At each temperature, elements are in their standard states at 1 bar. Since the absolute value of the free energy of any element can not be determined, this standard value of free energy Go in their most stable state is taken as zero. The standard free energy of formation of a compound is the standard reaction free energy for the formation of the compound from its elements in their standard states. E.g., the standard reaction free energies r Go for the following reactions at 298 K are
C + O 2 CO 2 , r Go = -394.4 kJ/mol
½ C + ½ O 2 CO, r Go = -137.2 kJ/mol.
(21.42)
The reaction free energies above are the standard free energies formation Gof for CO 2 and CO respectively.
The free energy changes for a reaction can be estimated from the free energies of formation of the reactants and products. Consider the following reaction,
CO (g) +1/2 O 2 (g) CO2 (g), (T = 298 K )
(21.43)
For this reaction rGo = – 394.4 –1/2 Gof (O2 ) – ( –137.2) = – 257.2 kJ/mol, since
