Among the following hydrogen halides, which compound forms the strongest hydrogen bonds between its molecules?

Introduction / Context:
Hydrogen bonding is a very important concept in physical chemistry because it explains the unusually high boiling points, viscosities and solubilities of many substances. In particular, hydrogen halides provide a good comparison, because the only structural difference is which halogen atom is bonded to hydrogen. This question asks you to identify which hydrogen halide among HI, HCl, HF and HBr forms the strongest hydrogen bonds between its molecules, testing your understanding of how electronegativity and molecular size influence hydrogen bonding strength.

Given Data / Assumptions:

• The compounds being compared are hydrogen iodide (HI), hydrogen chloride (HCl), hydrogen fluoride (HF) and hydrogen bromide (HBr).
• Each compound has one hydrogen atom bonded to a halogen atom.
• Hydrogen bonding occurs when hydrogen is bonded to a highly electronegative atom such as fluorine, oxygen or nitrogen.
• We consider intermolecular hydrogen bonds, not the strength of the covalent HX bond itself.

Concept / Approach:
Hydrogen bonds form when a hydrogen atom covalently bonded to a very electronegative atom (for example F, O or N) interacts with a lone pair on another electronegative atom. The strength of hydrogen bonding depends mainly on the electronegativity of the atom attached to hydrogen and on the small size that allows close approach of molecules. Among the halogens, fluorine is the most electronegative and the smallest in size. Therefore, an H–F bond produces a very polar molecule where hydrogen carries a strong partial positive charge and fluorine carries a strong partial negative charge, leading to very strong hydrogen bonding between HF molecules compared with other hydrogen halides.

Step-by-Step Solution:
Step 1: Recall that strong hydrogen bonding requires hydrogen bonded to a highly electronegative atom with at least one lone pair. Step 2: Identify the electronegativities of the halogens: fluorine is more electronegative than chlorine, bromine and iodine. Step 3: Because fluorine is very electronegative and small, the H–F bond is highly polar, so HF molecules attract one another strongly via hydrogen bonding. Step 4: HCl, HBr and HI have lower electronegativity of the halogen and larger atomic size, so the H–X bonds in these molecules are less polar and form weaker hydrogen bonds or almost none. Step 5: Therefore, among the given hydrogen halides, HF has the strongest hydrogen bonding between its molecules.

Verification / Alternative check:
A good way to verify this conclusion is to look at boiling points. Hydrogen fluoride has a significantly higher boiling point than HCl, HBr and HI, even though some of these contain heavier atoms. This high boiling point is due to strong intermolecular hydrogen bonding. In contrast, the other hydrogen halides behave more like typical polar covalent molecules with weaker dipole dipole forces and London dispersion forces. The anomalous behaviour of HF compared with other hydrogen halides is a standard textbook example used to illustrate hydrogen bonding and supports the conclusion that HF has the strongest hydrogen bonds.

Why Other Options Are Wrong:
Option A, hydrogen iodide (HI), involves iodine which is much larger and less electronegative, so hydrogen bonding is very weak or negligible. Option B, hydrogen chloride (HCl), is polar but does not form hydrogen bonds as strong as HF because chlorine is less electronegative and larger than fluorine. Option D, hydrogen bromide (HBr), is even less electronegative and larger, so it has weaker intermolecular attractions. Option E, a mixture of these hydrogen halides, does not change the basic fact that HF molecules form the strongest hydrogen bonds on their own compared with the other pure compounds listed.

Common Pitfalls:
A common mistake is to assume that the heaviest molecule must have the strongest intermolecular forces and therefore the strongest hydrogen bonds. While greater molar mass can increase dispersion forces, hydrogen bonding depends much more on electronegativity and bond polarity. Another pitfall is to think that all hydrogen halides show similar hydrogen bonding strength simply because they contain hydrogen and a halogen. To avoid these errors, remember the simple rule that strong hydrogen bonding occurs only when hydrogen is bonded to fluorine, oxygen or nitrogen and that fluorine gives particularly strong effects.

Final Answer:
Among the given hydrogen halides, the compound that forms the strongest hydrogen bonds between its molecules is Hydrogen fluoride (HF).