2

T

HE PREVIOUS CHAPTER ESTABLISHED THAT MATTER BEHAVES

ACCORDING TO CERTAIN RULES called equations of state. We can

now begin to understand the rules by which energybehaves. Even though we

will primarily be using gases as examples, the ideas of thermodynamics are ap-

plicable to all systems, whether solid, liquid, gas, or any combination of phases.

Thermodynamics was developed mostly in the nineteenth century. This was

after the acceptance of the modern atomic theory of Dalton but before the

ideas of quantum mechanics (which imply that the microscopic universe of

atoms and electrons follow different rules than the macroscopic world of large

masses). Therefore, thermodynamics mostly deals with large collections of

atoms and molecules. The laws of thermodynamics are macroscopicrules. Later

in the text we will cover microscopicrules (that is, quantum mechanics), but

for now remember that thermodynamics deals with systems we can see, feel,

weigh, and manipulate with our own hands.

2.1 Synopsis

First, we will define work, heat, and internal energy. The first law of thermo-

dynamics is based on the relationship between these three quantities. Internal

energy is one example of a state function. State functions have certain proper-

ties that we will find useful. Another state function, enthalpy, will also be in-

troduced. Changes in state functions will be considered, and we will develop

ways to calculate how internal energy and enthalpy change during a physical

or chemical process. We will also introduce heat capacities and Joule-Thomson

coefficients, both of which are related to temperature changes in systems. We

will end the chapter by recognizing that the first law of thermodynamics is lim-

ited in its predictions, and that other ideas—other laws of thermodynamics—

are needed to understand how energy interacts with matter.

2.2 Work and Heat

Physically,workis performed on an object when the object moves some dis-

tance sdue to the application of a force F. Mathematically, it is the dot prod-

uct of the force vector Fand the distance vector s:

24

2.1 Synopsis

2.2 Work and Heat

2.3 Internal Energy and the First

Law of Thermodynamics

2.4 Stae Functions

2.5 Enthalpy

2.6 Changes in State Functions

2.7 Joule-Thomson Coefficients

2.8 More on Heat Capacities

2.9 Phase Changes

2.10 Chemical Changes

2.11 Changing Temperatures

2.12 Biochemical Reactions

2.13 Summary

The First Law of

Thermodynamics