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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Kalibasa on October 10, 2009, 07:41:49 PM

Title: How do I set up a simple thermodynamic cycle?
Post by: Kalibasa on October 10, 2009, 07:41:49 PM

Here is the problem I'm looking at:

The heat of formation of 2-methylpropane is -32 kcal/mol. The heat of formation of butane is -30 kcal/mol. The heat of hydrogenation of 2-methylpropene is -28 kcal/mol. What is the heat of formation of 2-methylpropene?

Someone else was confused on this and the professor told us to set up a thermodynamic cycle, but I don't even see how to set it up. (I have read three sites explaining how to use these cycles, but they were all too physics/engineering based and I still don't get it at all). I esp. don't see how butane fits into the picture. Am I supposed to use butane in the cycle? If so, how do I work with heats of formation? (I thought those were supposed to be starting from scratch)

Any help would be great, I really am not getting anywhere on this...

Thanks!

Title: Re: How do I set up a simple thermodynamic cycle?
Post by: Yggdrasil on October 10, 2009, 07:46:27 PM

Here's how I would approach a problem like this:

1)  write out equations for each of the reactions you are given (including your desired reaction)
2)  add up these reactions to give you the desired reaction
3)  from how your add up the reactions in (2), add/subtract the correct heats of reaction to get your desired heat of reaction

Title: Re: How do I set up a simple thermodynamic cycle?
Post by: Kalibasa on October 10, 2009, 08:43:40 PM

Ha, I got so bent out of shape about the "cycles" that I missed how easy it was, I got it in two steps and now feel silly :) Thanks!

But I'm still wondering what my professor what talking about. Are thermodynamic cycles basically a fancier way of using Hess' Law?

Title: Re: How do I set up a simple thermodynamic cycle?
Post by: Yggdrasil on October 10, 2009, 11:18:25 PM

Basically.  The concept is very similar to Hess's Law.