Specific heat of real gases

All known gases, called real gases, are nonideal, which means that they do not obey the fundamental gas laws and the equation pv =RT [See under "Detonation (and Explosion), Equations of State , in this Volume]. Specific heats of "real gases vary with temperature and the product composition depends upon both temperature and pressure. 

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. 

The specific heats of a substance, in general, depend on two independent properties such as temperature and pressure. For an ideal gas, however, they depend on temperature only (Fig. 1-9). At low pressures all real gases approach ideal gas behavior, and therefore their specific heats depend on temperature only. 

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). 

UNIVERSAL REPRESENTATION OF THE REAL COMPONENTS OF THE SPECIFIC HEATS OF GASES. 

Variable of the equation of state of real gases, covolumes (m /kg) Specific heat capacity at a constant pressure (J/(kgK))... 

The ideal gas values can be used for the real gases at low pressures. At high pressures the effect of pressure on the specific heat may be appreciable. 

Edmister (1948) published a generalized plot showing the isothermal pressure correction for real gases as a function of the reduced pressure and temperature. His chart, converted to SI units, is shown as Eigure 3.2. Edmister s chart was based on hydrocarbons but can be used for other materials to give an indication of the likely error if the ideal gas specific heat values are used without corrections. 

In fact we find that real gases do not exhibit this behaviour and the specific heat is not equal to that of the perfect gas. For example, fig. 11.3 shows the molar heat capacity Cy of ammonia as a function of T and p. 

THE UNIVERSAL REPRESENTATION OF THE REAL PART OF THE SPECIFIC HEAT AT CONSTANT PRESSURE OF GASES. PH.D. 

The specific heat capacity of an ideal gas is the basic quantity for the enthalpy calculation, as it is independent from molecular interactions. It is also possible to define a real gas heat capacity, but for process calculations it is more convenient to account for the real gas effects with the enthalpy description of the equation of state used (see Section 6.2). In process calculations, the specific heat capacity of ideal gases mainly determines the duty of gas heat exchangers, and it has an influence on the heat transfer coefficient as well. 

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