For reactions in condensed phases (liquids and solids), the difference between H and U is usually quite small. This statement holds for reactions...

For reactions in condensed phases (liquids and solids), the difference between ΔH and ΔU 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 ΔH and ΔU can differ by a significant amount. A well-known example is the slow conversion of graphite to diamond under Earth's surface.

Calculate ΔH − ΔU for the conversion of 1 mole of graphite to 1 mole of diamond at a pressure of 3.00 × 104 atm. The densities of graphite and diamond are 2.25 g/cm3 and 3.52 g/cm3, respectively.

 kJ/mol