Problem 84 - A Complete Resource Book in Chemistry for JEE Main

Question

When water is allowed to freeze in the presence of Ar, Kr or Xe under pressure then (a) atoms to noble gas (G) get trapped in the crystal lattice of ice giving clathrates corresponding to the composition 8G. \(26 \mathrm{H}_{2} \mathrm{O}\) (b) these are called noble gas hydrates (c) both are correct here (d) all are incorrect here

Step-by-Step Solution

Answer

The correct answer is (c) both are correct here.

Step 1: Understanding the Problem

We need to determine which statement or statements about noble gases and ice are correct: whether atoms of noble gases get trapped in ice's crystal lattice, forming clathrates of specific composition, or if anything written is incorrect.

Step 2: Analyzing Statement (a)

Statement (a) suggests that noble gas atoms are trapped in the crystal lattice of ice, forming clathrates corresponding to the composition 8G. \(26 \mathrm{H}_{2} \mathrm{O}\). Clathrates are indeed a type of structure where small guest molecules (such as noble gases) become trapped within the lattice of another compound, like ice. The composition mentioned aligns with known clathrate structures.

Step 3: Analyzing Statement (b)

Statement (b) identifies these structures as noble gas hydrates. Noble gas hydrates are another name for the clathrates formed when noble gases are trapped within the ice lattice, aligning with the definition.

Step 4: Conclusion Based on Analysis

Since both (a) and (b) accurately describe known scientific phenomena related to noble gases and ice clathrates, both statements are correct. Therefore, option (c), indicating both are correct, is the accurate choice.

Key Concepts

Clathrate Hydrates

Clathrate hydrates are fascinating compounds where water molecules form a cage-like structure, trapping small gas molecules inside. These cages are formed by hydrogen-bonded water molecules that create a unique crystalline lattice. This structure has cavities or pockets suitable for encapsulating guest molecules like gases.

The process of forming these hydrates involves cooling the water in the presence of certain gases under pressure. The water molecules naturally arrange themselves into a lattice structure, and if gas molecules are present, they can become trapped within. This phenomenon creates a stable structure despite the weak Van der Waals forces holding the gas molecules inside.

Clathrate hydrates have significant implications:

• They can potentially affect gas storage technologies.
• Understanding them helps in studying natural gas reserves.
• They contribute to insights into energy resources in icy regions.

Noble Gas Hydrates

Noble gas hydrates are a specific form of clathrate hydrates where noble gases, such as argon (Ar), krypton (Kr), and xenon (Xe), are encased within the lattice of ice. These formations occur when water freezes in the presence of these gases under heightened pressure conditions.

The structure of noble gas hydrates results from the unique properties of noble gases. These gases are chemically inert due to their full valence electron shells, meaning they rarely react with other substances. Nevertheless, they can become physically trapped within the ice lattice. This entrapment forms a stable association between the water's crystalline structure and the otherwise unreactive gas.

Noble gas hydrates have fascinating physical characteristics:

• They exhibit lower densities due to the presence of gas molecules within the lattice.
• Their formation is one route for studying the behavior of molecules at low temperatures.
• They offer exciting possibilities for researching gas storage and transportation.

Crystal Lattice of Ice

The crystal lattice of ice is a beautifully ordered structure formed by water molecules. This lattice is crucial in the formation of substances like clathrate and noble gas hydrates. At a molecular level, ice adopts a hexagonal structure due to hydrogen bonding between water molecules.

This hexagonal arrangement is not just pleasing to look at but functional. It results in an open framework with cavities where small molecules, like noble gases, can become trapped. These spaces become the sites for forming clathrates when cold conditions and the presence of guest molecules are aligned.

The lattice itself is notable for several reasons:

• It reflects water's unique thermal properties, such as expansion upon freezing.
• It's central to understanding natural phenomena like the stability of polar ice caps.
• The crystalline rigidity provides a scaffold for capturing gas molecules.