In physics and thermodynamics, an equation of state is a relation between state variables.^{[1]} More specifically, an equation of state is a thermodynamic equation describing the state of matter under a given set of physical conditions. It is a constitutive equation which provides a mathematical relationship between two or more state functions associated with the matter, such as its temperature, pressure, volume, or internal energy. Equations of state are useful in describing the properties of fluids, mixtures of fluids, solids, and even the interior of stars.
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The 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 moderate 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.
Boyle's Law was perhaps the first expression of an equation of state. In 1662, the noted Irish physicist and chemist Robert Boyle performed a series of experiments employing a Jshaped 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:
The above relationship has also been attributed to Edme Mariotte and is sometimes referred to as Mariotte's law. However, Mariotte's work was not published until 1676.
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 GayLussac published results of similar experiments, indicating a linear relationship between volume and temperature:
Dalton's Law of partial pressure states that 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 n species as:
In 1834 Émile Clapeyron combined Boyle's Law and Charles' law into the first statement of the ideal gas law. Initially the law was formulated as pV_{m}=R(T_{C}+267) (with temperature expressed in degrees Celsius), where R is the gas constant. 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:
In 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.^{[2]} His new formula revolutionized the study of equations of state, and was most famously continued via the Redlich–Kwong equation of state and the Soave modification of RedlichKwong.
For a given amount of substance contained in a system, the temperature, volume, and pressure are not independent quantities; they are connected by a relationship of the general form:
In 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.
The classical ideal gas law may be written:
The ideal gas law may also be expressed as follows
where ρ is the density, γ = C_{p} / C_{v} is the adiabatic index (ratio of specific heats), e = C_{v}T is the internal energy per unit mass (the "specific internal energy"), C_{v} is the specific heat at constant volume, and C_{p} is the specific heat at constant pressure.
The Van der Waals equation of state may be written:
where V_{m} is molar volume, and a and b are substancespecific constants. They can be calculated from the critical properties p_{c},T_{c} and V_{c} (noting that V_{c} is a the molar volume at the critical point) as:
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 a is called the attraction parameter and b the repulsion parameter or the effective molecular volume. While the equation is definitely superior to the ideal gas law and does predict the formation of a liquid phase, the agreement with experimental data is limited for conditions where the liquid forms. While the van der Waals equation is commonly referenced in textbooks and papers for historical reasons, it is now obsolete. Other modern equations of only slightly greater complexity are much more accurate.
The van der Waals equation may be considered as the ideal gas law, “improved” due to two independent reasons:
Introduced 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 vaporliquid equilibria. However, it can be used in conjunction with separate liquidphase 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 onehalf of the ratio of the temperature to the critical temperature (reduced temperature):
Where ω is the acentric factor for the species.
for hydrogen:
In 1972 Soave replaced the 1/√(T) term of the RedlichKwong equation with a function α(T,ω) involving the temperature and the acentric factor. The α function was devised to fit the vapor pressure data of hydrocarbons and the equation does fairly well for these materials.
Note especially that this replacement changes the definition of a slightly, as the T_{c} is now to the second power.
In polynomial form:
where, ω is the acentric factor of the species, R is the universal gas constant and Z=PV/(RT) is compressibility factor.
The PengRobinson equation was developed in 1976 in order to satisfy the following goals:^{[3]}
For the most part the PengRobinson 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 PengRobinson equation are given on a separate article.
The Elliott, Suresh, and Donohue (ESD) equation of state was proposed in 1990.^{[4]} The equation seeks to correct a shortcoming in the PengRobinson 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 nonpolar 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:
and
where
The characteristic size parameter is related to the shape parameter c through
where
Noting the relationships between Boltzmann's constant and the Universal gas constant, and observing that the number of molecules can be expressed in terms of Avogadro's number and the molar mass, the reduced number density η can be expressed in terms of the molar volume as
The shape parameter q appearing in the Attraction term and the term Y are given by
where ε is the depth of the squarewell potential and is given by
The model can be extended to associating components and mixtures of nonassociating components. Details are in the paper by J.R. Elliott Jr., S.J. Suresh and M.D. Donohue cited above.
where a is associated with the interaction between molecules and b takes into account the finite size of the molecules, similarly to the Van der Waals equation.
The reduced coordinates are:
Although 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 B corresponds to interactions between pairs of molecules, C to triplets, and so on. Accuracy can be increased indefinitely by considering higher order terms.
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;
where
Values of the various parameters for 15 substances can be found in:
K.E. Starling, Fluid Properties for Light Petroleum Systems. Gulf Publishing Company (1973).
When 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 e is the internal energy per unit mass, γ is an empirically determined constant typically taken to be about 6.1, and p^{0} is another constant, representing the molecular attraction between water molecules. The magnitude of the correction is about 2 gigapascals (20000 atmospheres).
The equation is stated in this form because the speed of sound in water is given by c^{2} = γ(p + p^{0}) / ρ.
Thus water behaves as though it is an ideal gas that is already under about 20000 atmospheres (2 GPa) pressure, and explains why water is commonly assumed to be incompressible: when the external pressure changes from 1 atmosphere to 2 atmospheres (100 kPa to 200 kPa), the water behaves as an ideal gas would when changing from 20001 to 20002 atmospheres (2000.1 MPa to 2000.2 MPa).
This equation mispredicts the specific heat capacity of water but few simple alternatives are available for severely nonisentropic processes such as strong shocks.
An ultrarelativistic fluid has equation of state
where p is the pressure, μ is the mass density, and c_{s} is the speed of sound.
The 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), z is exp(μ/kT) where μ is the chemical potential, Li is the polylogarithm, ζ is the Riemann zeta function, and T_{c} is the critical temperature at which a BoseEinstein condensate begins to form.
PVT analysis for Oil and Gas

