Among the acids of the halogens, which is considered to be the most acidic?

• A. HBr
• B. HI
• C. HF
• D. HCl

Answer: (B)

In hydrohalic acids, how easily the molecule donates a proton depends on two things: the strength of the H–X bond and how stable the halide ion X− will be after dissociation. As you go down the group from chlorine to bromine to iodine, the H–X bond gets weaker and the resulting X− ion becomes larger and more polarizable. A weaker bond means easier proton release, and a larger, more polarizable anion stabilizes the negative charge better, making dissociation more favorable. These combined effects make hydroiodic acid the strongest acid in this series.

Fluorine is an exception to the trend. The H–F bond is extremely strong, and the fluoride ion, though highly basic, is very small and poorly stabilizable as a conjugate base, which keeps HF weak as an acid. Chlorine and bromine lie between HF and iodine in strength, with HCl and HBr being stronger than HF but weaker than HI.