Effectively, this provides a convenient definition of the zero value. In
the original formulation by Nerst, the entropy change accompanying any
physical or chemical transformation approaches zero as the temperature
approaches zero provided the substances are perfectly ordered. With this
definition, the absolute value of entropy may be either positive or negative
at any given value, with a zero value assigned to any temperature. As a
matter of convenience, this law is usually expressed such that the entropy
is defined to be always non-negative. With this choice for poising a zero
value for entropy, the third law is expressed as follows.
The third law of thermodynamics states that the entropy of all perfectly
crystalline substances is zero at a temperature of zero.
The third law is used for biological applications as it provides a basis for
the definition of entropies of materials relative to their crystalline state.
However, this law does not normally have a significant impact on bio-
logical systems as organisms live at room temperature and their properties
at T=0 are not relevant for understanding their cellular processes. The
exception arises when cellular components are investigated at low tem-
peratures for spectroscopic studies.
Gibbs energy
A process is spontaneous if the o 9 erall entropy change for the system is positi 9 e.
If the entropy is negative, then the process is not favored and thus not
spontaneous. However, the calculation of the entropy changes can be diffi-
cult as changes in both the system and surroundings must be considered.
In the 1800s, the American theoretician Josiah Willard Gibbs established
a new state function that is now termed Gibbs energy in his honor. The
state function not only provides a means of establishing both positive and
negative entropy changes but also is straightforward to establish for a given
system. The Gibbs energy can be used to not only determine whether a
reaction will proceed but also how much energy is released.
The o 9 erall entropy change,ΔStot, is the sum of the entropy changes from the
system, ΔS, and surroundings, ΔSsur. The entropy change of the surroundings
at constant pressure is just the entropy divided by the temperature (from
eqn 3.9):
(3.16)
So the total entropy change,ΔStot, at constant temperature and pressure is
given by:
Δ
Δ
S
H
sur T
=−
54 PARTI THERMODYNAMICS AND KINETICS
