Basic Chemical Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 13

2 Basic Chemical

Thermodynamics

ThediscussioninChapter1indicatedthatknowledgeofequilibriumbetweendifferent
compartments is important in environmental engineering.The fundamental principles
of thermodynamics are germane to the understanding of equilibrium in environmental
systems. There are four laws of thermodynamics that epitomize the entire subject of
thermodynamics.
In this chapter the discussion of thermodynamics is limited to its basic laws, since
a number of excellent references are available elsewhere (e.g., Lewis and Randall,
1961; Denbigh, 1981). Phase equilibrium, as it relates to bulk phases (single com-
ponent, homogeneous), will be discussed in this chapter, followed by an expanded
discussion of multicomponent and heterogeneous systems in Chapter 3. Since most
of the applications of thermodynamics in environmental engineering are confined to
a narrow range of temperatures ( 40 ◦ C) and atmospheric pressure, we

need not focus on extreme temperatures or pressures. A brief review of the concept
of equilibrium and the fundamental laws of thermodynamics is presented, followed
by the introduction of the concepts of free energy, chemical potential, and the Gibbs-
Duhem relationship. The effect of surface area on the total thermodynamic property
of a system is negligible except when the subdivision in phases is exceedingly small,
which occurs in many environmental systems. Therefore, an introductory discussion
of surface thermodynamics is also included.

40 ◦ Cto

∼− +

2.1 EQUILIBRIUM
The natural environment is a complex system. However, an environmental system can
be assessed by assuming that equilibrium exists between the different environmental
phases and that the properties are time-invariant. Equilibrium models are easy to
apply since they need only a few inputs. They are sometimes grossly inappropriate
and have to be replaced with kinetic models, which assume time-variant properties
for the phases. Kinetic models are complex and require a number of input parameters
that are poorly understood and sometimes unavailable for environmental situations.
Before beginning a discussion of the laws of thermodynamics, we need to define
some terms. To quote Lewis and Randall (1961), “Whatever part of the objective
world is the subject of thermodynamic discourse is customarily called a system.” The
system may be separated from the surroundings by a physical boundary like the walls
of a container, or maybe concrete.A system may be closed if there is no mass transfer
either to or from the surroundings, or open if either matter or energy can transfer
to and from the surroundings. For example, the Earth is an open system in contact
with the atmosphere around it with which it exchanges both matter and energy. A
system is said to be isolated if it cannot exchange either mass or energy with its

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