Which hydrohalic acid is the strongest acid among the following?

Which hydrohalic acid is the strongest acid among the following?

• A. HBr
• B. HI
• C. HF
• D. HCl

Answer: (B)

The strength of hydrohalic acids in water increases as you move from fluorine to iodine because the H–X bond becomes weaker and the conjugate base becomes more stable and better solvated. HF has a very strong H–F bond and the fluoride ion is small and highly basic, so it’s not easily stabilized in solution and deprotonation is hard. As you go to chlorine and bromine, the H–Cl and H–Br bonds are weaker, and the corresponding chloride and bromide ions are larger and better solvated, aiding proton loss. Iodine forms the largest, most polarizable conjugate base (I−), which is particularly well stabilized by water, making HI donate its proton most readily. So, among the listed acids, the strongest in aqueous solution is hydroiodic acid.

The strength of hydrohalic acids in water increases as you move from fluorine to iodine because the H–X bond becomes weaker and the conjugate base becomes more stable and better solvated. HF has a very strong H–F bond and the fluoride ion is small and highly basic, so it’s not easily stabilized in solution and deprotonation is hard. As you go to chlorine and bromine, the H–Cl and H–Br bonds are weaker, and the corresponding chloride and bromide ions are larger and better solvated, aiding proton loss. Iodine forms the largest, most polarizable conjugate base (I−), which is particularly well stabilized by water, making HI donate its proton most readily. So, among the listed acids, the strongest in aqueous solution is hydroiodic acid.