Page 24 - Physical Chemistry

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from contact with its surroundings causes no change in the properties of the system. Section 1.2
If condition (a) holds but (b) does not hold, the system is in a steady state. An exam- Thermodynamics
ple of a steady state is a metal rod in contact at one end with a large body at 50°C and
in contact at the other end with a large body at 40°C. After enough time has elapsed,
the metal rod satisfies condition (a); a uniform temperature gradient is set up along the
rod. However, if we remove the rod from contact with its surroundings, the tempera-
tures of its parts change until the whole rod is at 45°C.
The equilibrium concept can be divided into the following three kinds of equilib-
rium. For mechanical equilibrium, no unbalanced forces act on or within the system;
hence the system undergoes no acceleration, and there is no turbulence within the sys-
tem. For material equilibrium, no net chemical reactions are occurring in the system,
nor is there any net transfer of matter from one part of the system to another or be-
tween the system and its surroundings; the concentrations of the chemical species in
the various parts of the system are constant in time. For thermal equilibrium between
a system and its surroundings, there must be no change in the properties of the system
or surroundings when they are separated by a thermally conducting wall. Likewise, we
can insert a thermally conducting wall between two parts of a system to test whether
the parts are in thermal equilibrium with each other. For thermodynamic equilibrium,
all three kinds of equilibrium must be present.

Thermodynamic Properties
What properties does thermodynamics use to characterize a system in equilibrium?
Clearly, the composition must be specified. This can be done by stating the mass of
each chemical species that is present in each phase. The volume V is a property of the
system. The pressure P is another thermodynamic variable. Pressure is defined as the
magnitude of the perpendicular force per unit area exerted by the system on its sur-
roundings:
P F>A (1.1)*

where F is the magnitude of the perpendicular force exerted on a boundary wall of
area A. The symbol indicates a definition. An equation with a star after its number
should be memorized. Pressure is a scalar, not a vector. For a system in mechanical
equilibrium, the pressure throughout the system is uniform and equal to the pressure
of the surroundings. (We are ignoring the effect of the earth’s gravitational field, which
causes a slight increase in pressure as one goes from the top to the bottom of the sys-
tem.) If external electric or magnetic fields act on the system, the field strengths are
thermodynamic variables; we won’t consider systems with such fields. Later, further
thermodynamic properties (for example, temperature, internal energy, entropy) will be
defined.
An extensive thermodynamic property is one whose value is equal to the sum of
its values for the parts of the system. Thus, if we divide a system into parts, the mass
of the system is the sum of the masses of the parts; mass is an extensive property. So
is volume. An intensive thermodynamic property is one whose value does not depend
on the size of the system, provided the system remains of macroscopic size—recall
nanoscopic systems (Sec. 1.1). Density and pressure are examples of intensive prop-
erties. We can take a drop of water or a swimming pool full of water, and both sys-
tems will have the same density.
If each intensive macroscopic property is constant throughout a system, the sys-
tem is homogeneous. If a system is not homogeneous, it may consist of a number of
homogeneous parts. A homogeneous part of a system is called a phase. For example,
if the system consists of a crystal of AgBr in equilibrium with an aqueous solution
of AgBr, the system has two phases: the solid AgBr and the solution. A phase can con-
sist of several disconnected pieces. For example, in a system composed of several