Textbook Problem 4.59

[Alyssa Ly 2G]

Hello!

Can someone help me understand this problem?

"A technique used to overcome the unfavorable thermodynamics of one reaction is to “couple” that reaction to another process that is thermodynamically favored. For instance, the dehydrogenation of cyclohexane to form benzene and hydrogen gas is not spontaneous. Show that, if another molecule such as ethene is present to act as a hydrogen acceptor (that is, the ethene reacts with the hydrogen produced to form ethane), then the overall process is spontaneous."

Thank you so much!

[Ryan Blaydon 1K]

So this problem is quite similar to the enthalpy problems that we have previously done you need to combine the equations basically. This time however we will be using Delta G instead of Delta H.

[Jose Angelo Grajeda 2D]

Hi! I believe you would approach this problem as you would do the same question with ∆H's instead. Since enthalpy changes are additive, we can use Hess's Method to combine equations together, and add/subtract/multiply/divide ∆H values—in this case, we would do the same with the ∆G's (to make it spontaneous, we need the final ∆G to be negative) --> rearrange and add the reactions! Since ethene can combine with hydrogen to form ethane, this would release energy (because forming bonds releases energy). Therefore, if we couple this reaction with the original, we can make the overall process spontaneous (-∆G)!