The acentric factor is a conceptual number introduced by
Kenneth Pitzer in 1955, proven to be useful in the description of
fluids
In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any she ...
. It has become a standard for the phase characterization of single & pure components, along with other state description parameters such as
molecular weight
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
,
critical temperature,
critical pressure
In thermodynamics, a critical point (or critical state) is the end point of a phase equilibrium curve. The most prominent example is the liquid–vapor critical point, the end point of the pressure–temperature curve that designates conditions ...
, and
critical volume (or critical compressibility).
Pitzer defined from the relationship
:
where
is the
reduced saturation vapor pressure and
is the
reduced temperature.
The acentric factor is said to be a measure of the non-sphericity (centricity) of molecules.
As it increases, the
vapor curve is "pulled" down, resulting in higher
boiling points. For many monatomic fluids,
is close to 0.1, which leads to
. In many cases,
lies above the
boiling temperature of liquids at atmosphere pressure.
Values of can be determined for any fluid from accurate experimental vapor pressure data. The definition of gives values which are close to zero for the
noble gas
The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low ch ...
es
argon
Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as ...
,
krypton
Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often ...
, and
xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
.
is also very close to zero for molecules which are nearly spherical.
Values of correspond to vapor pressures above the
critical pressure
In thermodynamics, a critical point (or critical state) is the end point of a phase equilibrium curve. The most prominent example is the liquid–vapor critical point, the end point of the pressure–temperature curve that designates conditions ...
, and are non-physical.
The acentric factor can be predicted analytically from some
equations of state. For example, it can be easily shown from the above definition that a
van der Waals fluid has an acentric factor of about −0.302024, which if applied to a real system would indicate a small, ultra-spherical molecule.
Values of some common gases
See also
*
Equation of state
In physics, chemistry, and thermodynamics, an equation of state is a thermodynamic equation relating state variables, which describe the state of matter under a given set of physical conditions, such as pressure, volume, temperature, or intern ...
*
Reduced pressure
In thermodynamics, the reduced properties of a fluid are a set of state variables scaled by the fluid's state properties at its critical point. These dimensionless thermodynamic coordinates, taken together with a substance's compressibility fact ...
*
Reduced temperature
References
Gas laws
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