Talk:Fugacity

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Help with this template Comments: edit – history – watch – refresh I think it's a horrible explanation. Pressure-like property?? As far as I know pressure is a physical property. You can measure, apply and observe it. On the other hand fugacity is a measured but discrete property.

Definition of Fugacitiy

Fugacity is a pressure-like property, that allows us to apply the same equations from the perfect gas to a real gas.

In a perfect gas, the molecules are so distant apart, such that collisions only occur between molecules and walls.

However, in a real gas, interference between molecules does happen, and statistical properties such as entropy can no longer be linearly added together (eg: say, for a two fluid mixture).

The fugacity definition solves the problem, as it directly relates to the compressibility factor (Z), which quantifies how far our assumptions are from the perfect gas, pV = ZRT. In turn, it represents a way to "correct" the idealized pressure, so as to remain able to use the classic equations.

The compressibility factor can be found in charts for different substances.

The beauty and generality of it lies on that, for different substances, if the whole chart is normalized regarding a critical pressure (easily identified by an inflection point), all charts (of a given molecule shape) will look alike.

Contributed by E.Hwang, March 22nd, 2006.

[edit] Too many definitions

The above definition is clear enough. To me this means that fugacity is a theoretical (or imaginary) property of gases diamensionally the same as pressure, the magnitude of which is such that substituting its value for pressure gives correct relationships for the ideal gas law. Why do we complicate it so that no one can understand it?

This is not the same as what is said under 'Detail', that is Fugacity is the change of pressure required etc. A change of pressure is dimensionless and the dimension of fugacity ought to be [mass]/[distance][time]*[time]. Perhaps someone will tell me if I am totallly wrongLouisBB 22:19, 31 March 2007 (UTC)

[edit] 1) definitions not specific ; 2) is the sign in the formula for μ correct?

There should be a clear mathematical formula defining fugacity. I found none (admittedly, the page is more than disorderly now) so I added one following from the definition of chemical potential in terms of fugacity. So far I have seen chemical potential appear first... And I believe there should be a minus sign in this formula [should be f = exp( − μ / k_BT)] but I found no reference so I left an expression following from the above "definition of μ". If anyone can confirm there is a minus sign please correct it. I think the whole thermodynamics section should be rewritten anyway since it is much more customary to defime μ as the primary variable for a grand canonical ensemble.

[edit] definition

μ - chemical potencial is partial molar derivation of Gibs energy

fugacity (or its increment) was originaly by Lewis defined as folows: dμ_i = RT d Ln(f_i) - this is original definition

Lewis also stated that fugacity of some component in ideal solution is equal of partial pressure of this component. Then fugacity coeficien was defined bz formal integration between 2 states real and ideal solution with same T,p, and composition.

as f/(f*) where f* = p*, is fugacity odf i in ideal solution equal to its partial presure. —Preceding unsigned comment added by 212.5.210.202 (talk) 10:51, 21 September 2007 (UTC)