Specific heat of polyatomic gases

THE MOLECULAR STRUCTURE AND SPECIFIC HEAT OF POLYATOMIC GASES... 

In view, particularly, of our inadequate knowledge of the behaviour of the specific heats of polyatomic gases, it will only rarely be possible to calculate the coefficients ft or higher terms consequently the following still more simplified approximation formula may be employed temporarily ... 

Thus, one gets the molar specific heats of the gases composed of monatomic, diatomic, and polyatomic molecules as... 

The specific heat ratio defined in Eq. (1.9) is 5/3 for monatomic molecules and 9/7 for diatomic molecules. Since the excitations of rotational and vibrational freedoms occur only over restricted temperatures, the specific heats determined by kinetic theory are different from the specific heats determined by experiments. However, the results obtained by the theory are valuable to understand the behavior of molecules and the process of energy conversion in the thermochemistry of combustion. Figure 1-1 shows the specific heats of real gases appearing in combustion. The specific heats of monatomic gases remain constant when temperature is increased as are determined by kinetic theory. The specific heats of diatomic and polyatomic gases are increased as the rotational and vibrational modes are excited by the increases of temperature. 

Easily hquefiable diatomic gases (CI2), and all tri- and polyatomic gases, show a very distinct increase of both specific heats with the temperature. It would seem that our present theory of gases breaks down here utterly, and the failure of the theory becomes more apparent as we go to higher and higher temperatures. The deviations of real gases from the gas laws, which we discussed earlier in the book, are of a totally different Pier, Zeitschr. f. Elektrochemie 16, 897 (1910). 

---