Question

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

Answer 1

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):

`HA rightleftharpoons H^+ + A^-`

In aqueous solution, the acid, `HA` dissociates into `H^+` and `A^-`. The reaction is reversible and the chemical species, `HA`, `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=([H^+][A^-])/([HA])`

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

As the strength of acid increases, `HA` will dissociate more into `H^+` and `A^-`, therefore `[H^+]` and `[A^-]` 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_2O(l) rightleftharpoons HB^+ + OH^-`

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

`K_b=([HB^+][OH^-])/([B])`

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