3

A

LTHOUGH THE MATHEMATICAL AND CONCEPTUAL TOOLS

PROVIDED BY THE ZEROTH AND FIRST LAWS OF THERMO-

DYNAMICS ARE VERY USEFUL, we need more. There is a major question

that these laws cannot answer: Will a given process occur spontaneously?

Nothing in the previous chapters addresses spontaneity, which is an important

concept. Thermodynamics helps to understand the spontaneity of processes—

but only once we add more of its tools. These tools are called the second and

third laws of thermodynamics.

3.1 Synopsis

As useful as the first law of thermodynamics is, we will see that it is limited.

There are some questions that it cannot answer. First, we will consider some of

the limitations of the first law. We will then introduce efficiency and see how

it applies to engines, which are devices that convert heat into work. The sec-

ond law of thermodynamics can be expressed in terms of efficiency, so we will

introduce the second law at this point.

Our treatment of engines will suggest a new state function, called entropy.

Using its initial definition as a start, we will derive some equations that allow

us to calculate the entropy changes for various processes. After considering a

different way of defining entropy, we will state the third law of thermodynam-

ics, which makes entropy a unique state function in thermodynamics. Finally,

we will consider entropy changes for chemical reactions.

In this chapter, we focus almost exclusively on the entropy of the system,

not the surroundings. Most processes of interest to us involve some sort of in-

teraction between the system and the surroundings, but the system itself re-

mains the part of the universe of interest to us.

3.2 Limits of the First Law

Will a chemical or physical process occur spontaneously? A process occurring

inside a system is spontaneousif the surroundings are not required to perform

work on the system. For example, if you drop a rock from a waist-high height,

the rock will fall spontaneously. When the plunger of a spray can of hair spray

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3.1 Synopsis

3.2 Limits of the First Law

3.3 The Carnot Cycle and
Efficiency

3.4 Entropy and the Second Law
of Thermodynamics

3.5 More on Entropy

3.6 Order and the Third Law
of Thermodynamics

3.7 Entropies of Chemical
Reactions

3.8 Summary

The Second and Third Laws

of Thermodynamics