Page 56 - Physical Chemistry

lev38627_ch02.qxd 2/29/08 3:11 PM Page 37

CHAPTER
2

The First Law of

Thermodynamics

CHAPTER OUTLINE

2.1 Classical Mechanics

2.2 P-V Work
Chapter 1 introduced some of the vocabulary of thermodynamics and defined the im- 2.3 Heat
portant state function temperature. Another key state function in thermodynamics is
the internal energy U, whose existence is postulated by the first law of thermodynam- 2.4 The First Law of
ics; this law is the main topic of Chapter 2. The first law states that the total energy of Thermodynamics
system plus surroundings remains constant (is conserved). Closely related to the in-
ternal energy is the state function enthalpy H, defined in Sec. 2.5. Other important 2.5 Enthalpy
state functions introduced in this chapter are the heat capacities at constant volume and
at constant pressure, C and C (Sec. 2.6), which give the rates of change of the inter- 2.6 Heat Capacities
P
V
nal energy and enthalpy with temperature [Eq. (2.53)]. As a preliminary to the main 2.7 The Joule and
work of this chapter, Sec. 2.1 reviews classical mechanics. Joule–Thomson Experiments
The internal energy of a thermodynamic system is the sum of the molecular ener-
gies, as will be discussed in detail in Sec. 2.11. Energy is a key concept in all areas of 2.8 Perfect Gases and the
physical chemistry. In quantum chemistry, a key step to calculating molecular proper- First Law
ties is solving the Schrödinger equation, which is an equation that gives the allowed
energy levels of a molecule. In statistical mechanics, the key to evaluating thermody- 2.9 Calculation of First-Law
namic properties from molecular properties is to find something called the partition Quantities
function, which is a certain sum over energy levels of the system. The rate of a chem-
ical reaction depends strongly on the activation energy of the reaction. More generally, 2.10 State Functions and Line
the kinetics of a reaction is determined by something called the potential-energy Integrals
surface of the reaction. 2.11 The Molecular Nature of
The importance of energy in the economy is obvious. World consumption of
20
20
energy increased from 3.0 10 J in 1980 to 4.9 10 J in 2005, with fossil fuels Internal Energy
(oil, coal, natural gas) supplying 86% of the 2005 total. 2.12 Problem Solving
Energy transformations play a key role in the functioning of biological organisms.
2.13 Summary

2.1 CLASSICAL MECHANICS
Two important concepts in thermodynamics are work and energy. Since these con-
cepts originated in classical mechanics, we review this subject before continuing with
thermodynamics.
Classical mechanics (first formulated by the alchemist, theologian, physicist, and
mathematician Isaac Newton) deals with the laws of motion of macroscopic bodies
whose speeds are small compared with the speed of light c. For objects with speeds
not small compared with c, one must use Einstein’s relativistic mechanics. Since the
thermodynamic systems we consider will not be moving at high speeds, we need not
worry about relativistic effects. For nonmacroscopic objects (for example, electrons),
one must use quantum mechanics. Thermodynamic systems are of macroscopic size,
so we shall not need quantum mechanics at this point.