Page 77 - Instant notes
P. 77
Free energy 63
its elements in their standard states. Calculation of the standard free energy of a reaction
may be expressed as:
values may be obtained from standard tables. Substances with negative values of
are termed thermodynamically stable. Substances which have positive values of
are termed thermodynamically unstable. Thermodynamically unstable materials
may be synthesized under non-standard conditions and remain stable due to kinetic
factors, but cannot be formed directly from the elements in their standard states.
The standard reaction free energy is limited in its usefulness, as it requires that both
reactants and products be in their standard states. The reaction free energy is the change
in free energy when a reaction takes place under conditions of constant composition. The
difference may be illustrated by the reaction:
If, rather than measuring the standard free energy change, the free energy change is
measured when the molar ratio of the reaction components is 1:2, it is found to be −39.9
−1
kJ mol at 298 K. The difference arises because of the different conditions which the two
values relate to.
The reaction free energy varies markedly with composition, and is directly related to
the standard reaction free energy through the reaction quotient, Q (see Topic C1):
