_{(aq)}+{\text{A}}^{\text{-}}{}_{(aq)}$

The equilibrium expression for the above reaction is given below.

  $K_a=\frac{[{\text{H}}^{\text{+}}{\text{][A}}^{\text{-}}\text{]}}{[\text{HA]}}$

Where,

  $K_a$ is acid ionization constant,

  $[{\text{H}}^{\text{+}}\text{]}$  is concentration of hydrogen ion

  $[{\text{A}}^{\text{-}}\text{]}$  is concentration of acid anion

  $[\text{HA]}$ is concentration of the acid

The strength of an acid can be determined from the magnitude of ionization constant $K_a$

of that acid.

• When the $K_a$ is large, it indicates strong acid
• When the $K_a$ is small, it indicates weak acid