Page 91 - Essentials of physical chemistry

4 The First Law

of Thermodynamics

INTRODUCTION
In the previous chapter, we sharpened our computational skills and gained an appreciation for the
particle model of gases. We now turn our attention to matters of energy and energy flow according
to the laws of thermodynamics. In the Math Review chapter, we showed that energy can flow
between various forms of kinetic and potential energy but that overall energy is conserved and only
the form is changed. Many things can be said about thermodynamics. Mainly, thermodynamics
owes more to the steam engine than the steam engine owes to thermodynamics. That means that
Watt [1] and other inventors built steam engines and got them to work using raw mechanical
reasoning and then thermodynamics was developed=discovered to understand the principles of the
engine. We will try to help you gain a foundation of understanding if you will follow along and use
pencil and paper to write out some derivations rather than just read the text. It should be understood
that while physics majors develop expertise in electromagnetic theory far more than chemistry
majors, it is generally true that chemistry majors gain a better understanding of thermodynamics.
Chemical engineers use thermodynamics as their main expertise, although augmented by kinetics
and transport theory, so the chemistry professionals should take pride that thermodynamics is ‘‘their
thing,’’ their chance to shine in terms of the scientific method.
Thermodynamics is necessarily more abstract than the study of mechanical devices because it is
not always easy to see ‘‘heat.’’ You will soon see that thermodynamics is wonderful for providing
information about ‘‘after-minus-before’’ processes, but often it tells us little about the mechanism of
the process being considered. The good news is that we do not need to know the details of the
mechanism of a process, but the bad news is that often thermodynamics does not provide any means
to determine the mechanism. This adds to the mystery of thermodynamics since often one can
substitute an imaginary process with the same beginning and ending to obtain results without
knowing the real mechanism but you will not learn the mechanism. Overall, thermodynamics offers
sweeping principles that are important in all the sciences: chemistry, physics, astronomy, and even
biology. We will see that there is an ongoing dynamic between the tendency of energy to decrease
and a tendency of randomness to increase. Living systems are caught in this dynamic, so basic
metabolism is subject to thermodynamics.

HISTORICAL DEVELOPMENT OF THERMODYNAMICS
Although we want to treat thermodynamics from the beginning of quantitative relationships, it may
be worth mentioning more primitive ideas. Prior to 1798, one explanation of heat produced by
friction was that heat is the release of a substance trapped within materials. The substance was called
‘‘caloric.’’ Actually at a very shallow grade school level this idea has some merit but just what is
caloric, a liquid, a vapor, some sort of ‘‘igneous fluid?’’ The breakthrough came from an American
named Benjamin Thompson (1753–1814) (Figure 4.1) who was born in Woburn, Massachusetts. He
became a Major in the British Army at the age of 19 and he left with the British in 1776, after the
surrender of Boston. He later entered into the employ of the Bavarian Government and was given
the title of Count Rumford. His key experiment was performed in Munich where he was involved in
the manufacture of cannon [2]. In the 1700s, cannon were large solid pieces of cast brass or iron

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