Problem 102 - A Complete Resource Book in Chemistry for JEE Main

Question

The ease of liquefaction of noble gases increase in the order (a) \(\mathrm{Ar}>\mathrm{Kr}>\mathrm{Xe}>\mathrm{Ne}>\mathrm{He}\) (b) \(\mathrm{Xe}>\mathrm{Kr}>\mathrm{Ne}>\mathrm{Ar}>\mathrm{He}\) (c) \(\mathrm{Kr}>\mathrm{Xe}>\mathrm{He}>\mathrm{Ne}>\mathrm{Ar}\) (d) \(\mathrm{He}>\mathrm{Ne}>\mathrm{Ar}>\mathrm{Kr}>\mathrm{Xe}\)

Step-by-Step Solution

Answer

The correct order for ease of liquefaction is (d): He > Ne > Ar > Kr > Xe.

Step 1: Understanding the Concept

The ease of liquefaction of noble gases is influenced by their atomic mass and intermolecular forces such as Van der Waals forces. Generally, heavier noble gases are easier to liquefy than lighter ones.

Step 2: Arranging in Order Based on Atomic Weight

Let's arrange the noble gases in order of increasing atomic weight: 1. Helium (94m) - Atomic weight = 4 2. Neon (94m) - Atomic weight = 20 3. Argon (94m) - Atomic weight = 40 4. Krypton (94m) - Atomic weight = 84 5. Xenon (94m) - Atomic weight = 131 Given that heavier noble gases are easier to liquefy, the order of increasing ease of liquefaction is from lighter to heavier.

Step 3: Identifying the Correct Order

Based on our understanding and arrangement by atomic weight, the correct order for the ease of liquefaction is: \( ext{He} > ext{Ne} > ext{Ar} > ext{Kr} > ext{Xe} \) This matches option (d) from the choices given.

Key Concepts

Van der Waals Forces

Van der Waals forces are weak attractive forces that exist between atoms and molecules. These forces play a crucial role, especially in gases comprised of noble gases like helium, neon, argon, krypton, and xenon. Van der Waals forces arise due to temporary fluctuations in electron distribution. This creates momentary polarizations in atoms, causing a weak attraction between them.

In noble gases, these forces are the primary intermolecular attractions because these gases are nonpolar and do not form hydrogen bonds. The strength of Van der Waals forces is highly dependent on the surface area and size of the atom. Therefore, as the atomic size increases, so does the strength of these forces. This directly influences the ability to liquefy the gas, with larger atoms having stronger Van der Waals forces that facilitate the process of liquefaction.

These forces are essential to understand because they explain why heavier atoms, despite their inertness, are more susceptible to becoming liquids under increased pressure or decreased temperature.

Atomic Mass of Noble Gases

The atomic mass, or atomic weight, of noble gases is a measure of the total number of protons and neutrons in their nuclei. This is an important factor when analyzing their physical properties, like the ease of liquefaction. As you go down the group in the periodic table, from helium to xenon, the atomic weight increases. This increase has a direct influence on the properties of the gases.

The trend in increasing atomic mass corresponds to an increase in the atomic radius and, consequently, an increase in Van der Waals forces, as previously explained. As a result, heavier noble gases like xenon are easier to liquefy compared to lighter ones such as helium. This is because the larger mass tends to have a more significant electron cloud, which is more easily polarizable. This makes it an important concept when considering applications that require the liquefaction of noble gases, such as in refrigeration or in physics experiments.

Properties of Noble Gases

Noble gases are a group of chemical elements that are characterized by their lack of reactivity and are often found in a gaseous state at room temperature. These include helium, neon, argon, krypton, and xenon. Some common properties among them include:

• Colorless and odorless, making them difficult to detect without appropriate instruments.
• Low chemical reactivity due to having a full valence electron shell.
• Exist as monatomic gases, meaning they do not naturally form molecules with other atoms.
• Subject to Van der Waals forces, which are relatively weak compared to chemical bonds.

The liquefaction of noble gases is a major topic in studying their properties. Despite their weak intermolecular forces, the ability to liquefy them can dramatically increase when exposed to lowered temperatures or increased pressures. These processes are heavily influenced by their atomic masses, as larger and heavier noble gases are more susceptible to polarization. This makes understanding their liquefaction important for various industrial and scientific applications.