|Prediction of chemical reactivity
by concentrating on the system - the free energy functions (The Helmholtz
free energy, The Gibbs free energy)
> System in thermal equilibrium with surroundings at a temperature T
> There is a change in the system accompanied by a transfer of energy dq between system and surroundings
> From the Classius Inequality we have:
(a) Under the conditions of constant volume we have no p-V work,
i.e. in the absence of other forms of work:
......................... (constant V, no non-expansion work)i.e.:
(b) Under the conditions of constant pressure, in the absence of other forms of work other than p-V, we have :
......................... (constant p, no non-expansion work)i.e.:
The two inequalities above (i.e. and ) and are so important (they determine whether a process is going to occur or not) that we shall assign to them two new thermodynamic functions namely:
(a) The Helmholtz free energy, A, given by:
These two functions may be differentiated to give:
Which at the conditions of constant temperature T, we have:
DG plays a very important role in standard laboratory conditions are usually under constant pressure and temperature.
For practical reasons, the standard Gibbs free energy for a reaction
is defined in terms of the standard Gibbs energies of formation, DfG0,where
standard Gibbs energy of formation is the standard reaction Gibbs energy
for the formation of a compound from its elements in their reference states.
Gibbs energies formation of the elements in their reference states are
zero, because their formation is 'null' reaction.),as follows:
The standard Gibbs free energy relates to the reaction enthalpy and entropy as follows:
The experimental values of DrG0 are obtained either through calorimetric methods (finding DH directly, and DS from the heat capacities), or through equilibrium constants (discussed at a later stage). Other methods are also possible.
The spontaneity of a reaction at constant temperature and pressure is
related to the Gibbs free energy as follows (see Fig. 1):
(i) If (exergonic, from the Greek word for work-producing), the forward reaction is spontaneous.Similar arguments may be presented for DA.
(3) The relationships between the free energies and entropy, enthalpy, and the internal energy.
(a) From both inequalities, it may be clearly deduced that a positive dS favours the spontaneity of the process (i.e. a negative dA or dG)
(b) From the inequality at constant volume, we may deduce that a negative dU favours a negative dA. The interpretation for this should be that a process with a negative dU is accompanied by a positive entropy change of the surroundings, given by '-dU / T ' (i.e. the spontaneity is not driven by the desire for a lower value of dU, but for a higher value of dSSUR. ) Systems change spontaneously if the total entropy (system + surroundings) increases, not because they tend to a lower internal energy.
(b) Note that endothermic reactions do occur provided that . Such reactions happen because the entropy increase in the system is greater than the entropy decrease of the surroundings ().
(4) The free energies and the concept of maximum work
(a) The change in the Helmholtz free energy is equal to the maximum work accompanying a process, i.e.:
Because of this, A is sometimes called the 'maximum work function' or the 'work function' (Arbeit is the German word for work, hence the symbol A). We can show this by combining the first law with the Classius inequality -
(b) In analogy, dG refers to the maximum non-expansion work.
(See Atkins, p. 117).