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Entropy generation in chemical reactions

by Urmi Roy
Tags: chemical, entropy, generation, reactions
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Urmi Roy
#1
Nov22-13, 04:33 PM
P: 692
So from the first law for a closed system,

dU=dQ-dW=dQ-PdV

From the second law,

dS=dQ/T + Sgenerated (i.e. the entropy generated)

Putting expression of dQ from second law into first law,
dU=T*dS-T*Sgen-PdV

If s and v are constant,
dU= -T*Sgen>0

Hence dU<0
This is a derivation that was given in class

My questions are as follows:
1. When we say s is constant, does it mean there is not heat flow into/out of the system?

2.If dU<0, does the temperature decrease? I find it hard to understand how there can be a positive Sgen and decrease in temperature/ internal energy simultaneously!
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DrDu
#2
Dec4-13, 02:46 AM
Sci Advisor
P: 3,564
Quote Quote by Urmi Roy View Post
My questions are as follows:
1. When we say s is constant, does it mean there is not heat flow into/out of the system?

2.If dU<0, does the temperature decrease? I find it hard to understand how there can be a positive Sgen and decrease in temperature/ internal energy simultaneously!
1. No, you can imagine that dQ=Sgen/T
2. What happens if you mix salt and ice in a thermo jar?
Urmi Roy
#3
Dec8-13, 10:46 PM
P: 692
About your Q2...not too sure, but I know that having a salt in water lowers the freezing point of water...So the ice melts back?

DrDu
#4
Dec13-13, 07:58 AM
Sci Advisor
P: 3,564
Entropy generation in chemical reactions

Yes, and gets cooler!


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