Q. 12 The CORRECT order of boiling points for the hydrogen halides is
(A) HF > HI > HBr > HCl
(B) HF > HCl > HBr > HI
(C) HI > HBr > HCl > HF
(D) HI > HF > HBr > HCl

The correct answer is option (A) HF > HI > HBr > HCl.

Boiling points of hydrogen halides depend primarily on intermolecular forces, with actual values being HF (293 K), HI (238 K), HBr (206 K), and HCl (189 K). Hydrogen fluoride exhibits the highest due to strong hydrogen bonding from fluorine’s high electronegativity, while the others follow a trend driven by increasing London dispersion forces down the group.

Option Analysis

• (A) HF > HI > HBr > HCl: Correct, as HF’s hydrogen bonding outweighs HI’s strong van der Waals forces from its larger size and more electrons (molecular weights: HF 20, HCl 36.5, HBr 81, HI 128).

• (B) HF > HCl > HBr > HI: Incorrect, ignores the rise in dispersion forces from HCl to HI, which increases boiling points despite weaker polarity down the group.

• (C) HI > HBr > HCl > HF: Incorrect, overlooks HF’s exceptional hydrogen bonding that elevates its boiling point above even the heaviest halide.

• (D) HI > HF > HBr > HCl: Incorrect, misplaces HCl below HBr and disrupts HF’s top position due to H-bonding.

Hydrogen halides boiling points follow a unique order: HF > HI > HBr > HCl, crucial for CSIR NET Life Sciences and chemistry exams. This trend arises from intermolecular forces—hydrogen bonding dominates in HF, while London dispersion forces increase down the group for others.

Why HF Has the Highest Boiling Point

HF boils at 293 K (20°C) due to extensive hydrogen bonding (H-F⋯H-F), forming zigzag polymers that require significant energy to break. Fluorine’s electronegativity (4.0) creates a strong δ+ H and δ- F dipole, unlike weaker interactions in HCl (189 K), HBr (206 K), and HI (238 K).

Trend in HCl, HBr, and HI

Without significant H-bonding, these rely on dipole-dipole and London forces. Boiling points rise from HCl to HI as molecular size and electrons increase (18 vs. 36 vs. 70 vs. 106 electrons), enhancing temporary dipoles and van der Waals attractions.