Geological conditions are sometimes so extreme that quantities neglected in normal laboratory experi

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Geological conditions are sometimes so extreme that quantities neglected in normal laboratory experiments take on an overriding importance. For example, consider the formation of diamond under geophysically typical conditions. The density of graphite is 2.4 g/cm3 and that of diamond is 3.6 g/cm3 at a certain temperature and 500 k bar. By how much does ΔUtrans differs from ΔHtrans for the graphite to diamond transition?

Accepted Answer

$\Delta H - \Delta U = P.\Delta V = P\left\lbrack V_{D} - V_{G} \right\rbrack$

$= 500 \times 10^{3} \times 10^{5}\frac{N}{m^{2}}\left\lbrack \frac{12}{3.6} - \frac{12}{2.4} \right\rbrack \times 10^{- 6}\text{ }\text{m}^{3} $$$= - 8.33 \times 10^{4}\text{ J/mol}$$

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