Page 67 - Physical Chemistry

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Chapter 2 development of thermodynamics requires no knowledge of the nature of U. All that is
The First Law of Thermodynamics needed is some means of measuring the change in U for a process. This will be pro-
vided by the first law of thermodynamics.
In most applications of thermodynamics that we shall consider, the system will be
at rest and external fields will not be present. Therefore, K and V will be zero, and the
total energy E will be equal to the internal energy U. (The effect of the earth’s gravi-
tational field on thermodynamic systems is usually negligible, and gravity will usually
be ignored; see, however, Sec. 14.8.) Chemical engineers often deal with systems of
flowing fluids; here, K
0.
With our present knowledge of the molecular structure of matter, we take it for
granted that a flow of heat between two bodies involves a transfer of internal energy
between them. However, in the eighteenth and nineteenth centuries the molecular the-
ory of matter was controversial. The nature of heat was not well understood until about
1850. In the late 1700s, most scientists accepted the caloric theory of heat. (Some stu-
dents still do, unhappily.) Caloric was a hypothetical fluid substance present in matter
and supposed to flow from a hot body to a cold one. The amount of caloric lost by the
hot body equaled the amount gained by the cold body. The total amount of caloric was
believed to be conserved in all processes.
Strong evidence against the caloric theory was provided by Count Rumford in
1798. In charge of the army of Bavaria, he observed that, in boring a cannon, a virtu-
ally unlimited amount of heating was produced by friction, in contradiction to the
caloric-theory notion of conservation of heat. Rumford found that a cannon borer
driven by one horse for 2.5 hr heated 27 lb of ice-cold water to its boiling point.
Addressing the Royal Society of London, Rumford argued that his experiments had
proved the incorrectness of the caloric theory.
Rumford began life as Benjamin Thompson of Woburn, Massachusetts. At 19 he married
a wealthy widow of 30. He served the British during the American Revolution and settled
in Europe after the war. He became Minister of War for Bavaria, where he earned extra
money by spying for the British. In 1798 he traveled to London, where he founded the
Royal Institution, which became one of Britain’s leading scientific laboratories. In 1805 he
married Lavoisier’s widow, adding further to his wealth. His will left money to Harvard to
establish the Rumford chair of physics, which still exists.

Despite Rumford’s work, the caloric theory held sway until the 1840s. In 1842
Julius Mayer, a German physician, noted that the food that organisms consume goes
partly to produce heat to maintain body temperature and partly to produce mechanical
work performed by the organism. He then speculated that work and heat were both
forms of energy and that the total amount of energy was conserved. Mayer’s argu-
ments were not found convincing, and it remained for James Joule to deal the death
blow to the caloric theory.
Joule was the son of a wealthy English brewer. Working in a laboratory adjacent to
the brewery, Joule did experiments in the 1840s showing that the same changes produced
by heating a substance could also be produced by doing mechanical work on the sub-
stance, without transfer of heat. His most famous experiment used descending weights
to turn paddle wheels in liquids. The potential energy of the weights was converted to
kinetic energy of the liquid. The viscosity (internal friction) of the liquid then converted
the liquid’s kinetic energy to internal energy, increasing the temperature. Joule found
that to increase the temperature of one pound of water by one degree Fahrenheit requires
the expenditure of 772 foot-pounds of mechanical energy. Based on Joule’s work, the
first clear convincing statement of the law of conservation of energy was published by
the German surgeon, physiologist, and physicist Helmholtz in 1847.
The internal energy of a system can be changed in several ways. Internal energy
is an extensive property and thus depends on the amount of matter in the system. The