Which of the following hydrogen halides is the weakest acid in water?

Hydrogen halides are binary compounds of hydrogen with halogens: HF, HCl, HBr, and HI. Their acid strength in water depends on two main factors: the bond strength between hydrogen and the halogen and the stability of the halide ion after dissociation.

Among the hydrogen halides, HF (hydrofluoric acid) is the weakest acid. The reason is that the H–F bond is very strong due to the small size of fluorine and its high electronegativity. A strong H–F bond makes it difficult for HF to release H⁺ ions in water, resulting in weak acidity.

In contrast, HI has the weakest H–I bond, so it dissociates completely in water and acts as the strongest acid among the hydrogen halides. HCl and HBr have intermediate acid strengths. Therefore, the acid strength trend of hydrogen halides in aqueous solution is:

HF < HCl < HBr < HI

This trend highlights the importance of bond strength and halogen size in determining acidity. As we move down Group 17, the hydrogen-halogen bond becomes longer and weaker, making proton release easier and increasing acid strength.

HF’s weak acidity does not affect its reactivity in other chemical reactions, such as etching glass or reacting with silicates, but in aqueous solution, it behaves as a weak acid.

In summary, HF is the weakest acid in water among the hydrogen halides due to its strong H–F bond and the difficulty in releasing hydrogen ions. Understanding this trend is fundamental in inorganic chemistry for predicting the reactivity and properties of hydrogen halides.