[Hertzinger]

I understand that entropy is the "measurement of the degree of chaos" in an equation, and it's calcuable; however, when presented with a basic question such as:

O_2(g) + Cl_{2 (g)} ---> 2OCl_(g)

what side does the entropy favour?

There are no calculations given, just the equation.

[MrTeo]

Do you know the statistical interpretation of entropy?

[Hertzinger]

No, no information what so ever is giving. I've assumed that this question is unsolvable with the given information  . Anyone else got a crack at it?

[Jorriss]

The 'complexity' of the molecule also effects entropy to a degree, a factor which I am not sure is present here. A more complex molecule has more degrees of freedom and from that, more entropy.

[MrTeo]

The point of the question is: in how many ways could you arrange the molecules of your system before and after the reaction? e.g. Given a box divided into two halves (we'll call them A and B) what would be the possible configurations of our system? (try to write them if it helps you)

Before the reaction:                                  After the reaction:
4 possibilities                                           3 possibilities

Boltzmann tells us that:



where N is the number of possible microstates of our system, so:



In this case N₂<N₁ so ∆S<0.

[opti384]

@Teo: You are right with the Boltzmann's equation and the statistical definition of entropy, but I'm just curious. Jorriss talked about the complexity of the molecules and I'm just wondering if the complexity is solely negligible here so that you can only consider the microstates.

[Hertzinger]

Hey Mr. Teo, haven't had any run ins with Boltzmans equations yet. You just made me excited . Can you link me to some websites where I can learn the applications of the equation?
Thanks for the help this far guys! Let's keep the discussion going.

[MrTeo]

@Teo: You are right with the Boltzmann's equation and the statistical definition of entropy, but I'm just curious. Jorriss talked about the complexity of the molecules and I'm just wondering if the complexity is solely negligible here so that you can only consider the microstates.

Well, some time ago during an exam I heard a question about reactions with ∆H=0 (e.g. isotopic exchange reaction), the professor asked about the ∆S of this reaction:

CH₄+CD₄⇌2CH₂D₂

the candidate (helped with some suggestions) solved the question this way and neglected the change in the entropy of molecules... Probably we can do so due to the similarity of the molecular structures and atomic radii... anyway when working with much more difficult reactions I think the only way to find out ∆S would be Hess' law (in other words experimental data).

Hey Mr. Teo, haven't had any run ins with Boltzmans equations yet. You just made me excited . Can you link me to some websites where I can learn the applications of the equation?

As usual Wikipedia is an excellent source of information:
http://en.wikipedia.org/wiki/Entropy_(statistical_thermodynamics)

If you want to know more about this topic Fermi's "Thermodynamics" is a good and really understandable book to start with:
http://www.amazon.com/Thermodynamics-Enrico-Fermi/dp/048660361X