Page 64 - Instant notes

B4
ENTROPY

Key Notes
For any process in which energy is transferred from one body to
another, it is possible to transfer the energy in one of two ways. If
the energy is transferred reversibly to or from a system, it must be
possible to reverse the direction of the transfer through an
infinitesimal change in the conditions. In practice, this requires
that the energy be transferred infinitely slowly. An irreversible
process results from energy transfer which is not transferred
under these conditions.
Entropy is a thermodynamic property of a system, denoted as S. It
is a state function and is defined in terms of entropy changes
rather than its absolute value. For a reversible process at constant
temperature, the change in entropy, dS, is given by dS=dq rev /T.
For an irreversible process, dS>dq/T.
In addition to the thermodynamic definition of entropy, it is also
possible to refer to entropy in statistical terms. For any system,
the entropy is given by S=k B ln(W), where W is the number of
possible configurations of the system. This definition allows the
entropy to be understood as a measure of the disorder in a system.
The third law of thermodynamics states that the entropy of a
perfectly crystalline solid at the absolute zero of temperature is
zero. The entropy has a measurable absolute value for a system,
in contrast to the enthalpy and internal energy. There is no
requirement for standard entropies of formation to be defined, as
the absolute values of entropy may be used in all calculations.
Related topics The first law (B1) Entropy and change (B5)
Enthalpy (B2) Free energy (B6)
Thermochemistry (B3) Statistical thermodynamics (G8)

Reversible and irreversible processes

Any process involving the transfer of energy from one body to another may take place
either reversibly or irreversibly. In a reversible process, energy is transferred in such a
way as to ensure that at any point in the process the transfer er may be reversed by an
infinitesimally small change in the conditions. The system is therefore in equilibrium
throughout the transfer. In practice, this means that the energy must be transferred