Question
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Which among the following is the strongest acid?
A.HF
B.HCl
C.HBr
D.HI

Answer
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Hint:
In 1923, J.N. Bronsted and T.M. Lowry proposed a definition of acids and bases in terms of donation and receiving of protons. He said that acids are proton donors and bases are proton acceptors. Bronsted and Lowry concept is a wider concept and is not restricted only to neutral molecules. Even the ions can be regarded as acids and bases.

Complete answer:
Step 1
The hydrogen halides in the gaseous state are purely covalent and do not show acidic character. In aqueous solution, they undergo ionization to produce hydronium ions and thus behave as an acid.
\[HX + {H_2}O\xrightarrow{{}}{H_3}{O^ + } + {X^ - }\]
Step 2
The acid strength of halogen acids is just the reverse of the electronegative values of the halide ions present in the compound. The electronegativity difference is the maximum in HF as fluorine is the most electronegative halogen. Therefore, the percent ionic character in HF should have been the maximum thereby should have been the strongest halogen acid. But it is found to be the weakest halogen acid.
Step 3
The strength of halogen acid depends upon the ease with which the \[H - X\] bond present in it dissociates in aqueous solution to give \[{H_3}{O^ + }\] ions. The bond dissociation energy of the bond governs the ease of the breaking of \[H - X\] bond. The less the bond dissociation energy of the \[H - X\] bond, the more is the ease with which it dissociates in the aqueous solution and greater is the acid strength of the hydrogen halide.
Step 4
From the experimental data, we know that the bond dissociation energy decreases from \[H - F to H - I\]. As \[H - F\] bond is the strongest hence it requires maximum energy to dissociate. So, \[H - F\] is the weakest of the halogen acid and \[H - I\] is the strongest acid.

Hence, we can understand that option d) is the correct answer for the given question.

Note:HF does not show any reducing character while HI is a strong reducing agent. The reducing character of hydrogen halides increases from HF to HI.