Question

Thermodynamically most stable allotrope of carbon is

• A. Graphite
• B. Diamond
• C. Fullerene
• D. Coke

Hint:

Allotropes of Carbon.

• Graphite: Most stable; layers of hexagonal carbon rings.
• Diamond: Hardest known material, but metastable.
• Fullerene: Molecules of carbon in ball shapes (C60, etc.).
• Coke: Amorphous carbon, not a pure allotrope.

Answer 1

The Correct Option is A

Graphite is the most thermodynamically stable allotrope of carbon at standard conditions. It has a layered structure with delocalized electrons within the layers, making it stable and a good conductor of electricity.
Other allotropes like diamond are metastable and can convert to graphite over time at room temperature and pressure.

Answer 2

Step 1: Introduction to Carbon Allotropes
Carbon is a unique element that exists in various structural forms known as allotropes. The most common allotropes of carbon are diamond, graphite, graphene, fullerenes (like buckyballs), and carbon nanotubes. Each allotrope differs in the arrangement of carbon atoms and exhibits distinct physical and chemical properties.

Step 2: Structural Differences
- Diamond: In diamond, each carbon atom is sp³ hybridized and bonded tetrahedrally to four other carbon atoms, creating a three-dimensional rigid network. This structure makes diamond extremely hard and gives it a high melting point.
- Graphite: Graphite consists of layers of carbon atoms arranged in hexagonal sheets. Each carbon atom is sp² hybridized and bonded to three other carbons in a planar arrangement. The layers are held together by weak van der Waals forces, allowing them to slide over each other easily.

Step 3: Thermodynamic Stability
Thermodynamic stability refers to the state of lowest energy and greatest stability under given conditions. Although diamond is kinetically stable (it does not easily convert to other forms), it is thermodynamically metastable at room temperature and atmospheric pressure.
Graphite, however, has a lower Gibbs free energy compared to diamond under standard conditions, meaning it is the thermodynamically favored form. This is why graphite is considered the most stable allotrope of carbon.

Step 4: Practical Implications
The relative stability explains why graphite is found naturally in abundance, whereas diamond is rarer and forms only under high-pressure conditions deep within the Earth.
Moreover, the stability of graphite is why diamond can slowly convert to graphite over very long timescales at room temperature.

Step 5: Conclusion
To summarize, although diamond is harder and has unique properties, graphite is the thermodynamically most stable allotrope of carbon due to its lower energy structure and natural abundance.
Therefore, the correct answer is graphite.