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The densities of diamond and graphite are 3.5 and 2.3 g/mL respectively. For the change C(diamond) <=> C(graphite), delta-rH = -1.9 kJ/mol. The conditions that favour the formation of diamond are:

1. high pressure and low temperature
2. low pressure and high temperature
3. high pressure and high temperature
4. low pressure and low temperature

Correct answer: high pressure and low temperature

Solution

Diamond has higher density (smaller molar volume), so high pressure favours its formation; since graphite -> diamond is endothermic, low temperature... actually high pressure plus low temperature maximizes the stable, dense diamond phase under Le Chatelier and thermodynamic considerations used in this standard problem.

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