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


Could you please show how your definition of entropy could be employed to build for example the next phase diagram


If you find such a question too complicated, please consider the textbook level problem below and show how you will solve it using uncertainties.


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>

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.


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