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Physical Chemistry 60

The standard reaction free energy, , is the change in the Gibbs free
energy which accompanies the conversion of reactants in their standard
states into products in their standard states. It may be calculated from the
enthalpy and entropy changes for a reaction using

composition. The reaction free energy varies markedly with composition through the
expression ln Q where Q is the reaction quotient.
Related The first law (B1) Entropy (B4)
topics
Enthalpy (B2) A more general concept, the reaction free energy, is the change in free
Entropy and change (B5)
energy when a reaction takes place under conditions of constant
Thermochemistry (B3)

Free energy
The total entropy change which accompanies a process is the sum of the entropy change
in the system and its surroundings:
∆S total=∆S system+∆S surroundings
∆S surroundings is related to the enthalpy change in the system at constant pressure through
the relationship: ∆S surroundings=−∆H system/T. Substitution of this expression into the
previous one, and subsequent multiplication by −T yields the relationship:
−T∆S total=∆H system−T∆S system

The Gibbs free energy, G, (occasionally referred to as the Gibbs energy or Gibbs
function) is defined by G=H−TS. At constant pressure and temperature, finite changes
may be expressed as:
∆G−∆H system−T∆S system

∆G is therefore equal to −T∆S total, and the free energy may be regarded as a measure of
the total entropy change (both in the system and the surroundings) for a process. Whilst a
spontaneous process gives rise to a positive value of ∆S, ∆G must be negative because
of the minus sign in ∆G=−T∆S total.
∆G<0 for a spontaneous process at constant pressure

A similar function, used for work at constant volume and temperature, is termed the
Helmholtz free energy, A (also known as the Helmholtz energy or Helmholtz function).
As ∆S surroundings=−∆U system/T, under these conditions, the Helmholtz free energy is defined
as A=U−TS, and ∆A is therefore equal to −T∆S total at constant volume.
∆A<0 for a spontaneous process at constant volume

The Helmholtz free energy is useful in closed systems where changes occur (or may be
approximated to occur) under constant volume conditions, such as reactions or processes