Question

How does $pKa$ affect acidity?

Answer 1

The $pKa$ value is an important parameter to indicate the strength of an acid. $pKa$is defined as the negative log of the acid dissociation constant or $Ka$value. So, firstly to see how $pKa$ affects acidity, let’s see what exactly $pKa$and $Ka$means while dealing with the acids.

Complete step-by-step answer:
Firstly, let’s understand what Acid dissociation constant $Ka$means.
Consider the following reaction which represent dissociation of weak acid (acetic acid)
$C{{H}_{3}}COOH\,+\,{{H}_{2}}O\,C{{H}_{3}}CO{{O}^{-}}\,+\,{{H}^{+}}$
$Ka$ is the equilibrium constant of the dissociation reaction of an acid. This constant is a quantitative measure of strength of an acid.
$Ka$ for the above reaction can be written as,
$Ka=\,\dfrac{\left[ C{{H}_{3}}CO{{O}^{-}} \right]\,\left[ {{H}^{+}} \right]}{\left[ C{{H}_{3}}COOH \right]\,\left[ {{H}_{2}}O \right]}\,$
From the above expression it is clear, if $Ka$ value is increased it represents that the dissociation of the acid increases. The more the dissociation of acid, the more is the release of ${{H}^{+}}$ ion in the solution. And we know that, the higher the ${{H}^{+}}$, the higher is the acidic strength.
Therefore, acids with greater $Ka$ will be more acidic than the acids with smaller $Ka$
Now, we know that $pKa$ is a negative logarithm of the $Ka$.
$\therefore \,pKa=\,-\log \left[ Ka \right]$
The above expression signifies that if $Ka$will increase, then $pKa$which is the negative of $Ka$will decrease.

This clearly signifies the following two facts:
1. If $pKa$ is small $\left( Ka\,is\,greater \right)$this means the acid is mostly dissociated, so the acid is strong Acids with $pKa$less than/around $2$are strong acids.
2. If $pKa$ is large $\left( Ka\,is\,small \right)$, little dissociation has occurred, so the acid is weak. Acids with $pKa$ in the range of $-2\,to\,12$in water are weak acids.

Hence, we can conclude that acidity of a compound is more if its$pKa$is less and vice-versa.

Note: Different acids have different $pKa$ value and as a result their acidity varies. One should note that $pKa$ value is constant for each type of molecule. It is unaffected by the concentration of the solution. $Ka$, even for the acid is constant and changes with change in temperature only.