Thermodynamically the most stable form of carbon is

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Correct — D, Graphite. Under ordinary conditions — room temperature and one atmosphere — graphite is the allotrope of carbon with the lowest energy, and it is therefore the thermodynamically stable form. Chemistry formalises this by taking graphite as the reference state of the element carbon: the standard enthalpy of formation of graphite is defined as zero, and every other form of carbon is quoted relative to it. Diamond sits about 2 kilojoules per mole ABOVE graphite, which is small but positive, and that positive sign is the whole answer to this question. Anything above the reference state is, strictly speaking, metastable rather than stable. So of the four options, graphite is the one to which the others would convert if energy alone decided the outcome.

Why the other options are wrong — (a) Fullerenes — the cage molecules of which C60, buckminsterfullerene, is the best known — are genuine allotropes of carbon, but they are markedly higher in energy than graphite. Curving a flat hexagonal sheet into a closed cage forces strain into the carbon–carbon bonds, and that strain energy has to be paid for. They are the least stable of the three true allotropes on this list, not the most stable.  (b) This is the trap the question is built around, and it is a good one, because everything a student knows about diamond says 'stable': it is the hardest natural substance, it is chemically inert, and it lasts indefinitely. But hardness is a mechanical property and inertness at room temperature is a KINETIC fact, not a thermodynamic one. Diamond is about 2 kJ/mol higher in energy than graphite, so it is metastable; it does not turn into graphite in a jeweller's window only because breaking and rearranging its rigid three-dimensional network of carbon–carbon bonds carries an enormous activation barrier. Confusing 'does not change' with 'lowest energy' is precisely the error being tested. (At the very high pressures found deep in the Earth's mantle the balance reverses and diamond becomes the stable phase — which is why diamonds form there and not at the surface.)  (c) Coal is not a form of carbon in the sense the question means. It is not an allotrope at all but a heterogeneous sedimentary rock — a fossil fuel made of altered plant matter that is rich in carbon but also contains hydrogen, oxygen, nitrogen, sulphur, moisture and mineral ash in variable proportions. An allotrope must be the pure element in a definite structural arrangement; coal has neither the purity nor a single defined structure, so it cannot be compared with graphite, diamond and fullerenes on a thermodynamic scale at all.

Concept: Allotropes are different structural forms of the same element in the same physical state. Carbon has several, and they differ in how each atom bonds. In diamond every carbon is sp3 hybridised and bonded tetrahedrally to four neighbours in a rigid three-dimensional network — hence extreme hardness, very high thermal conductivity and electrical insulation. In graphite every carbon is sp2 hybridised and bonded to three neighbours within flat hexagonal sheets, leaving one electron per atom delocalised over the sheet; the sheets themselves are held to each other only by weak forces. That structure explains graphite's three familiar properties at once: it is soft and slippery because the sheets slide, it conducts electricity because of the delocalised electrons, and it is the low-energy arrangement because the sp2 sheet packs bonding most efficiently at ordinary pressure. Fullerenes are closed cages of sp2 carbon, and graphene is a single isolated graphite sheet.

State PCS 2022 Q54 — Science & Technology PYQ with Answer

State PCS 2022 Q54 — Science & Technology PYQ with Answer