Problem 104 - A Complete Resource Book in Chemistry for JEE Main

Question

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

Step-by-Step Solution

Answer

The correct answer is (c) both are correct.

Step 1: Understanding Clathrates

Clathrates are compounds in which molecules of one component are physically trapped within the crystal lattice of another substance. In this exercise, water molecules form a lattice structure, trapping noble gas atoms within. This forms a noble gas hydrate where the noble gas atoms (such as Ar, Kr, or Xe) are encased in the water lattice.

Step 2: Evaluating the Given Composition

The question specifies a composition of 8 noble gas atoms (denoted as G) and 26 water molecules in the form of 8G. \(26 \text{H}_2\text{O}\). This formulation indicates a specific type of clathrate structure. We need to verify if this composition is consistent with known noble gas hydrates.

Step 3: Confirming Noble Gas Hydrates

By definition, noble gas hydrates are indeed clathrate hydrates containing noble gas atoms trapped within a lattice of water molecules. These structures are known for certain compositions, such as the one given in option (a). It matches the structure of known hydrates for noble gases.

Step 4: Analyzing Each Proposition

Proposition (a) describes the formation of a specific type of clathrate hydrate with noble gases, which matches the known composition, so it is correct. Proposition (b) refers to the naming of these substances as noble gas hydrates, which aligns with the definition of hydrate clathrates. Hence, both statements (a) and (b) are correct.

Step 5: Choosing the Correct Answer

Since both propositions (a) and (b) are correct about clathrates and noble gas hydrates, the correct choice would be option (c), stating that both are correct.

Key Concepts

Clathrates

Clathrates are fascinating compounds where one type of molecule physically traps another. Imagine a lattice structure, similar to a large cage, where smaller guest molecules are held inside without forming traditional chemical bonds. In the context of noble gas hydrates, the water molecules form a crystalline structure that encapsulates noble gas atoms like argon (Ar), krypton (Kr), or xenon (Xe).

These trapped noble gas atoms are the guests, and they fit snugly within the spaces of the cage-shaped water ice lattice. The process doesn’t involve any chemical reactions or exchanges of electrons, but rather a physical encapsulation due to the shape and size compatibility.

• Noble gases are perfect guests because they are non-reactive.
• The water lattice supports a stable environment at suitable temperatures and pressures.
• Common in natural environments, these structures can hold certain gases under conditions that would otherwise cause them to dissipate.

In summary, clathrate formation is an intriguing method where physical forces and geometry play key roles in trapping molecules.

Crystal Lattice

The concept of a crystal lattice is essential in understanding clathrates. Specifically, it's the ordered, repeating arrangement of atoms or molecules in a crystalline material. In a lattice, each component is arranged in a precise geometric pattern that maximizes space utilization and stability.

For noble gas hydrates, the crystal lattice is composed of water molecules arranged in a way that forms hollow spaces or cages. These cages are just the right size to trap noble gas atoms, enabling them to remain stable even at low temperatures and high pressures. The arrangement can be visualized as a network of water molecules forming a matrix.

• Each unit cell of the lattice contributes to the overall structure.
• Cage sizes vary, allowing different noble gases to fit based on their size.
• This regular pattern helps retain the gas molecules within the structure under specific environmental conditions.

Understanding the lattice framework is key to understanding why noble gas hydrates remain stable and how they can be efficiently formed or isolated.

Hydrate Formation

Hydrate formation is the process where water molecules encase other substances, usually gases, inside a solid structure. This process is pivotal in forming noble gas hydrates, also known as clathrate hydrates, under conditions that facilitate such structures. Typically, low temperatures and high pressures are ideal for hydrate formation.

During the formation of noble gas hydrates, water molecules bond with each other through hydrogen bonds, creating a stable ice-like structure capable of capturing noble gas atoms. This formation doesn’t alter the state of the water when it traps the gas; rather, it provides a stable matrix for the trapped gas.

• Hydrate formation is driven by temperature and pressure fluctuations.
• The presence of a guest molecule, like a noble gas, initiates the crystallization process in the water lattice.
• This process can naturally occur in environments such as ocean floors or permafrost regions, where conditions are favorable.

Understanding hydrate formation helps explain the significance of clathrates in both natural and controlled environments, highlighting their potential uses in gas storage and capture.