Problem 191 - A Complete Resource Book in Chemistry for JEE Main

Question

Graphite is a soft, solid, lubricant, extremely difficult to melt. The reason for this anomalous behaviour is that graphite (a) has molecules of variable molecular masses like polymers (b) has carbon atoms arranged in large plates of rings of strongly bound carbon atoms with weak interplate bonds (c) is an allotropic form of diamond (d) is a non crystalline substance

Step-by-Step Solution

Answer

The correct answer is (b).

Step 1: Understand the Properties of Graphite

Graphite is known for its softness and lubricating properties, while also being difficult to melt. This indicates that while each layer is stable, the interlayer forces are weaker, allowing them to slide over each other easily.

Step 2: Analyze the Structure of Graphite

Graphite consists of carbon atoms arranged in flat, parallel, two-dimensional layers or planes. Each carbon atom is bonded to three others in the same plane, forming hexagonal rings. These layers are held together by weak van der Waals forces.

Step 3: Evaluate the Options

Option (a) is incorrect, as graphite's molecular mass does not vary like polymers. Option (c) is incorrect because graphite and diamond are different allotropic forms of carbon and have distinct structures and properties. Option (d) is incorrect because graphite is crystalline, with its atoms arranged in a regular pattern.

Step 4: Select the Correct Answer

Given the analysis, option (b) accurately depicts graphite as having strong in-plane covalent bonds and weak van der Waals forces between the layers, explaining its properties.

Key Concepts

Allotropic forms of carbon

Carbon is a unique element that exists in different structural forms, known as allotropes. Each allotrope has distinct physical properties due to variations in atomic arrangement, although the chemical composition remains the same.
The most well-known allotropes of carbon include:

• Diamond: In diamond, each carbon atom bonds covalently to four other carbon atoms in a three-dimensional tetrahedral structure. This strong bonding makes diamond extremely hard and gives it a high melting point.
• Graphite: Unlike diamond, graphite consists of carbon atoms bonded in flat layers, forming hexagonal rings. Its unique structure allows the layers to slide over each other, resulting in a soft and smooth texture.
• Fullerenes and Carbon Nanotubes: These are spherical or cylindrical arrangements of carbon atoms. They have unique properties used in advanced materials and nanotechnology.

Graphite, as an allotropic form of carbon, showcases the incredible versatility of carbon atoms to form various structures, leading to vastly different physical properties.

Van der Waals forces

Van der Waals forces are weak, intermolecular forces that occur between molecules. Despite their weakness compared to covalent or ionic bonds, they play a crucial role in determining the physical properties of substances. In the case of graphite:

• These forces hold the layers of carbon atoms together. Each layer is strong due to covalent bonds within the layer, but the layers themselves are linked by van der Waals forces, allowing them to slide over one another.

This makes graphite an excellent lubricant. The layers slip past each other easily, reducing friction when applied as a lubricant. Van der Waals forces also contribute to the unique combination of properties in graphite, such as its softness and ability to conduct electricity parallel to the layers. Understanding these forces helps explain why materials like graphite have layered structures that influence their physical characteristics.

Graphite properties

Graphite is renowned for several distinct properties that make it useful in various applications. These properties are a direct result of its layered structure and bonding characteristics:

• Lubrication: The ability of the layers in graphite to slide over one another due to weak van der Waals forces makes it an efficient solid lubricant. Graphite is often used in applications where liquid lubricants might not be suitable.
• Electrical Conductivity: Graphite conducts electricity along the layers. This is because some electrons are delocalized across the planes, allowing for electron movement, which is critical in the manufacturing of electrodes and batteries.
• High Melting Point: Though soft and slippery, graphite's atoms within a single layer are strongly bound. This covalent bonding gives graphite a high melting point, making it stable under extreme heat.

Graphite's combination of softness, electrical conductivity, and thermal stability leads to its use in pencils, batteries, and as a moderator in nuclear reactors. Each property arises from the interaction of strong covalent bonds within layers and weaker van der Waals forces between them.