Chemistry concept Q

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highlighter
Posts: 6
Joined: Sat May 25, 2019 9:32 pm

Chemistry concept Q

Post by highlighter » Thu Jun 27, 2019 1:29 pm

Currently reviewing chem material & got confused about entropy.
I answered B for Q1 (p.86) thinking that entropy change is positive because conceptually, by turning into N2O4, there are less molecules, which means more "order". But the answer is D stating that entropy change is negative. I thought -/+ indicated whether something was high or low entropy (more +, more order), (more -, less order --> more spontaenous). Can someone tell me where I went wrong in my thinking process? Thank you!
avo
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Joined: Wed Jun 12, 2019 6:01 pm

Re: Chemistry concept Q

Post by avo » Thu Jun 27, 2019 7:05 pm

Hi highlighter (nice username by the way, I have paintroller syndrome in CARS!)

So according to the same book on page 78 on the 4th paragraph:
"Entropy is related to the number of configurations a system can possibly have, with more potential configurations corresponding to greater entropy"

For me, it was easier to visualize "configuration" as a means of movement. So take the three states of matter of water for example (solid, liquid, gas). Solids/Ice generally can't move around because it can really only assume one type of configuration; it's in a fixed state. With liquids, water molecules can move around still, but are localized within the volume it takes up. When water becomes gas vapor, these water molecules can now spread out and go wherever. In this way, we see that entropy can increase moving from solid to liquid to gas.

Going back to the question at hand on page 86, we see that the smog reaction at the bottom of the passage has 2 reactants forming 1 product. With only one molecule formed, there isn't a lot of ways to change configurations. That leads to another concept of entropy, where entropy will increase if we have more molecules in the products than in the reactants. With more molecules, we have more variability of movement/configuration between the molecules, and thus an increase in entropy.

It's a long-winded answer, but hopefully I was able to help! And if I got something wrong, hopefully one of the NS tutors can help the both of us out haha. Good luck, you got this!
highlighter
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Joined: Sat May 25, 2019 9:32 pm

Re: Chemistry concept Q

Post by highlighter » Fri Jun 28, 2019 1:28 pm

Thanks avo for your reply/answer. It makes sense but I'm still unsure about the use of positive & negatives when indicating entropy. The more negative a number for H, it indicates that the reaction is more exothermic. So I was wondering if a negative entropy means less or more order @_@
NS_Tutor_Will
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Re: Chemistry concept Q

Post by NS_Tutor_Will » Fri Jun 28, 2019 2:04 pm

Increasing entropy means more disorder and would correspond with a positive ∆S value. If something becomes more ordered, then entropy would decrease and ∆S would be negative.

Note, however, that we have to be finicky about what we're talking about when we say entropy is increasing. With a protein, for instance, when it properly folds, the protein is becoming more ordered and is therefore losing entropy. So there's an entropic cost in the protein itself to properly folding. That said, proteins fold, in part, because of the interactions in the solvent that they are in. So a protein folding up would enable more favorable reactions to occur within the solvent (i.e. no solvation layer would be necessary). That means that in the solvent the entropy is increasing and protein folding is therefore a favorable process.

I hope this is helpful!
NS_Tutor_Mathias
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Re: Chemistry concept Q

Post by NS_Tutor_Mathias » Fri Jun 28, 2019 5:01 pm

highlighter wrote:
Fri Jun 28, 2019 1:28 pm
Thanks avo for your reply/answer. It makes sense but I'm still unsure about the use of positive & negatives when indicating entropy. The more negative a number for H, it indicates that the reaction is more exothermic. So I was wondering if a negative entropy means less or more order @_@
A way to remember or conceptualize this is by the definition of Gibb's free energy:
deltaG = deltaH - TdeltaS

Where a negative value for deltaG indicates spontaneity. Exothermic and increases in disorder are the more 'spontaneous' state (this is very much not a rigorous way of looking at things, but helpful, trust me!). Therefore, positive deltaS represents an increase in disorder and contributes somewhat to the spontaneity of a reaction.
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