Problem 4 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

HF has highest boiling point among hydrogen halides, because it has: [Main April 9, 2019 (II)] (a) strongest van der Waals' interactions (b) lowest ionic character (c) strongest hydrogen bonding (d) lowest dissociation enthalpy

Step-by-Step Solution

Answer

(c) strongest hydrogen bonding

Step 1: Identify Key Concept

The exercise asks us to determine the reason HF has the highest boiling point compared to other hydrogen halides. Boiling point is typically influenced by the strength of intermolecular forces.

Step 2: Recognize Types of Intermolecular Forces

The primary intermolecular forces are van der Waals interactions, hydrogen bonding, and ionic character. Hydrogen bonding is a significant force influencing boiling points among molecules capable of such bonding.

Step 3: Analyze Role of Hydrogen Bonding

HF is capable of forming hydrogen bonds due to the high electronegativity of fluorine. Hydrogen bonds are typically much stronger than van der Waals forces, greatly influencing HF's boiling point.

Step 4: Eliminate Distractors

(a) van der Waals' interactions and (d) dissociation enthalpy do not primarily affect the boiling point as much as hydrogen bonding does. (b) While ionic character can affect boiling points, HF’s high hydrogen bonding predominance is much more significant here.

Step 5: Conclude the Correct Answer

Based on the analysis, the strongest hydrogen bonding in HF compared to other hydrogen halides is the primary reason for its highest boiling point.

Key Concepts

Intermolecular Forces

Intermolecular forces are the attractions that occur between molecules, playing a crucial role in determining the physical properties of substances, such as boiling and melting points. There are several types of intermolecular forces, including:

• Dispersion Forces: Also known as London dispersion forces, these are the weakest type of intermolecular force and occur due to temporary, instantaneous dipoles formed in molecules.
• Dipole-Dipole Interactions: These occur between molecules that have permanent dipoles, due to differences in electronegativity between atoms within the molecule.
• Hydrogen Bonding: A strong type of dipole-dipole interaction, hydrogen bonds occur when hydrogen is covalently bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine.

Hydrogen bonding is especially significant as it can considerably raise the boiling point of substances, like in the case of hydrogen fluoride (HF), due to it forming stronger attractions compared to other intermolecular forces.

Boiling Point

The boiling point of a substance is the temperature at which it transitions from a liquid to a gas. This physical property is heavily influenced by the strength of intermolecular forces present within the substance.

Stronger intermolecular forces result in higher boiling points because more energy is required to separate the molecules from one another and convert the liquid into a gas. For example:

• Hydrogen Bonding: Substances capable of hydrogen bonding, like HF, tend to have significantly higher boiling points compared to substances that do not exhibit such bonding.
• Van der Waals Forces: These weaker forces typically result in lower boiling points, making them less significant for molecules with stronger interactions.

Hydrogen bonding directly contributes to the exceptionally high boiling point of HF compared to other hydrogen halides, allowing it to remain a liquid at higher temperatures.

Hydrogen Halides

Hydrogen halides are binary compounds consisting of hydrogen and halogen atoms. The common hydrogen halides include HF, HCl, HBr, and HI. These compounds are known for their acid-forming properties and differences in boiling points.

Among hydrogen halides, hydrogen fluoride (HF) stands out for its unique properties due to its ability to form hydrogen bonds. The high electronegativity of fluorine allows for strong hydrogen bonding, elevating HF's boiling point significantly above that of other hydrogen halides, such as HCl, which experiences primarily weaker van der Waals forces.

In contrast, the other hydrogen halides, like HCl, HBr, and HI, do not engage in significant hydrogen bonding, leading to lower boiling points. This variance in boiling points demonstrates the critical role of hydrogen bonding in influencing the physical properties of substances and the importance of molecular interactions in chemistry.