Explain why HF is a weak acid, whereas HCl, HBr, and HI are all strong acids.

Acids are classified into strong or weak based on how much they ionize in water. Strong acids almost completely ionize, while weak acids only partially ionize. For a hydrogen halide, this ionization is represented by the following equilibrium: \(HX \leftrightarrows H^+ + X^-\), where X is the halogen.

One of the main factors affecting the strength of an acid is the bond strength between hydrogen and the halogen, which is represented by the H-X bond. The weaker the bond, the easier it is for the halide to dissociate into its ions, hence a stronger acid. In the case of hydrogen halides, the bond strength generally decreases down the periodic table, meaning that as the halogen atoms get larger, the bond strength decreases.

In the case of HF, the bond between hydrogen and fluorine is actually quite strong because fluorine is small and highly electronegative. The electron pair shared by the hydrogen and fluorine is attracted very strongly by the fluorine atom, resulting in a fairly strong bond. This makes HF less likely to ionize, hence a weaker acid.

In contrast, the H-X bonds in HCl, HBr, and HI are weaker due to a larger halogen atom. As the atomic size of the halogens increases, the electron cloud gets more diffused, making H-Cl, H-Br, and H-I bonds weaker. This allows these compounds to ionize more readily, classifying them as strong acids.

Another factor to consider is the polarizability of the halides. Polarizability refers to the ability of a molecule's electron cloud to be distorted by an external electric field, such as the electric field from water molecules. Larger halides are more polarizable, meaning their electron clouds can be more easily distorted, which results in easier ionization and a stronger acid. HF, with its small and less polarizable fluoride ion, is more stable and less prone to dissociation. In conclusion, HF is a weak acid due to its relatively strong H-F bond and low polarizability compared to the other hydrogen halides. In contrast, HCl, HBr, and HI are strong acids due to their weaker H-X bonds and increased polarizability as we move down the halogen group in the periodic table.