Entropy generation in chemical reactions

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Discussion Overview

The discussion revolves around the generation of entropy in chemical reactions, particularly focusing on the implications of the first and second laws of thermodynamics. Participants explore the relationship between internal energy, entropy generation, and temperature changes in closed systems.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant presents a derivation involving the first and second laws of thermodynamics, leading to the conclusion that if entropy generation is positive, internal energy must decrease.
  • Questions are raised about the meaning of constant entropy (s) and whether it implies no heat flow into or out of the system.
  • Another participant suggests that a positive entropy generation (Sgen) can coexist with a decrease in internal energy (dU<0), prompting further inquiry into the relationship between these quantities.
  • A participant introduces a scenario involving mixing salt and ice in a thermodynamic jar, noting that this process lowers the freezing point of water, which may relate to the discussion of entropy and energy changes.
  • Another participant agrees that the ice melts and suggests that this process results in a cooling effect.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the implications of constant entropy and the relationship between positive entropy generation and decreasing internal energy. Multiple competing views remain on these concepts, and the discussion has not reached a consensus.

Contextual Notes

Limitations include potential misunderstandings of thermodynamic principles, particularly regarding the implications of constant entropy and the interactions between temperature, internal energy, and entropy generation.

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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