# Re: Entropy: A Guide for the Perplexed

```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.```
```
Jason
```
```
```
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.
```
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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