Page 48 - Modern physical chemistry

3

Gases and

Collective Properties

3. 1 Macroscopic Properties

FROM THE SIZE OF AVOGADRO'S NUMBER, we know that even small amounts of mate-
rial contain an enormous number of formula units. As a consequence, a person cannot
follow the detailed motions of the molecules, radicals, ions, or electrons therein. Never-
theless, one can get at the collective behavior of these.
The section of material chosen for study is called a system. The neighboring material
that interacts with the system in various ways is called the surroundings. The collective
behavior produces properties that can be observed. In a uniform material, some of these
vary directly with the amount considered. Such a property is said to be extensive. As exam-
ples, we have volume, mass, energy of thermal agitation, moles of each constituent. In
contrast, other cbservable properties are independent of the amount of material present.
Such a property is said to be intensive. As examples, we have density, pressure, temper-
ature, concentrations. The macroscopic state of a uniform system is defined when enough
of these collective properties are given so that all other such properties are fixed.
Those that are picked to determine the state of the system form the independent vari-
ables. The properties dependent on the values of the chosen independent variables are
called dependent variables.
The volume elements over which intensive properties vary continuously, if at all, form
a phase. A system containing a single uniform phase is said to be lwmogeneous. A system
containing two or more phases is said to be heterogeneous.
Phases are classified as solid, liquid, or gaseous following the criteria in Section 1.2,
In any given phase, interactions among the molecules or ions tend to eliminate any local
orderly motion or concentration of energy that might be introduced. Transients tend to
disappear leaving a steady or equilibrium state.

3.2 Elements of Ideal-Gas Theory
A given gas may be considered at a low enough density so that its molecules do not inter-
act appreciably with each other except during collisions. Such a phase, where the volume
of the molecules and the attractions among them are negligible, is called an ideal gas.
Whether or not a given phase behaves as an ideal gas, collisions between molecules
break down any orderly motion or concentration of energy that might be introduced. The

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