C(diamond) → C(graphite) ΔG° = -2.9 kJ/mol_rxn Which of the following best explains why the reaction — Thermodynamics Chemistry Question

STEPS:

1. Analyze the thermodynamic favorability of the reaction: The standard Gibbs free energy change for the conversion of diamond to graphite is given as $\Delta G^\circ = -2.9\text{ kJ/mol}_{\text{rxn}}$. Because the value of $\Delta G^\circ$ is negative, the reaction is thermodynamically favorable (spontaneous) at room temperature.
2. Distinguish between thermodynamics and kinetics (thermodynamic vs. kinetic control): Thermodynamics dictates the ultimate favorability and equilibrium position of a reaction, but the value of $\Delta G^\circ$ does not indicate or influence the rate (speed) of a reaction. The rate of a chemical process is a kinetic property governed entirely by the reaction pathway and its activation energy barrier.
3. Examine the chemical structure of the reactant: Diamond is a covalent network solid featuring carbon atoms that are held together by exceptionally strong covalent bonds extending in four different directions to form a rigid, three-dimensional tetrahedral lattice.
4. Relate molecular structure to activation energy: Converting diamond into graphite requires breaking these highly stable carbon-carbon bonds within the network lattice. Due to the immense strength of these three-dimensional bonds, the reaction possesses an extremely large activation energy barrier.
5. Conclude the kinetic behavior of the system: At room temperature, reactant particles lack the thermal energy required to overcome this massive activation energy barrier. Because the reaction rate is infinitesimally slow, the system is under kinetic control, explaining why the spontaneous transition of diamond to graphite is not observed. This identifies Option C as the correct choice.

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WHY_OTHERS_WRONG:

• Option A is incorrect: The magnitude of $\Delta G^\circ$ indicates thermodynamic favorability, not kinetics. A reaction with a relatively small $\Delta G^\circ$ value is not slow because of that value; reaction speed is governed by activation energy, not the change in free energy.
• Option B is incorrect: A change in the system's entropy does not govern or affect the kinetic rate of a reaction. Furthermore, carbon atoms in diamond's rigid crystalline network are highly ordered, whereas graphite's layered sheets are more mobile and less ordered, meaning the conversion of diamond to graphite actually involves an *increase* in entropy, not a decrease.
• Option D is incorrect: This option directly contradicts the thermodynamic data provided. Because $\Delta G^\circ$ is negative, the reaction is thermodynamically favorable. The only reason it does not proceed is kinetic, not thermodynamic.