Question

The ${{H}^{+}}\text{ and }{{\text{F}}^{-}}$ ion exists in solid state and also in liquid HF solution but not in dilute aqueous solution because:
[A] In aqueous medium there is hydrogen bonding but each HF molecule hydrogen bond with the much more prevalent ${{H}_{2}}O$ present instead of the other HF molecules and ${{H}_{3}}{{O}^{+}}$ and ${{F}^{-}}$ is more likely to be formed.
[B] HF is a weaker acid than ${{H}_{2}}O$.
[C] HF has H-bonding
[D] None of these

Answer 1

Hint: To answer this question, remember that hydrogen fluoride is a weak acid. Approach using the concept of hydrogen bonding and how it affects bond strength as well as bond length. Hydrogen bonding takes place between an electropositive and an electronegative atom.

Complete step by step answer:
 We know that HF i.e. hydrogen fluoride is a weak acid. We also know acids are compounds containing at least one hydrogen atom that is covalently bonded to the central electronegative atom. Strength of acid is described by its ability to lose the hydrogen ion.

Now let us discuss why ${{H}^{+}}\text{ and }{{\text{F}}^{-}}$ ions do not exist in dilute aqueous solution.
When we dissolve HF in water, the dipole existing inside the water molecule interacts with the HF molecule by surrounding it and stabilizes the polar H-F by weakening the covalent H-F bond.
It gains stabilization by the formation of hydrogen bonding that is formed between the oxygen of water molecules and hydrogen of HF. Due to the weakening of H-F bond, the bond length increases and then fully breaks and leads to the formation of hydronium ions and fluoride ions.
Thus, HF does not exist as ${{H}^{+}}\text{ and }{{\text{F}}^{-}}$ in aqueous solution.

Therefore, the correct answer is option [A] In aqueous medium there is hydrogen bonding but each HF molecule hydrogen bond with the much more prevalent ${{H}_{2}}O$ present instead of the other HF molecules and ${{H}_{3}}{{O}^{+}}$ and ${{F}^{-}}$ is more likely to be formed.
So, the correct answer is “Option A”.

Note: As fluorine is smaller in size and highly electronegative therefore there exists huge electronegativity difference between hydrogen and fluorine atoms thus making the bond highly polar and this makes it a weak acid.
We should know that weak acids do not dissociate in water themselves. They need a strong base which will extract the hydrogen ion from it whereas strong acids readily dissociate in water because they furnish their hydrogen ions themselves without the help of a base.