Problem 28 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is not correct consequences of Schottky defect? (a) stability of crystal decreases (b) density decreases (c) electrical conductivity increases (d) lattice energy of crystal increases

Step-by-Step Solution

Answer

(d) Lattice energy of crystal increases.

Step 1: Understanding Schottky Defects

A Schottky defect is a type of point defect in a crystal structure where an equal number of cations and anions are missing from the lattice. It typically occurs in ionic solids to maintain charge neutrality.

Step 2: Analyzing Consequences on Stability and Density

When ions are missing from the lattice due to a Schottky defect, it directly reduces the number of particles in the crystal structure, leading to decreased stability as the crystal becomes less organized. The absence of these particles also causes a reduction in the crystal's density since the mass is lower for the given volume.

Step 3: Assessing Electrical Conductivity

While Schottky defects involve ions being absent, these vacant sites can facilitate ion mobility within the lattice. As ions move through these vacancies, it can enhance ionic conductivity, thus increasing electrical conductivity.

Step 4: Evaluating Effect on Lattice Energy

Lattice energy is the energy released when ions form a crystalline lattice. Schottky defects do not increase lattice energy; in fact, they generally show a tendency to reduce lattice energy, as fewer ions are present to contribute to the energy release in the lattice formation.

Key Concepts

Point Defects in Crystals

Point defects in crystals are imperfections found in the crystal lattice where the precise order of the ions or atoms is disrupted. One common type of point defect is the Schottky defect. In this defect, equal numbers of cations and anions are missing, keeping the overall charge balanced. These missing ions create vacancies throughout the crystal. Point defects play a crucial role in determining the physical properties of materials, including mechanical strength and electrical conductivity.

• Types of point defects: Schottky defects, Frenkel defects, and interstitials.
• Importance: Affect properties like conductivity, optical properties, and durability.

Crystal Stability

Crystal stability refers to how well a crystal structure maintains its arrangement under various conditions. The presence of point defects can significantly influence stability. In the case of Schottky defects, missing ions can create a less ordered structure. This irregularity can lead to decreased stability because the even balance and symmetry that contribute to a stable lattice are disrupted.

• Impact of Schottky defects: Decreased stability due to missing ions.
• Influencing factors: Temperature, pressure, and chemical composition can also impact stability.

Ionic Conductivity

Ionic conductivity in crystals is a measure of how readily ions can move through the crystal lattice. With Schottky defects, vacant sites are created, which allow ions to move through the crystal more freely. This movement increases the material's ionic conductivity. When ions have more pathways to travel through the lattice, electrical conductivity is enhanced.

• Vacancy flow: Ion movement is facilitated by vacant lattice sites.
• Applications: Used in solid-state batteries and fuel cells for efficient ion transport.

Lattice Energy

Lattice energy is the total energy released when oppositely charged ions form a crystalline solid from their gaseous states. In a perfect crystal, numerous ions contribute to a high lattice energy. However, with Schottky defects, there are fewer ions, thus fewer contributions to the overall lattice energy. This results in a decrease rather than an increase in lattice energy, as there is less energy released in the formation process.

• Role in lattice formation: Determines structural strength and stability.
• Effect of Schottky defects: Reduces the overall lattice energy due to fewer ions.