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🧊thermodynamics ii review

Redlich-Kwong

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

The Redlich-Kwong equation is an empirical thermodynamic model that describes the behavior of real gases by accounting for non-ideal interactions between molecules. It improves upon the ideal gas law by including a temperature-dependent volume correction and a term that accounts for the attraction between molecules, making it useful in predicting vapor-liquid equilibrium and fugacity in various systems.

5 Must Know Facts For Your Next Test

  1. The Redlich-Kwong equation is expressed as $$P = \frac{RT}{V - b} - \frac{a}{V(V + b)T^{0.5}}$$, where 'P' is pressure, 'R' is the gas constant, 'T' is temperature, 'V' is molar volume, 'a' represents attraction forces, and 'b' accounts for molecular size.
  2. This equation provides better accuracy than the ideal gas law for gases at high pressures and low temperatures, where deviations from ideal behavior are significant.
  3. The Redlich-Kwong equation can be used to derive expressions for fugacity, enabling calculations of chemical equilibria in mixtures.
  4. It has been widely adopted in chemical engineering for process design and simulation due to its simplicity and effectiveness in predicting phase behavior.
  5. The equation was developed in 1949 by Otto Redlich and David M. Kwong, serving as a foundational model for later equations of state, including the Peng-Robinson equation.

Review Questions

  • How does the Redlich-Kwong equation improve upon the ideal gas law when modeling real gases?
    • The Redlich-Kwong equation improves upon the ideal gas law by incorporating two critical factors: a volume correction that accounts for the finite size of molecules and a term that addresses intermolecular attractions. These modifications allow the equation to more accurately predict the behavior of gases under conditions where non-ideal interactions are significant, such as at high pressures or low temperatures. This results in better predictions for properties like vapor-liquid equilibrium and fugacity.
  • Discuss the significance of fugacity in relation to the Redlich-Kwong equation and vapor-liquid equilibrium.
    • Fugacity is crucial when using the Redlich-Kwong equation because it provides a way to correct the pressure exerted by a real gas to account for non-ideal behaviors. In vapor-liquid equilibrium calculations, knowing the fugacity allows for accurate determination of phase compositions and chemical potentials. The Redlich-Kwong equation helps derive expressions for fugacity, making it an essential tool in studying phase behavior in chemical processes.
  • Evaluate the impact of the Redlich-Kwong equation on modern chemical engineering practices, particularly in relation to phase behavior predictions.
    • The Redlich-Kwong equation has had a lasting impact on modern chemical engineering by providing a practical method for predicting phase behavior in real gas systems. Its ability to handle non-ideal behaviors makes it invaluable for process design, simulation, and optimization in industries such as petrochemicals and natural gas. The foundation it laid for subsequent equations of state has enabled engineers to refine models further, leading to improved accuracy in predicting vapor-liquid equilibria and enhancing the efficiency of chemical processes.