Question

Statement I : BP order is HI<HBr<HCl<HF.
Statement II : MP order is HI<HF<HCl<HBr.

• A. Statement I and Statement II are correct
• B. Statement I is correct and Statement II is wrong
• C. Statement I is wrong and Statement II is correct
• D. Both Statement I and Statement II are incorrect

Hint:

For hydrides of groups 15, 16, and 17, always remember that the first member (NH$_3$, H$_2$O, HF) has an unexpectedly high boiling point due to hydrogen bonding.
For the others, the trend follows the increase in molecular weight.

Answer 1

The Correct Option is D

To address the given statements and verify their correctness regarding boiling point (BP) and melting point (MP) orders for hydrogen halides, we need to consider the physical properties and molecular interactions of these compounds.

1. Analyzing Statement I: BP order is HI < HBr < HCl < HF.
   • The boiling point of a compound is influenced by factors such as molecular weight and strength of intermolecular forces. In hydrogen halides, the dominant force is hydrogen bonding, particularly in HF.
   • HF has strong hydrogen bonding compared to others, which increases its boiling point significantly. Hence, its boiling point should be the highest among the hydrogen halides.
   • The expected order for boiling points, considering dipole-dipole interactions and molecular weights, is: \(HF > HI > HBr > HCl\).
   • Therefore, the given BP order in Statement I is incorrect.
2. Analyzing Statement II: MP order is HI < HF < HCl < HBr.
   • The melting point of hydrogen halides is also influenced by hydrogen bonding and molecular size. HF, although capable of forming strong hydrogen bonds, often has lower melting points than heavier compounds because of its smaller size and packing efficiency.
   • The expected order for melting points typically considers both molecular weights and the strength of interactions: \(HCl < HF < HBr < HI\).
   • Hence, the order in Statement II is incorrect.

Both provided statements about the order of boiling points and melting points of hydrogen halides are incorrect when verified against chemical principles.