I Availability at fixed pressure and temperature

AI Thread Summary
The discussion centers on the thermodynamic relationships involving changes in Helmholtz free energy (A), Gibbs free energy (G), enthalpy (H), and internal energy (U) under specific conditions of pressure (P) and temperature (T). It highlights confusion regarding the expressions for dA and dG, particularly the use of P_0 versus P in equations at constant pressure and temperature. The participants clarify that at fixed pressure, P equals P_0, but T may not always equal T_0, as illustrated by examples like ice melting. The conversation also touches on the significance of reversible processes in these thermodynamic equations. The thread concludes with a critique of teaching methods related to these concepts, emphasizing the importance of understanding changes in A and G under specified constraints.
laser1
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We define ##dA=dU+P_0dV-T_0dS \leq 0##. In my notes it says if you fix pressure and entropy, ##dA=dH##. I don't get this, because at constant T and S, I get ##dA=dU+P_0V##. It seems that somehow, ##P_0=P##. Is this correct, or am I missing something?

Second question about this:
1730534394097.png

If ##T_0=T## and ##P_0=P## then ##dG=0## which is probably false. But the above image confuses me also. Why can we write ##dH## as ##dU+P_0dV## rather than ##dU+PdV##? Thanks
 
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At fixed pressure, P=Po
 
Chestermiller said:
At fixed pressure, P=Po
Cool, thanks, that explains the first part. What about the second? At fixed pressure, P=Po. At fixed temperature, T=To. This means that dG = 0, which doesn't make sense, because why not just write that rather than ##dU-T_0dS+P_0dV=dG##?
 
After thinking for a bit, can I say that P=Po and T=To are only true when processes are reversible, i.e., when at constant entropy? I am not sure why that would be correct, but it would resolve all my problems in my OP!
 
See Chater 7 of Fundamentals of Engineering Thermodynamics by Moran et al, on Exergy.
 
Chestermiller said:
See Chater 7 of Fundamentals of Engineering Thermodynamics by Moran et al, on Exergy.
Cheers. I have looked briefly at it, but is my following argument below correct?

##P = P_o ## always at fixed ##P##.
##T## is not always ##T_0## at constant ##T##. Counterexample being ice melting. ##T## is not ##T_o## yet ##T## is constant.

##G=H-TS##
##dG = dU + PdV - TdS## at constant ##T## and ##P##. However, there is a correction term in ##dG##, which is ##(T-To)dS##.

Hence,
$$dG = dU + PdV - TdS + (T-To)dS$$
and substituting ##P = P_0## always,
I get the equation in my textbook,
$$dG = dU + P_0dV-T_0dS$$
 
What are you trying to prove?
 
Chestermiller said:
What are you trying to prove?
In the post #1 in the image after "Second question about this:" it has an expression for ##dG## which is $$dG=dU-T_0dS+P_0dV$$ I want to prove that.
 
laser1 said:
In the post #1 in the image after "Second question about this:" it has an expression for ##dG## which is $$dG=dU-T_0dS+P_0dV$$ I want to prove that.
What is the significance of the word "availability" in your thread title?
 
  • #10
Chestermiller said:
What is the significance of the word "availability" in your thread title?
I have attached the definition of it from Blundell and Blundell
1730893136297.png
 
  • #11
laser1 said:
I have attached the definition of it from Blundell and Blundell
View attachment 353194
I'm sorry. I can't help you with this. I totally disagree with the author's approach to teaching this material. I strongly oppose the use of differentials for a system that undergoes an irreversible process. If you want to get a better picture of how A and G change under certain well-specified constraints on T and P, see Chapter 1 of Principles of Chemical Equilibrium by Denbigh.
 
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