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## Equation of state*For the use of this concept in cosmology, see Equation of state (cosmology)*
In physics and thermodynamics, an
## OverviewThe most prominent use of an equation of state is to predict the state of gases and liquids. One of the simplest equations of state for this purpose is the ideal gas law, which is roughly accurate for gases at low pressures and high temperatures. However, this equation becomes increasingly inaccurate at higher pressures and lower temperatures, and fails to predict condensation from a gas to a liquid. Therefore, a number of much more accurate equations of state have been developed for gases and liquids. At present, there is no single equation of state that accurately predicts the properties of all substances under all conditions. In addition to predicting the behavior of gases and liquids, there are also equations of state for predicting the volume of solids, including the transition of solids from one crystalline state to another. There are equations that model the interior of stars, including neutron stars. A related concept is the perfect fluid equation of state used in cosmology. ## Historical## Boyle's law (1662)Boyle's Law was perhaps the first expression of an equation of state. In 1662 Robert Boyle, an Irishman, performed a series of experiments employing a J-shaped glass tube, which was sealed on one end. Mercury was added to the tube, trapping a fixed quantity of air in the short, sealed end of the tube. Then the volume of gas was carefully measured as additional mercury was added to the tube. The pressure of the gas could be determined by the difference between the mercury level in the short end of the tube and that in the long, open end. Through these experiments, Boyle noted that the gas volume varied inversely with the pressure. In mathematical form, this can be stated as: *pV*= constant
The above relationship has also been attributed to Edme Mariotte and is sometimes referred to as ## Charles's law or Law of Charles and Gay-Lussac (1787)In 1787 the French physicist Jacques Charles found that oxygen, nitrogen, hydrogen, carbon dioxide, and air expand to the same extent over the same 80 kelvin interval. Later, in 1802, Joseph Louis Gay-Lussac published results of similar experiments, indicating a linear relationship between volume and temperature: *V*_{1}/*T*_{1}=*V*_{2}/*T*_{2}
## Dalton's law of partial pressures (1801)Dalton's Law of Partial Pressure: The pressure of a mixture of gases is equal to the sum of the pressures of all of the constituent gases alone. Mathematically, this can be represented for *Pressure*=_{total}*Pressure*+_{1}*Pressure*+ ... +_{2}*Pressure*_{n}
## The ideal gas law (1834)In 1834 Émile Clapeyron combined Boyle's Law and Charles' law into the first statement of the R(T+267) (with temperature expressed in degrees Celsius). However, later work revealed that the number should actually be closer to 273.2, and then the Celsius scale was defined with 0 °C = 273.15 K, giving:
_{C}*p**V*_{m}=*R*(*T*_{C}+ 273.15)
## Van der Waals equation of stateIn 1873, J. D. van der Waals introduced the first equation of state derived by the assumption of a finite volume occupied by the constituent molecules. ## Major equations of stateIn the following equations the variables are defined as follows. Any consistent set of units may be used, although SI units are preferred. Absolute temperature refers to use of the Kelvin (K) or Rankine (°R) temperature scales, with zero being absolute zero. *p*= pressure*V*= volume*n*= number of moles of a substance*V*=_{m}*V*/*n*=**molar volume**, the volume of 1 mole of gas or liquid*T*= absolute temperature*R*= ideal gas constant (8.314472 J/(mol·K))*p*= pressure at the critical point_{c}*V*= molar volume at the critical point_{c}*T*= absolute temperature at the critical point_{c}
## Classical ideal gas lawThe classical ideal gas law may be written: *p**V*=*n**R**T*
The ideal gas law may also be expressed as follows *p*= ρ(γ − 1)*e*
where ρ is the density, γ = ## Cubic equations of state## Van der Waals equation of stateThe Van der Waals equation of state may be written: - , note that
*V*_{m}is molar volume.
Where Also written as Proposed in 1873, the van der Waals equation of state was one of the first to perform markedly better than the ideal gas law. In this landmark equation The van der Waals equation may be considered as the ideal gas law, “improved” due to two independent reasons: - Molecules are thought as particles with volume, not material points. Thus
*V*cannot be too little, less than some constant. So we get (*V*−*b*) instead of*V*. - While ideal gas molecules do not interact, we consider molecules attracting others within a distance of several molecules' radii. It makes no effect inside the material, but surface molecules are attracted into the material from the surface. We see this as diminishing of pressure on the outer shell (which is used in the ideal gas law), so we write (
*p*+ something) instead of*p*. To evaluate this ‘something’, let's examine an additional force acting on an element of gas surface. While the force acting on each surface molecule is ~ρ, the force acting on the whole element is ~ρ^{2}~.
