Q. 11 For the complete combustion of graphite and diamond in oxygen individually, the standard enthalpy change (ΔH°298) values are −393.5 kJ mol−1 and −395.4 kJ mol−1, respectively. Then, the ΔH°298 for the conversion of graphite into diamond is
The standard enthalpy change for converting graphite to diamond is +1.9 kJ/mol, calculated using combustion data via Hess’s law.
Core Concept
Graphite serves as the standard state for carbon, making its formation enthalpy zero. Diamond’s higher energy state results in a positive ΔH for the reaction C(graphite) → C(diamond). The given combustion values confirm diamond releases more heat upon burning, indicating graphite’s greater stability.
Step-by-Step Solution
Consider these combustion reactions at 298 K:
C(diamond) + O₂(g) → CO₂(g) ΔH° = -395.4 kJ/mol
Target: C(graphite) → C(diamond) ΔH° = ?
- Reverse the diamond equation:
CO₂(g) → C(diamond) + O₂(g) ΔH° = +395.4 kJ/mol - Add to graphite equation:
C(graphite) + O₂(g) → CO₂(g) ΔH° = -393.5 kJ/mol
CO₂(g) → C(diamond) + O₂(g) ΔH° = +395.4 kJ/mol
C(graphite) → C(diamond) ΔH° = +1.9 kJ/mol
Positive ΔH means the process is endothermic, aligning with diamond’s metastability.
Options Analysis
| Option | Value (kJ/mol) | Explanation |
|---|---|---|
| (A) | +1.9 | Correct. Matches ΔH = ΔHcomb(diamond) – ΔHcomb(graphite) = -395.4 – (-393.5) = +1.9 |
| (B) | -1.9 | Incorrect. Reverses sign; applies to diamond → graphite |
| (C) | +3.8 | Incorrect. Possible double calculation error |
| (D) | -3.8 | Incorrect. Wrong sign and doubled magnitude |
Why It Matters
This calculation demonstrates Hess’s law application in thermochemistry, key for stability predictions in allotropes. Relevant for exams like JEE Main, NEET, and undergraduate chemistry.
