Equilibrium and Nonequilibrium Systems by kira73

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									 Equilibrium and Nonequilibrium Systems

Equilibrium and Nonequilibrium Systems
It is our experience that if a physical system is isolated, its state-specified by
macroscopic variables such as pressure, temperature and chemical
composition
 Table 1.1 The van der Waals constants a and b for some gases




Source: These values can be obtained from the critical constants in data
source [B]. A more extensive listing of van der Waals constants can be
found in data source [Fl.
States of Matter and the van der Waals Equation
 One of the simplest transformations of matter is the melting of solids or the
 vaporization of liquids. In thermodynamics the various states of matter-
 solid, liquid, gas-are often referred to as phases. At a given pressure every
 compound has a definite temperature, Tmelb at which it melts and a definite
 temperature, Tbail, at which it boils. In fact, these properties can be used to
 identify a compound or separate the constituents of a mixture. With the
 development of the thermometer, these properties could be studied with
 precision.
 Joseph Black and James Watt discovered another interesting phenomenon
 associated with the changes of phase: at the melting or the boiling
 temperature, the heat supplied to a system does not produce an increase in
 temperature; it only converts the substance from one phase to another. This
 heat that lays "latent" or hidden without increasing the temperature was
 called the latent heat. When a liquid solidifies or a vapor solidifies, this heat
 is given out to the surroundings
van derWaals realized that two main factors were to be added to the ideal gas
equation: the effect of molecular attraction and the effect of molecular size. The
intermolecular forces would add a correction to the ideal gas pressure whereas
the molecular size would decrease the effective volume. In the case of the ideal
gas there is no intermolecular attraction. As illustrated in Fig. 1.4, the
intermolecular attraction decreases the pressure from its ideal value. If Preal is
the pressure of a real gas and Pideal is the corresponding pressure of the ideal
gas, i.e. the pressure in the absence of intermolecular forces, then Pideal ==
Preal +8p,
where 8p is the correction. Since the pressure is proportional to the number
density (NIV) (as can be seen from the ideal gas equation), 8p should be
proportional to (NIV). In addition, the total force on each molecule close to the
van del' Waals considered molecular interaction and molecular size to improve
the ideal gas equation. (a) The pressure of a real gas is less than the ideal gas
pressure because intermolecular attraction decreases the speed of the molecules
approaching the wall. Therefore Preal == Pideal - 8p. (b) The
volume available to molecules is less than the volume of the container due to the
finite size of the molecules. This "excluded“ volume depends on the total number
of molecules. Therefore Videal == V - bN

								
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