Question

How does equilibrium affect the strength of an acid?

Answer 1

The tendency of an acid HA to dissociate into an anion ${{A}^{-}}$ and a proton ${{H}^{+}}$, is known as acid strength.
A strong acid (like HCl) has the tendency to dissociate almost completely in a solution.
$HA\xrightarrow{{}}{{H}^{+}}+{{A}^{-}}$
A weak acid (like $C{{H}_{3}}COOH$), on the other hand does not dissociate completely in a solution, and the dissociated acid and the subsequent undissociated product are present together in equilibrium.
$HA\rightleftharpoons {{H}^{+}}+{{A}^{-}}$

Complete answer:
The strength or weakness of an acid can be determined by pK the value, where K is the equilibrium constant of an ionization reaction.
Now, we know that a system is said to be in equilibrium when its rate of forward reaction is equal to the rake of the reverse reaction. For a given reaction,
$aA+bBcC+dD$
The equilibrium constant K can be given by the following formula.
$K=\dfrac{{{[C]}^{c}}{{[D]}^{d}}}{{{[A]}^{a}}{{[B]}^{b}}}$
So, the equilibrium constant K of dissociation of acid equilibrium will be
$K=\dfrac{[{{H}^{+}}][{{A}^{-}}]}{[HA]}$
We know that strong acids dissociate almost completely, and there is almost negligible amount of undissociated acid, so the value of equilibrium constant K would be large.
Whereas, since weak acids only partially dissociate and still have high concentration of undissociated molecules left, it would have a very low value of equilibrium constant K.
Now, an acid is considered a strong acid if it has a low value of pK, and a weak acid if it has a high value of pK.
We know that the value of $pK=-{{\log }_{10}}K$.
So, an acid which has a high value of equilibrium constant K and hence has a low value of pK would be considered a strong acid. Any acid which has a value of pK less than -2 is a strong acid.
On the other hand, an acid which has a low value of equilibrium constant K and hence has a high value of pK would be considered a weak acid. Any acid which has a value of pK in the range of -2 and 12 is a weak acid.

Note:
It should be noted that there are various factors that determine the strength of an acid. An acid having a higher degree of dissociation will act as a strong acid. Also, if the strength of the H-A bond of an acid is weak, or the polarity of the H-A bond is high, the acid will have higher strength, as these factors increase the degree of dissociation of the acid.