The Redlich-Kwong equation (Otto Redlich, J. N. S. Kwong, 1949) is a cubic equation of state loosely modeled after the van der Waals equation. However, there is no particular theoretical basis behind the modifications that were adopted.

The Redlich-Kwong equation has been used extensively, and has been the basis for several succesful modified forms (such as the Soave equation). Nevertheless, it is currently considered that modifying the Redlich-Kwong equation has reached the point of diminishing returns, and alternative methods to improving equations of state have to be developed.

The equation can NOT be used for liquid phase systems, or vapor-liquid equilibria. The advantages of the Redlich-Kwong equation are that it is a relatively simple equation, and that it does not require much information about the substance or the components of a mixture.

The basic form of the Redlich-Kwong equation is:

(P + a/ (T½ V (V + b)) (V - b) = RT

The constants a and b can be given in terms of critical constants, Tc and Pc:

a = 0.42748 R2Tc2.5 / Pc

a = 0.08664 RTc / Pc

Otto Redlich, J. N. S. Kwong, Chem. Rev., vol. 44, pp. 233-244, 1949.

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