c04 JWBS043-Rogers September 13, 2010 11:24 Printer Name: Yet to Come

4

THERMOCHEMISTRY

Einstein once said, “Some things are simple but not easy.” Although he did not

have thermochemistry in mind, his comment applies to this time-honored branch

of physical chemistry. Thermochemistry is simple: Run a chemical reaction and

measure the temperature change (if any). But it is not easy. Anyone trying to do

this job at state-of-the-art precision will soon be enmeshed in technical problems

that try the patience of Job. Entire institutes of experimental science exist just for

precise measurement of the heat of chemical reactions. If governments are willing to

spend millions of dollars to support acquisition of thermochemical data, there must

be some significant advantage to be gained from them. That, in part, will be discussed

in this chapter. Today, computers play a large role in this field. The most significant

advance in thermochemistry in the last decade is the calculation of thermochemical

quantities from quantum mechanical first principles. That also will be introduced in

this chapter.

4.1 CALORIMETRY

A calorimeter is a device intended to measure heat given out or taken up when

a chemical reaction or a physical change, such as a change of state, takes place.

A Styrofoam coffee cup with a thermometer is a crude calorimeter. The word has

historical significance; it was once supposed to measure the amount of “caloric”

flowing into or out of a system. We have long since discarded the caloric theory but

Concise Physical Chemistry,by Donald W. Rogers

Copyright©C2011 John Wiley & Sons, Inc.

56