In the examples used in this book, we shall deal mainly with gases with small simple molecules at elevated temperatures and at relatively low pressures.
At P = 1.0 bar, the volumes of O 2, CO 2, and CH 3CH 2OH all deviate from the ideal value by < 0.03% (again assuming no dissociation), and at P = 10.0 bar, the deviations are still all < 0.06%.
However, at T = 1000 K, the deviations from ideality are much smaller. From these examples, it can be seen that, except in the case of the smallest molecules, deviations from ideal gas behavior must be taken into account in calculations near room temperature, particularly at elevated pressures. This is particularly true in the case of polar molecules such as ethanol. (These values were calculated from the first virial coefficient as given by the Tsonopoulos equation of state assuming no dissociation.) With larger molecules, the possibilities for intermolecular interactions are greater, and so the deviations from ideal behavior become larger. For the real gases O 2, CO 2, and CH 3CH 2OH (ethanol), the volumes under these conditions deviate from the ideal value by 0.06%, 0.5%, and 5.6%, respectively, while at 300 K and P = 10.0 bar, the deviations from the ideal volume (2.494 L) are 0.6%, 4.9%, and 56%, respectively. (2.32), the volume of an ideal gas is 24.94 L. (2.32).Īs examples of typical deviations from ideal behavior, consider 1.0 mol of a gas at T = 300 K and P = 1.0 bar. At higher pressures and lower temperatures, the molecules are in closer proximity, attractive and repulsive interactions become more important, and the equation of state of the gas becomes more complex than Eq.
Real gases approach ideal gas behavior as the pressure decreases and as the temperature increases. (2.32), can be derived from the kinetic theory of gases if it is assumed that the gaseous molecules do not interact with each other except through elastic collisions. Ideal gases were discussed in Section 2.4.1. Pelton, in Phase Diagrams and Thermodynamic Modeling of Solutions, 2019 2.4.2 A Note on Nonideal Gases
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