Entropy question

[lex1489]

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Coming from a Destroyer question, which has higher entropy, C4H10 or C3H8?
The book says C4H10 has more atoms and therefore greater molecular complexity than the smaller C3H8.

I thought it should be the opposite-- Since C4H10 decreases chaos/disorder by "locking up" more atoms, it should have less entropy. Can someone correct my logic? Thanks!

 

[Avery07]

I agree with the book.

Think of it like a skyscraper. Which has greater entropy, one with 10 stories or one with 100 stories?

 

[lex1489]

Should I think of it like, which has the greater possibility for disorder upon destruction? It just seems to me that building something very large is bringing order to more matter than building something small, and the more order you bring to matter, the less entropy.

 

[Avery07]

Should I think of it like, which has the greater possibility for disorder upon destruction? It just seems to me that building something very large is bringing order to more matter than building something small, and the more order you bring to matter, the less entropy.

I had the same thinking as the you for the longest time.. it makes sense that it appears to be ordered in our mind but in fact, in creating this order, we are setting up the ability for it to fall to pieces in a sense.

Which has a higher entropy? A full bookshelf in a library or a pile or books?

The answer is the bookshelf. Those books represent as you put, the potential for disaster or disorder. The pile of books really can't be disordered any more..

I know it doesn't make sense but once you convince yourself that this is the case, the problems become much easier.

 

[lex1489]

Got it. Thanks for the help, Avery!

 

[UCB05]

entropy is best represented microscopically as degrees of freedom. The idea here is that the larger molecule has more C-C and C-H bonds which can undergo rotation and vibration and more molecular conformations. That's why it has a higher entropy.

You're also right that a large molecule represents a reduction in systemic entropy from smaller building blocks, since degrees of freedom in the form of translational motion are lost, but this question specifically deals with entropy of two molecules, not the change in entropy from formation of the two. Just imagine which is more chaotic. The larger molecule has more twists and turns going on than the smaller one.