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

(b) has carbon atoms arranged in large plates of rings with weak interplate bonds.

Step 1: Identify Strong and Weak Bonds

Graphite's structure includes large plates of rings with strong covalent bonds between carbon atoms within each layer. However, the layers themselves are held together by weaker van der Waals forces. Understand that these weak interlayer bonds allow layers to slide over each other, making graphite a good lubricant.

Step 2: Assess Melting Difficulty

The strong covalent bonds within the layers require a lot of energy to break, thus making graphite extremely difficult to melt. This characteristic is due to the robust intra-layer bonding between carbon atoms.

Step 3: Evaluate All Options

Option (a) is incorrect as graphite does not have variable molecular masses. Option (b) is correct, as it accurately describes the layered structure with strong in-plane bonds and weak out-of-plane bonds. Option (c) is incorrect because being an allotropic form of diamond does not explain the lubricant property. Option (d) is incorrect, graphite has a crystalline structure.

Step 4: Select the Correct Answer

Based on the evaluation, the correct choice is (b) because it best explains the structure and properties of graphite.

Key Concepts

Covalent Bonds

Covalent bonds are a type of strong chemical bond where two atoms share one or more pairs of valence electrons. In graphite, each carbon atom forms covalent bonds with three other carbon atoms. This configuration creates a flat, hexagonal lattice structure, which is extremely stable.

These bonds within each layer of graphite give rise to its strength and resilience. However, they differ in behavior from the bonds holding different layers together. The strength of covalent bonds is one reason behind graphite's high melting point, as we need a significant amount of energy to break them.

In essence, thanks to these powerful covalent bonds, graphite maintains its structural integrity even under high temperature conditions, making it incredibly difficult to melt.

Van der Waals Forces

Van der Waals forces are weak intermolecular forces that occur between molecules. In graphite, these forces work between the layers of carbon atoms. While covalent bonds hold the atoms within a layer tightly, van der Waals forces are much weaker and allow the layers to slide over each other.

This slidable characteristic is what makes graphite an effective lubricant. When you write with a pencil, for example, the layers of graphite slide off onto the paper.

• These forces arise because of temporary changes in electron distribution, causing slight electric charges in atoms that attract each other.
• They’re essential for the properties of graphite, particularly its lubricating abilities.

Understanding van der Waals forces helps explain why graphite’s layers can move in relation to one another while remaining attached.

Allotropic Forms

Allotropy refers to the occurrence of an element in more than one form, each with different physical characteristics. Carbon is an excellent example, appearing in several allotropes, including graphite and diamond.

These allotropes vary drastically due to the different arrangements of the carbon atoms. In diamond, each carbon atom forms strong covalent bonds with four other carbon atoms in a 3D lattice, explaining its hardness.

Graphite, alternatively, has carbon atoms in layers bonded by covalent bonds while each layer interacts with others through weaker van der Waals forces. This structural distinction highlights the versatility and uniqueness of carbon's allotropic forms.

Lubricant Properties

Lubricants reduce friction between two surfaces. Graphite's layered structure, coupled with its ability to slide, makes it an exceptional solid lubricant.

When used as a lubricant, it's particularly beneficial where traditional oil-based lubricants might fail, such as environments with extreme temperatures. The lubricating property comes from the weak van der Waals forces between the layers, which allows them to easily glide over one another.

• Works in high temperature and pressure environments.
• Prevents metals from sticking and reduces wear and tear.

This makes graphite invaluable in many industrial applications, showcasing its unique ability to operate under harsh conditions.