The acentric factor is a conceptual number introduced by

Kenneth Pitzer Kenneth Sanborn Pitzer (January 6, 1914 – December 26, 1997) was an American physical and theoretical chemist, educator, and university president. He was described as "one of the most influential physical chemists of his era" whose work "sp ...

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 shear ...

. 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 or Critically may refer to: *Critical, or critical but stable, medical states **Critical, or intensive care medicine * Critical juncture, a discontinuous change studied in the social sciences. * Critical Software, a company specializing i ...

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 :

\omega = - \log_ (p^_r) - 1,  T_r = 0.7

where

p^_r = \frac

is the reduced saturation vapor pressure and

T_r = \frac

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,

p_r^ T_r = 0.7

is close to 0.1, which leads to

\omega \to 0

. In many cases,

T_r = 0.7

lies above the

boiling temperature The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding env ...

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 che ...

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 a ...

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 ...

\omega

is also very close to zero for molecules which are nearly spherical. Values of correspond to vapor pressures above the

, and are non-physical. The acentric factor can be predicted analytically from some

equations 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 in ...

. 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

References

Gas laws {{thermodynamics-stub

Values of some common gases

See also

References