Entropy generation in chemical reactions

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SUMMARY

The discussion centers on entropy generation in chemical reactions, specifically the relationship between internal energy (dU), heat transfer (dQ), and work done (dW) in a closed system. The derivation presented indicates that when entropy generation (Sgen) is positive, the internal energy (dU) decreases, leading to a temperature drop. The participants clarify that constant entropy (s) does not imply the absence of heat flow, and they explore the implications of mixing salt and ice, which lowers the freezing point and results in cooling.

PREREQUISITES
  • Understanding of the first and second laws of thermodynamics
  • Familiarity with concepts of internal energy (dU) and entropy (S)
  • Knowledge of heat transfer mechanisms (dQ) in closed systems
  • Basic principles of phase changes and freezing point depression
NEXT STEPS
  • Study the implications of the first and second laws of thermodynamics in closed systems
  • Explore the mathematical derivation of entropy generation in chemical reactions
  • Investigate the effects of solutes on freezing point depression in solutions
  • Learn about the relationship between temperature, internal energy, and entropy in thermodynamic processes
USEFUL FOR

Students and professionals in chemistry, chemical engineering, and thermodynamics who seek to deepen their understanding of entropy generation and its implications in chemical reactions.

Urmi Roy
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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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Urmi Roy said:
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?
 
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?
 
Yes, and gets cooler!
 

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