Page 25 - Physical chemistry eng

2 CHAPTER 1 Fundamental Concepts of Thermodynamics

Given that the microscopic nature of matter is becoming increasingly well under-
stood using theories such as quantum mechanics, why is a macroscopic science like
thermodynamics relevant today? The usefulness of thermodynamics can be illustrated
by describing four applications of thermodynamics which you will have mastered after
working through this book:
• You have built a plant to synthesize NH 3 gas from N 2 and H 2 . You find that the
yield is insufficient to make the process profitable and decide to try to improve the
NH 3 output by changing the temperature and/or the pressure. However, you do not
know whether to increase or decrease the values of these variables. As will be
shown in Chapter 6, the ammonia yield will be higher at equilibrium if the temper-
ature is decreased and the pressure is increased.
• You wish to use methanol to power a car. One engineer provides a design for an
internal combustion engine that will burn methanol efficiently according to the
reaction CH OH(l) + 3>2O (g) : CO (g) + 2H O(l) . A second engineer
3
2
2
2
designs an electrochemical fuel cell that carries out the same reaction. He claims
that the vehicle will travel much farther if powered by the fuel cell than by the inter-
nal combustion engine. As will be shown in Chapter 5, this assertion is correct, and
an estimate of the relative efficiencies of the two propulsion systems can be made.
• You are asked to design a new battery that will be used to power a hybrid car.
Because the voltage required by the driving motors is much higher than can be gen-
erated in a single electrochemical cell, many cells must be connected in series.
Because the space for the battery is limited, as few cells as possible should be used.
You are given a list of possible cell reactions and told to determine the number of
cells needed to generate the required voltage. As you will learn in Chapter 11, this
problem can be solved using tabulated values of thermodynamic functions.
• Your attempts to synthesize a new and potentially very marketable compound have
consistently led to yields that make it unprofitable to begin production. A supervi-
sor suggests a major effort to make the compound by first synthesizing a catalyst
that promotes the reaction. How can you decide if this effort is worth the required
investment? As will be shown in Chapter 6, the maximum yield expected under
equilibrium conditions should be calculated first. If this yield is insufficient, a cata-
lyst is useless.

The Macroscopic Variables Volume,
1.2 Pressure, and Temperature

We begin our discussion of thermodynamics by considering a bottle of a gas such as He
or CH 4 . At a macroscopic level, the sample of known chemical composition is com-
z pletely described by the measurable quantities volume, pressure, and temperature for
which we use the symbols V, P, and T. The volume V is just that of the bottle. What
physical association do we have with P and T?
Pressure is the force exerted by the gas per unit area of the container. It is most eas-
ily understood by considering a microscopic model of the gas known as the kinetic the-
ory of gases. The gas is described by two assumptions: the atoms or molecules of an
v z
ideal gas do not interact with one another, and the atoms or molecules can be treated as
v
v y point masses. The pressure exerted by a gas on the container confining the gas arises
y from collisions of randomly moving gas molecules with the container walls. Because
v x the number of molecules in a small volume of the gas is on the order of Avogadro’s
number N , the number of collisions between molecules is also large. To describe pres-
A
sure, a molecule is envisioned as traveling through space with a velocity vector v that
can be decomposed into three Cartesian components: v , v , and v as illustrated in
x x y z
Figure 1.1.
2
FIGURE 1.1 The square of the magnitude of the velocity v in terms of the three velocity
Cartesian components of velocity. The components is
particle velocity v can be decomposed into #
2
2
2
three velocity components: v x , v y , and v z . v = v v = v + v + v 2 z (1.1)
x
y