Real gases are non-ideal gases whose molecules occupy space and have interactions; consequently, they do not adhere to the ideal gas law. To understand the behaviour of real gases, the following must be taken into account:
compressibility effects; variable specific heat capacity; van der Waals forces; non-equilibrium thermodynamic effects; issues with molecular dissociation and elementary reactions with variable composition For most applications, such a detailed analysis is unnecessary, and the ideal gas approximation can be used with reasonable accuracy. On the other hand, real-gas models have to be used near the condensation point of gases, near critical points, at very high pressures, to explain the Joule–Thomson effect, and in other less usual cases. The deviation from ideality can be described by the compressibility factor Z.
Models
Van der Waals model
Real gases are often modeled by taking into account their molar weight and molar volume:
R T = ( p + a V m 2 ) ( V m − b ) p = R T V m − b − a V m 2 {\displaystyle {\begin{aligned}RT&=\left(p+{\frac {a}{V_{\text{m}}^{2}}}\right)\left(V_{\text{m}}-b\right)\\p&={\frac {RT}{V_{m}-b}}-{\frac {a}{V_{m}^{2}}}\end{aligned}}}
Where p is pressure, T is temperature, R is the ideal gas constant, and Vm is the molar volume. a and b are parameters that are determined empirically for each gas, but are sometimes estimated from their critical temperature (Tc) and critical pressure (pc) using these relations:
a = 27 R 2 T c 2 64 p c , b = R T c 8 p c {\displaystyle {\begin{aligned}a&={\frac {27R^{2}T_{\text{c}}^{2}}{64p_{\text{c}}}},&b&={\frac {RT_{\text{c}}}{8p_{\text{c}}}}\end{aligned}}}
The constants at the critical point can be expressed as functions of the parameters a and b:
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