Problem 21 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Read the following statement and explanation and answer as per the options given below : Assertion : The value of van der Waals'constant ' \(\mathrm{a}\) ' is larger for ammonia than for nitrogen. Reason : Hydrogen bonding is present in ammonia. [1998 - 2 Marks] (a) If both assertion and reason are correct, and reason is the correct explanation of the assertion. (b) If both assertion and reason are correct, but reason is not the correct explanation of the assertion. (c) If assertion is correct but reason is incorrect. (d) If assertion is incorrect but reason is correct.

Step-by-Step Solution

Answer

(a) Both assertion and reason are correct, and reason is the correct explanation of the assertion.

Step 1: Understanding the Assertion

The assertion states that the value of van der Waals' constant \(a\) is larger for ammonia compared to nitrogen. This constant 'a' measures the strength of the intermolecular forces of attraction. Larger values of 'a' generally indicate stronger intermolecular forces or greater polarity in the molecules.

Step 2: Analyzing the Reason

The reason provided is that hydrogen bonding is present in ammonia. Hydrogen bonding is a type of strong intermolecular force that occurs when hydrogen is bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine.

Step 3: Assessing the Assertion

Comparing ammonia \((\text{NH}_3)\) with nitrogen \((\text{N}_2)\), ammonia has stronger intermolecular forces due to hydrogen bonding, leading to a higher van der Waals' constant 'a' than nitrogen, which only has weak van der Waals forces.

Step 4: Verifying the Reason

The reason given is correct because ammonia forms hydrogen bonds, which contribute to the increased strength of its intermolecular forces. This aligns with the fact that van der Waals' 'a' would be larger for ammonia compared to nitrogen.

Step 5: Matching Assertion and Reason

Since the presence of hydrogen bonding in ammonia directly explains why its van der Waals' constant 'a' is larger than that of nitrogen, both the assertion and the reason are correct, and the reason correctly explains the assertion.

Key Concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction and repulsion between molecules that dictate various physical properties of substances. They are generally weaker than the strong covalent, ionic, or metallic bonds found within molecules or compounds but are critically important in determining the state, boiling points, and melting points of substances. There are several types of intermolecular forces, which include:

• Van der Waals Forces: These are weak attractions, including London dispersion forces and dipole-dipole interactions. They arise from temporary shifts in the electron density of molecules, leading to weak polarization.
• Hydrogen Bonding: A specific and stronger type of dipole-dipole interaction occurring when hydrogen is directly bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine. It creates a significant attraction between molecules.
• Ionic Interactions: Though often considered part of ionic bonding, the attractions between oppositely charged ions in solution can also be viewed as intermolecular forces.

Understanding these forces helps us predict how molecules will behave in different environments.

Hydrogen Bonding

Hydrogen bonding is a type of strong intermolecular force that occurs when a hydrogen atom is covalently bonded to one of the most electronegative elements, such as nitrogen, oxygen, or fluorine. This results in a significant dipole, where the hydrogen atom takes on a partial positive charge, while the electronegative element takes on a partial negative charge.

This situation allows for hydrogen from one molecule to be attracted to the electronegative atom of another molecule. This attraction leads to a hydrogen "bond," which is not a true covalent or ionic bond, but a considerably strong interaction relative to other types of intermolecular forces.

Hydrogen bonding is responsible for many remarkable properties, such as:

• The high boiling point of water compared to other group 16 hydrides.
• The unique open structure of ice, leading to its lower density than liquid water.
• The higher boiling point of ammonia compared to other group 15 hydrides, like nitrogen gas.

These effects underscore the importance of hydrogen bonds in both chemistry and biology.

Polarity of Molecules

The polarity of a molecule is determined by the distribution of electron density across its structure. When electrons are shared unequally between atoms due to differences in electronegativity, it creates a polar covalent bond and results in a dipole moment.

A molecule with a significant dipole moment is termed polar, meaning it has a partial positive charge on one end and a partial negative charge on the other. Several factors can influence polarity, including:

• Electronegativity: The difference in electronegativity between bonded atoms. The bigger the difference, the more polar the bond.
• Molecular Geometry: The spatial arrangement of bonds around the central atom. Even if a molecule has polar bonds, its geometry may make it nonpolar if the dipole moments cancel each other out.
• Bond Length and Number: Longer bonds and higher bond order can affect how charge is distributed within the molecule.

Polarity has profound consequences on the physical properties, solubility, and reactivity of substances. For example, polar molecules tend to dissolve in polar solvents like water, explaining why water is such a versatile solvent.