CHAPTER 2 SYSTEMS AND THEIR PROPERTIES
2.4 THESTATE OF THESYSTEM 49
state on this longer time scale. This consideration of time scale is similar to the one we
apply to the persistence of deformation in distinguishing a solid from a fluid (Sec.2.2.1).
Even if a system is not in internal equilibrium, it can be in an equilibrium state if a
change of state is prevented by an imposed internal constraint or the influence of an external
field. Here are five examples of such states.
A system with an internal adiabatic partition separating two phases can be in an equi-
librium state that is not in thermal equilibrium. The adiabatic partition allows the two
phases to remain indefinitely at different temperatures. If the partition is rigid, it can
also allow the two phases to have different pressures, so that the equilibrium state
lacks mechanical equilibrium.
An experimental system used to measure osmotic pressure (Fig.12.2on page 372 )
has a semipermeable membrane separating a liquid solution phase and a pure solvent
phase. The membrane prevents the transfer of solute from the solution to the pure
solvent. In the equilibrium state of this system, the solution has a higher pressure than
the pure solvent; the system is then in neither transfer equilibrium nor mechanical
equilibrium.
In the equilibrium state of a galvanic cell that is not part of a closed electrical circuit
(Sec.3.8.3), the separation of the reactants and products and the open circuit are
constraints that prevent the cell reaction from coming to reaction equilibrium.
A system containing mixed reactants of a reaction can be in an equilibrium state
without reaction equilibrium if we withhold a catalyst or initiator or introduce an
inhibitor that prevents reaction. In the example above of a mixture of H 2 and O 2
gases, we could consider the high activation energy barrier for the formation of H 2 O
to be an internal constraint. If we remove the constraint by adding a catalyst, the
reaction will proceed explosively.
An example of a system influenced by an external field is a tall column of gas in a
gravitational field (Sec.8.1.4). In order for an equilibrium state to be established in
this field, the pressure must decrease continuously with increasing elevation.
Keep in mind that regardless of the presence or absence of internal constraints and
external fields, the essential feature of an equilibrium state is this: if we isolate the system
while it is in this state,the state functions do not change over time.
Three additional comments can be made regarding the properties of equilibrium states.
1.It should be apparent that a system with thermal equilibrium has a single value of
T, and one with mechanical equilibrium has a single value ofp, and this allows the
state to be described by a minimal number of independent variables. In contrast, the
definition of a nonequilibrium state with nonuniform intensive properties may require
a very large number of independent variables.
2.Strictly speaking, during a time period in which the system exchanges energy with the
surroundings its state cannot be an equilibrium state. Energy transfer at a finite rate
causes nonuniform temperature and pressure within the system and prevents internal
thermal and mechanical equilibrium. If, however, the rate of energy transfer is small,
then at each instant the state can closely approximate an equilibrium state. This topic
will be discussed in detail in the next chapter.