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Author: Subject: Thermodynamics - Heat of Vaporization change with pressure
FluoroPunch
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[*] posted on 14-4-2019 at 19:08
Thermodynamics - Heat of Vaporization change with pressure


I'm a little unsure about how it's physically possible for the heat of vaporization of a substance to change as the pressure is reduced.

As I understand, this is the case so I'm not doubting that, I just don't understand how this is possible. From what I know, enthalpy is a state function, so the path taken should not matter, only the end state. This doesn't seem to agree with the heat of vaporization changing as the pressure is reduced. Shouldn't the energy to turn a liquid into a gas be constant since it's a path function? Perhaps its simply because the intermolecular forces do not prevent vaporization as much when the pressure is reduced. It just doesn't make sense thermodynamically...

Could anyone clarify? Maybe I'm just tired or going insane...or missing something simple?
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[*] posted on 14-4-2019 at 19:18


The enthalpy of vaporization will change slightly with pressure. Enthalpy is a state function, but a low-pressure end state isn't the same as a high-pressure end state. I don't know how to calculate the deltaH for gas going from one pressure to another, but I wouldn't be surprised if it's non-zero.



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[*] posted on 14-4-2019 at 19:44


Quote: Originally posted by DraconicAcid  
The enthalpy of vaporization will change slightly with pressure. Enthalpy is a state function, but a low-pressure end state isn't the same as a high-pressure end state. I don't know how to calculate the deltaH for gas going from one pressure to another, but I wouldn't be surprised if it's non-zero.


So if I'm understanding correctly, what you're implying is that there's some enthalpy change due to the pressure change? That makes sense... So its kind of like how after a reaction you have to bring the reactants to the same temperature at the start of reaction to find out the total heat of reaction, only in this case you would have to bring the pressure back to what it was initially after vaporization/condensation?
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[*] posted on 14-4-2019 at 20:01


Quote: Originally posted by FluoroPunch  
Quote: Originally posted by DraconicAcid  
The enthalpy of vaporization will change slightly with pressure. Enthalpy is a state function, but a low-pressure end state isn't the same as a high-pressure end state. I don't know how to calculate the deltaH for gas going from one pressure to another, but I wouldn't be surprised if it's non-zero.


So if I'm understanding correctly, what you're implying is that there's some enthalpy change due to the pressure change? That makes sense... So its kind of like how after a reaction you have to bring the reactants to the same temperature at the start of reaction to find out the total heat of reaction, only in this case you would have to bring the pressure back to what it was initially after vaporization/condensation?


Yes. If you want standard delta(H), all the gases have to be at 1 atm before and after.




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[*] posted on 14-4-2019 at 20:41


Alright that was the missing piece of the puzzle. Thanks a bunch
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[*] posted on 15-4-2019 at 08:53


The definition of enthalpy is "internal energy" + PV. So why wouldn't the enthalpy change with pressure.



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[*] posted on 15-4-2019 at 13:02


Quote: Originally posted by Magpie  
The definition of enthalpy is "internal energy" + PV. So why wouldn't the enthalpy change with pressure.


Oof...you're right. Guess I'm getting rusty on the basics :( I think that's explains it best, it is a function of pressure.
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[*] posted on 15-4-2019 at 16:27


From a theoretical view (definitely not my forte!), I would think that a substance would vaporize more easily (require less energy) at a lower pressure because the liquid molecules would have to "fight against" fewer vapor molecules to enter the gas phase (vaporize).
(Is that pretty deep for me, or what?:D) Regards, Charlie A
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[*] posted on 16-4-2019 at 09:48


Quote: Originally posted by CharlieA  
From a theoretical view (definitely not my forte!), I would think that a substance would vaporize more easily (require less energy) at a lower pressure because the liquid molecules would have to "fight against" fewer vapor molecules to enter the gas phase (vaporize).
(Is that pretty deep for me, or what?:D) Regards, Charlie A


Haha yeah that's one way to think about it I guess. I didn't have a problem with how it happened, I was just unsure how to reconcile it with the other stuff :P
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