Question

Why is HF a weak acid while HCl a strong acid?
A. $HF$ produces very few ${H^ + }$ ions in water
B. $HCl\;$ dissociates completely to produce many ${H^ + }$ ions in water
C. both A and B
D. None of the above.

Answer 1

Acid strength is the measure of the ability of the acid to lose its ${H^ +}$ ions. Acids which ionises completely into its ions are called strong acids. A strong acid is one which is $100\%$ ionized in solution and weak acid produces very few ${H^+}$ ions in water.

Complete step by step solution:
The degree of dissociation of an acid depends on the strength of the H-A bond. The weaker the bond, the lesser the energy required to break it. Hence, the acid is strong.
$HF$ is the only hydrohalic acid that is not a strong acid. $HF$ ionizes in ​an aqueous solution
$HF{\text{ }} + {\text{ }}{H_2}O\;\;\;\underset {} \leftrightarrows \;\;\;{H_3}{O^{+\;}} + {F^- }$
In Hydrogen fluoride the ${H_3}{O^{+\;}}$and ${F^-}\;$ ions are strongly attracted to each other and form the strongly bound. The hydronium ion attached to the fluoride ion isn't free to function as an acid, thus limiting the strength of $HF$ in water.
In $HCl\left( {aq} \right),$ when $HCl$ is dissolved in ${H_2}O$, it completely dissociates into ${H^+}$(aq) and $Cl^-$(aq) ions. It dissociates $100\%$ into ions thereby called a strong acid.
Hence, $HF$ is a weak acid while $HCl$ is a strong acid as It dissociates completely to produce many ${H^+}$ ions in water

**So correct option is (B)

Note: **
In periodic table ionic size increases as we move down$\left( {{\text{ }}F} \right)$ is at the top of the halogens, the ${F^-}\;ion$ is the smallest halide and its electrons are concentrated around its nucleus, so the $H - F$ bond is relatively short and is more stable and difficult to break. As moving down $Cl$ is larger and more electrons, and therefore the $H - Cl$ bond is longer and weaker and $H - Cl$ bond and ions dissociates in solution.