Question

What is the pKa range for a strong acid?

Answer 1

Hint: To solve this question we first need to know what is a strong acid. A strong acid tends to dissociate almost completely in a solution.
\[HA+S\rightleftharpoons S{{H}^{+}}+{{A}^{-}}\]
Here, HA is the acid and S is the solvent molecule.
Some examples of strong acids are hydrochloric acid (HCl), nitric acids ($HN{{O}_{3}}$), sulfuric acid (${{H}_{2}}S{{O}_{4}}$), hydrobromic acid (HBr), etc.

Complete answer:
The tendency of an acid HA to dissociate into an anion ${{A}^{-}}$ and a proton ${{H}^{+}}$is known as acid strength.
The pKa value determines the strength or weakness of an acid, where K is the equilibrium constant of an ionization reaction.
Now, when the rate of forward reaction is equal to the rate of the reverse reaction, the system is said to be in equilibrium. For a given reaction,
\[aA+bB\rightleftharpoons cC+dD\]
The equilibrium constant K can be given by the following formula.
\[K=\dfrac{{{[C]}^{c}}{{[D]}^{d}}}{{{[A]}^{a}}{{[B]}^{b}}}\]
So, for the dissociation of acid, the equilibrium constant K will be
\[K=\dfrac{[S{{H}^{+}}][{{A}^{-}}]}{[HA][S]}\]
We know that strong acids dissociate almost completely, and there is an almost negligible amount of undissociated acid, so the value of equilibrium constant K would be large.
We know that the value of $pKa=-{{\log }_{10}}K$.
Since a strong acid has a high value of equilibrium constant K, it will have a low value of pKa.
Any acid which has a value of pKa less than -2 is a strong acid.
For example, nitric acid ($HN{{O}_{3}}$) has a pKa of approximately -1.4

Additional Information:
The relation between pKa and the pH of a substance can be given by the Henderson-Hasselbalch Equation.
\[\begin{align}
  & pH=pKa+\log \dfrac{\text{conjugate base}}{\text{weak acid}} \\
 & pH=pKa+\log \dfrac{[{{A}^{-}}]}{[HA]} \\
\end{align}\]
Substances having a pH in the range of 0 to 7 are considered to be acids. The lower the pH, the stronger the acid.
For example, nitric acid ($HN{{O}_{3}}$) has a pH of approximately 3.01.

Note:
It should be noted that various factors determine the strength of an acid.
- Acid having a higher degree of dissociation will be a strong acid.
- Acid having a weak H-A bond will be a strong acid.
- Acid having a highly polar H-A bond will be a strong acid.