Page 25 - Physical Chemistry

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Chapter 1 AgBr crystals in equilibrium with an aqueous solution, all the crystals are part of the
Thermodynamics same phase. Note that the definition of a phase does not mention solids, liquids, or
gases. A system can be entirely liquid (or entirely solid) and still have more than one
phase. For example, a system composed of the nearly immiscible liquids H O and
2
CCl has two phases. A system composed of the solids diamond and graphite has two
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phases.
A system composed of two or more phases is heterogeneous.
The density r (rho) of a phase of mass m and volume V is

r m>V (1.2)*
Figure 1.4 plots some densities at room temperature and pressure. The symbols s, l,
and g stand for solid, liquid, and gas.
Suppose that the value of every thermodynamic property in a certain thermody-
namic system equals the value of the corresponding property in a second system.
The systems are then said to be in the same thermodynamic state. The state of a
thermodynamic system is defined by specifying the values of its thermodynamic prop-
erties. However, it is not necessary to specify all the properties to define the state.
Specification of a certain minimum number of properties will fix the values of all other
properties. For example, suppose we take 8.66 g of pure H O at 1 atm (atmosphere)
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pressure and 24°C. It is found that in the absence of external fields all the remaining
properties (volume, heat capacity, index of refraction, etc.) are fixed. (This statement
ignores the possibility of surface effects, which are considered in Chapter 7.) Two
thermodynamic systems each consisting of 8.66 g of H O at 24°C and 1 atm are in the
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same thermodynamic state. Experiments show that, for a single-phase system con-
taining specified fixed amounts of nonreacting substances, specification of two addi-
tional thermodynamic properties is generally sufficient to determine the thermody-
namic state, provided external fields are absent and surface effects are negligible.
A thermodynamic system in a given equilibrium state has a particular value for
each thermodynamic property. These properties are therefore also called state
functions, since their values are functions of the system’s state. The value of a state
function depends only on the present state of a system and not on its past history. It
doesn’t matter whether we got the 8.66 g of water at 1 atm and 24°C by melting ice
Figure 1.4 and warming the water or by condensing steam and cooling the water.
Densities at 25°C and 1 atm. The
scale is logarithmic.
1.3 TEMPERATURE

Suppose two systems separated by a movable wall are in mechanical equilibrium with
each other. Because we have mechanical equilibrium, no unbalanced forces act and
each system exerts an equal and opposite force on the separating wall. Therefore each
system exerts an equal pressure on this wall. Systems in mechanical equilibrium with
each other have the same pressure. What about systems that are in thermal equilibrium
(Sec. 1.2) with each other?
Just as systems in mechanical equilibrium have a common pressure, it seems
plausible that there is some thermodynamic property common to systems in thermal
equilibrium. This property is what we define as the temperature, symbolized by u (theta).
By definition, two systems in thermal equilibrium with each other have the same temper-
ature; two systems not in thermal equilibrium have different temperatures.
Although we have asserted the existence of temperature as a thermodynamic state
function that determines whether or not thermal equilibrium exists between systems,
we need experimental evidence that there really is such a state function. Suppose that
we find systems A and B to be in thermal equilibrium with each other when brought
in contact via a thermally conducting wall. Further suppose that we find systems B and