On 15.04.2011 21:44 meekerdb said the following:

Entropy and information are related. In classical thermodynamics the relation is between what constraint you impose on the substance and dQ/T. You note that it is calculated assuming constant pressure - that is a constraint; another is assuming constant energy. In terms of the phase space in a statistical mechanics model, this is confining the system to a hypersurface in the the phase space. If you had more information about the system, e.g. you knew all the molecules were moving the same direction (as in a rocket exhaust) that you further reduce the part of phase space and the entropy. If you knew the proportions of molecular species that would reduce it further. In rocket exhaust calculations the assumption of fixed species proportion is often made as an approximation - it's referred to as a frozen entropy calculation. If the species react that changes the size of phase space and hence the Boltzmann measure of entropy.## Advertising

Brent

First how do you define information? According to Shannon? Then if we consider a thermodynamic system, the Second Law dS >= dQ/T

`does not impose constraints as such. It is held for any closed system`

`and for any process. The only assumption here is that the system`

`possesses a temperature. If one can define temperature than the entropy`

`follows according to the Second Law unambiguously and I do not see how`

`one additionally will need information, whatever it means.`

`If you speak about reaction chemistry, let us consider a simple exercise`

`from classical thermodynamics.`

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. So, the`

`entropy is there. What do you mean when you state that information is`

`also involved? Where is in this example the related information, again`

`whatever it is?`

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