Page 23 - Physical Chemistry

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Chapter 1 An open system is one where transfer of matter between system and surroundings
Thermodynamics can occur. A closed system is one where no transfer of matter can occur between sys-
tem and surroundings. An isolated system is one that does not interact in any way with
its surroundings. An isolated system is obviously a closed system, but not every closed
system is isolated. For example, in Fig. 1.2, the system of liquid water plus water vapor
in the sealed container is closed (since no matter can enter or leave) but not isolated
(since it can be warmed or cooled by the surrounding bath and can be compressed or
expanded by the mercury). For an isolated system, neither matter nor energy can be
transferred between system and surroundings. For a closed system, energy but not
matter can be transferred between system and surroundings. For an open system, both
matter and energy can be transferred between system and surroundings.
A thermodynamic system is either open or closed and is either isolated or non-
isolated. Most commonly, we shall deal with closed systems.

Walls
A system may be separated from its surroundings by various kinds of walls. (In
Fig. 1.2, the system is separated from the bath by the container walls.) A wall can be
either rigid or nonrigid (movable). A wall may be permeable or impermeable,
where by “impermeable” we mean that it allows no matter to pass through it. Finally,
a wall may be adiabatic or nonadiabatic. In plain language, an adiabatic wall is one
that does not conduct heat at all, whereas a nonadiabatic wall does conduct heat.
However, we have not yet defined heat, and hence to have a logically correct devel-
opment of thermodynamics, adiabatic and nonadiabatic walls must be defined without
reference to heat. This is done as follows.
W Suppose we have two separate systems A and B, each of whose properties are ob-
served to be constant with time. We then bring A and B into contact via a rigid, imper-
meable wall (Fig. 1.3). If, no matter what the initial values of the properties of A and B
A B are, we observe no change in the values of these properties (for example, pressures, vol-
umes) with time, then the wall separating A and B is said to be adiabatic. If we gener-
ally observe changes in the properties of A and B with time when they are brought in con-
tact via a rigid, impermeable wall, then this wall is called nonadiabatic or thermally
Figure 1.3 conducting. (As an aside, when two systems at different temperatures are brought in
Systems A and B are separated by contact through a thermally conducting wall, heat flows from the hotter to the colder sys-
a wall W. tem, thereby changing the temperatures and other properties of the two systems; with an
adiabatic wall, any temperature difference is maintained. Since heat and temperature are
still undefined, these remarks are logically out of place, but they have been included to
clarify the definitions of adiabatic and thermally conducting walls.) An adiabatic wall is
an idealization, but it can be approximated, for example, by the double walls of a Dewar
flask or thermos bottle, which are separated by a near vacuum.
In Fig. 1.2, the container walls are impermeable (to keep the system closed) and
are thermally conducting (to allow the system’s temperature to be adjusted to that of
the surrounding bath). The container walls are essentially rigid, but if the interface
between the water vapor and the mercury in the manometer is considered to be a
“wall,” then this wall is movable. We shall often deal with a system separated from its
surroundings by a piston, which acts as a movable wall.
A system surrounded by a rigid, impermeable, adiabatic wall cannot interact with
the surroundings and is isolated.

Equilibrium
Equilibrium thermodynamics deals with systems in equilibrium. An isolated system
is in equilibrium when its macroscopic properties remain constant with time. A non-
isolated system is in equilibrium when the following two conditions hold: (a) The
system’s macroscopic properties remain constant with time; (b) removal of the system