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Q77
(NDA-I/2026)
Science & Technology › Basic Science (Physics, Chemistry, Biology)
Which one of the following forms of carbon is thermodynamically most stable?
Result
Your answer:
—
·
Correct:
B
Explanation
Among the allotropes of carbon, graphite is the thermodynamically most stable form at standard temperature (298 K) and pressure (1 atm). In graphite, carbon atoms are sp2 hybridized and arranged in hexagonal layers. The presence of delocalized π-electrons and the specific bonding arrangement result in the lowest Gibbs free energy (ΔGf° = 0 kJ/mol) compared to other forms.
- Diamond is metastable; it has a higher free energy than graphite (ΔG ≈ 2.9 kJ/mol). While it is the hardest natural substance, it will theoretically convert to graphite over extremely long periods.
- Fullerenes (like C60) have significantly higher heat of formation and are less stable due to bond strain in their caged structures.
- Coal is an impure, amorphous form of carbon and is not a pure crystalline allotrope.
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