## Redlich-Kwong equation of stateIntroduced in 1949 the Redlich-Kwong equation of state was a considerable improvement over other equations of the time. It is still of interest primarily due to its relatively simple form. While superior to the van der Waals equation of state, it performs poorly with respect to the liquid phase and thus cannot be used for accurately calculating vapor-liquid equilibria. However, it can be used in conjunction with separate liquid-phase correlations for this purpose. The Redlich-Kwong equation is adequate for calculation of gas phase properties when the ratio of the pressure to the critical pressure (reduced pressure) is less than about one-half of the ratio of the temperature to the critical temperature (reduced temperature): ## Soave modification of Redlich-KwongWhere ω is the acentric factor for the species. for hydrogen: In 1972 Soave replaced the Note especially that this replacement changes the definition of ## Peng-Robinson equation of statewhere, ω is the acentric factor of the species and The Peng-Robinson equation was developed in 1976 in order to satisfy the following goals: - The parameters should be expressible in terms of the critical properties and the acentric factor.
- The model should provide reasonable accuracy near the critical point, particularly for calculations of the compressibility factor and liquid density.
- The mixing rules should not employ more than a single binary interaction parameter, which should be independent of temperature pressure and composition.
- The equation should be applicable to all calculations of all fluid properties in natural gas processes.
For the most part the Peng-Robinson equation exhibits performance similar to the Soave equation, although it is generally superior in predicting the liquid densities of many materials, especially nonpolar ones. The departure functions of the Peng-Robinson equation are given on a separate article. ## Elliott, Suresh, Donohue equation of stateThe Elliott, Suresh, and Donohue (ESD) equation of state was proposed in 1990. The equation seeks to correct a shortcoming in the Peng-Robinson EOS in that there was an inaccuracy in the van der Waals repulsive term. The EOS accounts for the effect of the shape of a non-polar molecule and can be extended to polymers with the addition of an extra term (not shown). The EOS itself was developed through modeling computer simulations and should capture the essential physics of the size, shape, and hydrogen bonding. where: *c*= a “shape factor”
## Non-cubic equations of state## Dieterici equation of stateWhere The reduced coordinates are: ## Virial equations of state## Virial equation of stateAlthough usually not the most convenient equation of state, the virial equation is important because it can be derived directly from statistical mechanics. If appropriate assumptions are made about the mathematical form of intermolecular forces, theoretical expressions can be developed for each of the coefficients. In this case It can also be used to work out the Boyle Temperature (the temperature at which B = 0 and ideal gas laws apply) from a and b from the Van der Waals equation of state. If you use the value for B shown below; ## The BWRS equation of statewhere *p*= pressure*ρ*= the molar density
Values of the various parameters for 15 substances can be found in: K.E. Starling, ## Other equations of state of interest## Stiffened equation of stateWhen considering water under very high pressures (typical applications are underwater nuclear explosions, sonic shock lithotripsy, and sonoluminescence) the stiffened equation of state is often used: where The equation is stated in this form because the speed of sound in water is given by Thus water behaves as though it is an ideal gas that is This equation mispredicts the specific heat capacity of water but few alternatives are available for severely nonisentropic processes such as strong shocks. ## Ultrarelativistic equation of stateAn ultrarelativistic fluid has equation of state where ## Ideal Bose equation of stateThe equation of state for an ideal Bose gas is where α is an exponent specific to the system (e.g. in the absence of a potential field,
α=3/2), ## Equations of state for solids## See also## Bibliography- Elliot & Lira, (1999).
*Introductory Chemical Engineering Thermodynamics,*Prentice Hall.
## References**^**Perrot, Pierre (1998).*A to Z of Thermodynamics*. Oxford University Press.__ISBN 0-19-856552-6__.**^**van der Waals, J. D. (1873).*On the Continuity of the Gaseous and Liquid States (doctoral dissertation)*. Universiteit Leiden.**^**(1976) "A New Two-Constant Equation of State".*Industrial and Engineering Chemistry: Fundamentals***15**: 59-64.
Categories: Thermodynamics | Chemical engineering | Fluid mechanics | Phases of matter |
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Equation_of_state". A list of authors is available in Wikipedia. |