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

Question

For reactions in condensed phases (liquids and solids), the difference between AH and AU 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 All 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 All - Delta U for the conversion of 1 mole of graphite to 1 mole of diamond at a pressure of 4.00 X atm. The densities of graphite and diamond are 2.25 g/cm3 and 3.52 g/cm^3, respectively. -0.0768 kj/mol

Explanation / Answer

P=4x10^4 atm 1mole = MW

mass of graphite =12.g V= mass /d = 12 /2.25=5.333 cm^3

mass of diamond =12.4 g V=mass/d=12.42 /3.52=3.528cm^3

delta V= 3.528 - 5.333= - 1.805cm^3=1.805 x10^-3 liter

H= U +PV

Delta H - delta U = Pdelta V = 4x10^4 x-1.805x10^-3 = - 7.22x10 J/mol= - 0.0722 KJ

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