Basic Chemical Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 14

surroundings. The universe is considered an isolated system. A system is said to be
adiabatic if it cannot exchange heat with its surroundings. A system is composed of
spatially uniform entities called phases . Air, water, and soil are three environmental
phases.The properties thatcharacterizeasystemaredefinedbytemperature,pressure,
volume, density, composition, surface tension, viscosity, and so on. There exist two
types of properties: intensive and extensive . Intensive properties are those that do
not require any reference to the mass of the system and are nonadditive. Extensive
properties are dependent on the mass of the system and are additive. Any change in
system properties is termed a process .
Despite its name, thermodynamics does not deal with the dynamics of a system,
but its equilibrium state . The word equilibrium implies a state of balance. When a
system is in such a state that upon slight disturbances it returns to its original state,
it is said to be in a state of equilibrium . This state of equilibrium is one in which
the system is time-invariant in properties. A thermodynamic system can experience
thermal, mechanical, or chemical equilibrium.
If a system undergoes no apparent changes, it is said to be stable . However, there
are systems that are apparently stable because the rates of change within them are
imperceptible. Such systems are called inert . The degree of stability of a system can
vary. For example, a topic placed flat on a table is at its state of rest and is stable.
However, when pushed over the table to the ground it reaches another state of rest,
which is also stable.
The fundamental principles of thermodynamics can be stated in terms of what are
called system variables, modes of energy transfer , and characteristic state functions
(Stumm and Morgan, 1981). The system variables are five in number, namely, tem-
perature T , pressure P , volume V , moles n , and entropy S . There are only two modes
of energy transfer: heat, q , transferred to a system from its surroundings; and work, w ,
done on the system by its surroundings. A state function is one that depends only on
the initial and final states of the system and not on the path by which the system may
pass between states. Only one such characteristic state function, internal energy U ,is
necessary to define the system. However, three other characteristic state functions are
also used in thermodynamics. They are Helmholtz free energy A , Gibbs free energy
G , and enthalpy H .
2.2 FUNDAMENTAL LAWS OF THERMODYNAMICS
There are four fundamental laws on which the edifice of thermodynamics is built.
These are called zeroth, first, second, and third laws. These laws, which lead to
modern thermodynamics, are the work of several scientists over a considerable time
span (see Table 2.1).
The unique feature of these laws is that although derived through generalizations
resultingfromnumerousexperimentaldata,theyhavenotyetbeendisproved.Einstein
(1949) remarked that “ ... classical thermodynamics ... is the only physical theory of
universal content concerning which I am convinced that, within the framework of the
applicability of its basic concepts, it will never be overthrown ... .” Thermodynamics
is, as Einstein called it, “a theory of principle” based on “empirically observed general