On 06.02.2012 17:44 Jason Resch said the following:

I think entropy is better intuitively understood as uncertanty. The entropy of a gas is the uncertanty of the particle positions and velocities. The hotter it is the more uncertanties there are. A certain amount of information is required to eliminate this uncertanty.## Advertising

Jason

`Could you please show how your definition of entropy could be employed`

`to build for example the next phase diagram`

http://www.calphad.com/graphs/Metastable%20Fe-C%20Phase%20Diagram.gif

`If you find such a question too complicated, please consider the`

`textbook level problem below and show how you will solve it using`

`uncertainties.`

Evgenii ———————————————–

`Problem. Given temperature, pressure, and initial number of moles of`

`NH3, N2 and H2, compute the equilibrium composition.`

`To solve the problem one should find thermodynamic properties of NH3, N2`

`and H2 for example in the JANAF Tables and then compute the equilibrium`

`constant.`

From thermodynamics tables (all values are molar values for the standard pressure 1 bar, I have omitted the symbol o for simplicity but it is very important not to forget it): Del_f_H_298(NH3), S_298(NH3), Cp(NH3), Del_f_H_298(N2), S_298(N2), Cp(N2), Del_f_H_298(H2), S_298(H2), Cp(H2) 2NH3 = N2 + 3H2 Del_H_r_298 = Del_f_H_298(N2) + 3 Del_f_H_298(H2) – 2 Del_f_H_298(NH3) Del_S_r_298 = S_298(N2) + 3 S_298(H2) – 2 S_298(NH3) Del_Cp_r = Cp(N2) + 3 Cp(H2) – 2 Cp(NH3) To make life simple, I will assume below that Del_Cp_r = 0, but it is not a big deal to extend the equations to include heat capacities as well. Del_G_r_T = Del_H_r_298 – T Del_S_r_298 Del_G_r_T = – R T ln Kp

`When Kp, total pressure and the initial number of moles are given, it is`

`rather straightforward to compute equilibrium composition. If you need`

`help, please just let me know.`

———————————————–

On Feb 5, 2012, at 12:28 PM, Evgenii Rudnyi <use...@rudnyi.ru> wrote:On 05.02.2012 17:16 Evgenii Rudnyi said the following:On 24.01.2012 22:56 meekerdb said the following:In thinking about how to answer this I came across an excellent paper by Roman Frigg and Charlotte Werndl http://www.romanfrigg.org/writings/EntropyGuide.pdf which explicates the relation more comprehensively than I could and which also gives some historical background and extensions: specifically look at section 4. Brent

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