Page 118 - Physical chemistry understanding our chemical world

INTRODUCTION TO THERMODYNAMICS: INTERNAL ENERGY 85

Furthermore, because internal energy is a state function, the over-
The ‘first law of ther-
all change in U is zero following a series of changes described by
modynamics’ says
a closed loop. As an example, imagine three processes: a change
energy can neither be
from A → B, then B → C and finally from C → A. The only rea-
created nor destroyed,
son why the net value of U for this cycle is zero is because we only converted from
have neither lost nor picked up any energy over the cycle. We can one form to another.
summarize this aspect of physical chemistry by saying, ‘energy can-
not be created or destroyed, only converted’ – a vital truth called
the first law of thermodynamics.
If we measure U over a thermodynamic cycle and obtain a non-zero value,
straightaway we know the cycle is either incomplete (with one or more processes not
accounted for) or we employed a sloppy technique while measuring U.

Aside

William Rankine was the first to propose the first law of thermodynamics explicitly, in
1853 (he was famous for his work on steam engines). The law was already implicit in the
work of other, earlier, thermodynamicists, such as Kelvin, Helmholtz and Clausius. None
of these scientists sought to prove their theories experimentally; only Joule published
experimental proof of the first law.

Why is the water at the top of a waterfall cooler than
the water at its base?

The mechanical equivalence of work and energy

Two of the architects of modern thermodynamics were William Thompson (better
known as Lord Kelvin) and his friend James Prescott Joule – a scientist of great
vision, and a master of accurate thermodynamic measurement, as well as being some-
thing of an English eccentric. For example, while on a holiday in Switzerland in 1847,
Thompson met Joule. Let Thompson describe what he saw:

I was walking down from Chamonix to commence a tour of Mont Blanc, and whom
should I meet walking up but Joule, with a long thermometer in his hand and a carriage
with a lady in, not far off. He told me that he had been married since we parted in
Oxford [two weeks earlier] and that he was going to try for the elevation of temperature
in waterfalls.

Despite it being his honeymoon, Joule possessed a gigantic thermometer fully 4 to
5 feet in length (the reports vary). He spent much of his spare time during his honey-
moon in making painstaking measurements of the temperature at the top and bottom
of elongated Swiss waterfalls. He determined the temperature difference between the