Understanding the Helmholtz energy.

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SUMMARY

The discussion centers on the Helmholtz energy and its relationship with entropy and temperature. Participants explore the concept that heat can be absorbed from the environment when entropy increases, specifically referencing the equation TdS, which represents the heat change of the surroundings. The conversation highlights the confusion surrounding the derivation of maximum expansion work under constant volume and temperature conditions, as outlined in Atkins' "Physical Chemistry." Key insights include the understanding that an increase in entropy does not necessarily correlate with a rise in temperature, as demonstrated by the example of free expansion in ideal gases.

PREREQUISITES
  • Understanding of Helmholtz energy and its significance in thermodynamics.
  • Familiarity with the concepts of entropy and microstates in statistical mechanics.
  • Knowledge of the principles of heat transfer and thermodynamic processes.
  • Basic comprehension of physical chemistry, particularly the works of Atkins.
NEXT STEPS
  • Study the derivation of Helmholtz energy equations in "Physical Chemistry" by Atkins.
  • Explore the relationship between entropy and temperature in thermodynamic systems.
  • Investigate the concept of free expansion in ideal gases and its implications for entropy changes.
  • Learn about the statistical interpretation of microstates and their role in entropy calculations.
USEFUL FOR

This discussion is beneficial for students of physical chemistry, thermodynamics enthusiasts, and anyone seeking to deepen their understanding of the relationship between Helmholtz energy, entropy, and temperature in thermodynamic systems.

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I'm having some trouble understanding this concept. Why is it that you sometimes can get heat for free from the environment? Like suppose you have a system, on which you make an energyconsuming proces which creates entropy. Then you subtract TΔS because apparently heat can enter when the entropy increases - why is that? Does the temperature drop, when we increase the entropy?
Can you guys please explain in terms of microstates too, as it is that way I feel I understand it the best.
 
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Although I may not have a complete solution to your problem
I too have a lot of doubts in the derivations.I refer to Physical Chemistry by Atkins and the way they derive the equation and then extract the physical meaning out of results is puzzling. Like while deriving we consider, they ssume to be having V constant and T constant and later after the expression comes they say its the maximum expansion work a system can erform. If V is constant how can expansion work be performed.?

Coming to your query
From what I know ( I maybe wrong)
TdS is actually the heat change of surrounding.
Whenever we calculate the net change in entropy we calculate for the universe.
If the entropy of system increase entropy of surrounding should decrease as if heats flows into system the surrounding loses heat).
I think subtacting is done for that issue.

(NOTE: I maybe wrong.I am still in high school. I am just trying to share an idea or point out a speculation)
 
Also in general its not neccesary that temperature should rise when entropy increases.
It can remain contstant and can decrease too.
Its like providing heat to an ideal gas.
Providing heat increases its entropy.
Now if the added heat cam change its Internal energy or be used to do work or a combination of both.
So we can't really .However, since heat was provided entropy increases.


A classic example is free expansion where T remains same but entropy changes due to volume increase (and therefore more freedom in movement
 

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