Problem 17 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is the chemically inactive, allotropic form of carbon? (a) coal (b) diamond (c) animal charcoal (d) charcoal

Step-by-Step Solution

Answer

The chemically inactive, allotropic form of carbon is diamond.

Step 1: Understanding Allotropy

Allotropy refers to the existence of an element in more than one physical form. Carbon is known for having several allotropes, including diamond, graphite, and amorphous forms like coal and charcoal.

Step 2: Activity of Carbon Allotropes

Chemically inactive (or inert) allotropes of carbon do not react easily with other chemicals. Among the options, diamond is an allotrope of carbon that is known for being chemically inert. This is due to its highly stable and rigid three-dimensional lattice structure.

Step 3: Identifying the Chemically Inactive Form

Coal, charcoal, and animal charcoal are amorphous forms of carbon and are generally reactive due to their complex and less stable structures. On the other hand, diamond is renowned for its lack of chemical reactivity, thus qualifying as the chemically inactive form of carbon.

Key Concepts

Chemically Inactive Carbon

Chemically inactive carbon refers to carbon allotropes that do not easily engage in chemical reactions. These forms are often called inert allotropes and possess stable structures that contribute to their inactivity. Among carbon allotropes, diamond stands out as the chemically inactive form. Despite being made entirely of carbon, diamonds are resistant to reacting with most chemicals. This resistance is due to their unique bonding structure, where each carbon atom is tetrahedrally bonded to four other carbon atoms, creating an extremely stable three-dimensional network. This structure gives diamonds their renowned hardness and chemical inactivity. This makes diamond an exception among carbon allotropes, as many other forms, such as graphite and amorphous carbon, are relatively more reactive.

Amorphous Carbon Forms

Amorphous carbon refers to types of carbon allotropes that lack a crystalline structure. Unlike diamond and graphite, which have distinct lattice structures, amorphous carbon forms are made up of irregularly arranged atoms. This randomness in atomic arrangement gives amorphous carbon different properties. Common examples of amorphous carbon include coal, charcoal, and soot. These materials usually exhibit distinct properties such as:

• High surface area, making them useful for adsorption applications.
• Porous structures that contribute to their reactivity.
• Lack of a fixed shape, allowing them to be molded into various forms.

The reactivity of amorphous carbon is due to the presence of free electrons and the incomplete bonding of carbon atoms, which can form bonds with other elements more readily than the structured forms like diamond.

Diamond Structure

The diamond structure is specifically associated with the crystalline form of carbon known as diamond. This lattice structure is unparalleled in its stability, which gives diamond its unique properties. The diamond lattice is essentially an infinite network of carbon atoms bonded in a symmetrical arrangement, where each atom is tetrahedrally connected to four others. This bonding creates a robust three-dimensional structure that provides:

• Exceptional hardness, making diamonds the hardest natural substance.
• High thermal conductivity, allowing effective heat dissipation.
• Chemical inertness, preventing reactions with most substances.

The rigidity and regularity of the diamond structure also contribute to its optical properties, such as brilliance and refractive qualities, making diamonds both functional in industrial applications and prized in jewelry.