# Real gas

Real gases are non-hypothetical gases whose molecules occupy space and have interactions; consequently, they adhere to gas laws. 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

Isotherms of real gas

Dark blue curves – isotherms below the critical temperature. Green sections – metastable states.

The section to the left of point F – normal liquid.
Point F – boiling point.
Line FG – equilibrium of liquid and gaseous phases.
Section FA – superheated liquid.
Section F′A – stretched liquid (p<0).
Section AC – analytic continuation of isotherm, physically impossible.
Section CG – supercooled vapor.
Point G – dew point.
The plot to the right of point G – normal gas.
Areas FAB and GCB are equal.

Red curve – Critical isotherm.
Point K – critical point.

Light blue curves – supercritical isotherms

### Van der Waals model

Real gases are often modeled by taking into account their molar weight and molar volume

or alternatively:

Where p is the pressure, T is the temperature, R the ideal gas constant, and Vm 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:

With the reduced properties the equation can be written in the reduced form:

### Redlich–Kwong model

Critical isotherm for Redlich-Kwong model in comparison to van-der-Waals model and ideal gas (with V0=RTc/pc)

The Redlich–Kwong equation is another two-parameter equation that is used to model real gases. It is almost always more accurate than the van der Waals equation, and often more accurate than some equations with more than two parameters. The equation is

or alternatively:

where a and b two empirical parameters that are not the same parameters as in the van der Waals equation. These parameters can be determined:

Using the equation of state can be written in the reduced form:

with

### Berthelot and modified Berthelot model

The Berthelot equation (named after D. Berthelot)[1] is very rarely used,

but the modified version is somewhat more accurate

### Dieterici model

This model (named after C. Dieterici[2]) fell out of usage in recent years

with parameters a, b, and

### Clausius model

The Clausius equation (named after Rudolf Clausius) is a very simple three-parameter equation used to model gases.

or alternatively:

where

where Vc is critical volume.

### Virial model

The Virial equation derives from a perturbative treatment of statistical mechanics.

or alternatively

where A, B, C, A, B, and C are temperature dependent constants.

### Peng–Robinson model

Peng–Robinson equation of state (named after D.-Y. Peng and D. B. Robinson[3]) has the interesting property being useful in modeling some liquids as well as real gases.

### Wohl model

Isotherm (V/V0->p_r) at critical temperature for Wohl model, van der Waals model and ideal gas model (with V0=RTc/pc)
Untersuchungen über die Zustandsgleichung, pp. 9,10, Zeitschr. f. Physikal. Chemie 87

The Wohl equation (named after A. Wohl[4]) is formulated in terms of critical values, making it useful when real gas constants are not available, but it cannot be used for high densities, as for example the critical isotherm shows a drastic decrease of pressure when the volume is contracted beyond the critical volume.

or:

or, alternatively:

where

with
, where are (respectively) the molar volume, the pressure and the temperature at the critical point.

And with the reduced properties one can write the first equation in the reduced form:

### Beattie–Bridgeman model

[5]This equation is based on five experimentally determined constants. It is expressed as

where

This equation is known to be reasonably accurate for densities up to about 0.8 ρcr, where ρcr is the density of the substance at its critical point. The constants appearing in the above equation are available in the following table when p is in kPa, v is in , T is in K and R = 8.314 [6]

Gas A0 a B0 b c
Air 131.84410.019310.04611−0.0011014.34×104
Argon, Ar 130.78020.023280.039310.05.99×104
Carbon dioxide, CO2 507.28360.071320.104760.072356.60×105
Helium, He 2.18860.059840.014000.040
Hydrogen, H2 20.0117−0.005060.02096−0.04359504
Nitrogen, N2 136.23150.026170.05046−0.006914.20×104
Oxygen, O2 151.08570.025620.046240.0042084.80×104

### Benedict–Webb–Rubin model

The BWR equation, sometimes referred to as the BWRS equation,

where d is the molar density and where a, b, c, A, B, C, α, and γ are empirical constants. Note that the γ constant is a derivative of constant α and therefore almost identical to 1.

## Thermodynamic expansion work

The expansion work of the real gas is different than that of the ideal gas by the quantity .

## References

1. D. Berthelot in Travaux et Mémoires du Bureau international des Poids et Mesures – Tome XIII (Paris: Gauthier-Villars, 1907)
2. C. Dieterici, Ann. Phys. Chem. Wiedemanns Ann. 69, 685 (1899)
3. Peng, D. Y. & Robinson, D. B. (1976). "A New Two-Constant Equation of State". Industrial and Engineering Chemistry: Fundamentals. 15: 59–64. doi:10.1021/i160057a011.
4. A. Wohl (1914). "Investigation of the condition equation". Zeitschrift für Physikalische Chemie. 87: 1–39.
5. Yunus A. Cengel and Michael A. Boles, Thermodynamics: An Engineering Approach 7th Edition, McGraw-Hill, 2010, ISBN 007-352932-X
6. Gordan J. Van Wylen and Richard E. Sonntage, Fundamental of Classical Thermodynamics, 3rd ed, New York, John Wiley & Sons, 1986 P46 table 3.3
• Kondepudi, D. K.; Prigogine, I. (1998). Modern thermodynamics: From heat engines to dissipative structures. John Wiley & Sons. ISBN 978-0-471-97393-5.
• Hsieh, J. S. (1993). Engineering Thermodynamics. Prentice-Hall. ISBN 978-0-13-275702-7.
• Walas, S. M. (1985). Fazovyje ravnovesija v chimiceskoj technologii v 2 castach. Butterworth Publishers. ISBN 978-0-409-95162-2.
• Aznar, M.; Silva Telles, A. (1997). "A Data Bank of Parameters for the Attractive Coefficient of the Peng-Robinson Equation of State". Brazilian Journal of Chemical Engineering. 14 (1): 19–39. doi:10.1590/S0104-66321997000100003.
• Rao, Y. V. C (2004). An introduction to thermodynamics. Universities Press. ISBN 978-81-7371-461-0.
• Xiang, H. W. (2005). The Corresponding-States Principle and its Practice: Thermodynamic, Transport and Surface Properties of Fluids. Elsevier. ISBN 978-0-08-045904-2.