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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: cypher on November 19, 2006, 08:59:51 AM

Title: Calculating Entropy
Post by: cypher on November 19, 2006, 08:59:51 AM

hey guys,
I'm stuck on this question:

Rubidium has a heat of vaporization of 69.0kj/mol at its boiling point (686 degrees celcius).
Calculate ?S for this process, Rb(l)------->Rb(g), at 1 atm and 686 degrees celcius.


Ok, I think the question wants the ?S of the universem I'm not sure, what does it mean when they just say "?S"?
So I first tried to calculate the ?S of the system and the ?S of the surronding.

converted 69.kj/mol to .069 j/m. Then subtracted reactants from products in above equation = .069-.069=0. So the ?S for the system is 0...I think?

and I don't really know how to proceeed with the surrounding. I have a formula here, but ?H was not provided, and I can't seem to find it.

help would be greatly appreciated, thanks.

Title: Re: Calculating Entropy
Post by: cypher on November 19, 2006, 12:52:50 PM

wow....anybody?

Title: Re: Calculating Entropy
Post by: Yggdrasil on November 19, 2006, 03:14:26 PM

The question is asking for ?S of the system.  Here are some hints to help you out:

1)  ?G = ?H - T?S
2)  What is special about the boiling point? (Hint: think about equilibrium)
3)  For a process at equilibrium, what can you say about ?G?