# What does the ionization constant of an acid or a base indicate about either the acid or the base?

Jul 6, 2018

It tells you about the strength of the acid or base in a solution, meaning the extent of the forward reaction compared to the reverse reaction.

#### Explanation:

Let's look at the ionization of an acid, HA (all species here are aqueous):

$H A r i g h t \le f t h a r p \infty n s {H}^{+} + {A}^{-}$

In aqueous solution, the acid, $H A$ dissociates into ${H}^{+}$ and ${A}^{-}$. The reaction is reversible and the chemical species, $H A$, ${H}^{+}$ and ${A}^{-}$ are in equilibrium when the forward and reverse reactions are equal rates, giving the appearance of constant concentrations.

Therefore, the acid ionization constant can be determined from this equation:

${K}_{a} = \frac{\left[{H}^{+}\right] \left[{A}^{-}\right]}{\left[H A\right]}$

where the square brackets indicate the equilibrium concentration for each chemical species.

As the strength of acid increases, $H A$ will dissociate more into ${H}^{+}$ and ${A}^{-}$, therefore $\left[{H}^{+}\right]$ and $\left[{A}^{-}\right]$ will be higher than in a weaker acid, making the value of ${K}_{a}$ higher.

Therefore,

• the higher the value of ${K}_{a}$ , the stronger it is as an acid in a solution.
• the lower the value of ${K}_{a}$ , the weaker it is as an acid in a solution.

Similar rationale can be applied to the ionization of a general base, B, except that the equation will look like this:

$B + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s H {B}^{+} + O {H}^{-}$

Therefore, the base ionization constant can be determined from this equation:

${K}_{b} = \frac{\left[H {B}^{+}\right] \left[O {H}^{-}\right]}{\left[B\right]}$

where as before, we have equilibrium concentrations for each species.