For reactions in condensed phases (liquids and solids), the difference between .

Question

For reactions in condensed phases (liquids and solids), the difference between .H and U is usually quite small. This statement holds for reactions carried out under atmospheric conditions. For certain geochemical processes. however, the external pressure may be so great that delta H and delta U can differ by a significant amount. A Well-known example is the slow conversion of graphite to diamond under Earth?s surface Calculate for the conversion of 1 mole of graphite to 1 mole of diamond at a pressure of 1.00 x10^4 atm. The densities of graphite and diamond are 2.25 g/cm^3 and 3.52 gcm^3, respectively.

Explanation / Answer

deltaH - deltaU = P x deltaV

volume of graphite = mass / density = 12 / 2.25 = 5.33 cm3 = 0.00533 L

volume of diamond = mass / density = 12 / 3.52 = 3.41 cm3 = 0.00341 L

Graphite -----------------> Diamond

deltaV = 0.00341 - 0.00533 = -1.92 x 10-3 L

deltaH - deltaU = (4.00 x 104 x -1.92 x 10-3)

deltaH - delatU = -76.8 J/mol

deltaH - delatU = -0.0768 kJ/mol

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