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Author: Subject: Heat and Desiccating Organic Solvents
lollerskatez
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[*] posted on 31-3-2021 at 13:00
Heat and Desiccating Organic Solvents


I'm a little confused on this issue, since it seems that most organic solvents would evaporate when mixed with desiccating agents to make them anhydrous. My math could be way off, though.

So, for example:I have a solution consisting of 10 moles of acetone and 1 mole of water (roughly ~3% water, or 18.01/580.8). I mix that with 1 mole of sodium sulfate. Now, the specific heat of acetone 125 J/mol K and the heat of hydration of sodium sulfate is -81 kJ/mol. Assuming my math is right, the acetone increase in temperature by ((81000/125)/10), or 64.8 C.

The boiling point of acetone is 56 C, so shouldn't it completely boil off due to the temperature increase?
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[*] posted on 31-3-2021 at 13:56


Acetone also has a heat of hydration, and you "lose" that energy.

Also, you need to take care what mole you are talking about.
Hydrating a mole of sodium sulphate takes ten moles of water.

And, if you heat sodium sulphate decahydrate to about 33 degrees, it melts- absorbing a lot of heat.
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RustyShackleford
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[*] posted on 31-3-2021 at 14:09


You also have the specific heat of the sodium sulfate monohydrate aswell as the enthalpy of vaporization of acetone (31.27kJ/mol).
Quick math (assuming starting at 25C and that the Cp of sodium sulfate monohydrate is the same as sodium sulfate, 128.2J/mol-K) says that to heat up the system to the bp of acetone would take (56-25)(10*125+128)=42718J. So the "excess energy" would be 81-42.7=38kJ. Which would only be enough to boil off 38/31.27= 1.22mol of Acetone.
In reality it will be less due to the monohydrate Cp being higher than assumed, the heat capacity of the beaker/vessel aswell as cooling from surrounding airflow.

Edit: As Unionised said, there will also be some energy associated with the intermolecular forces in the acetone/water mix, the separation of which will result in a absorption of energy (which would otherwise go toward the heating/boiling)
Also idk if that value you gave for the heat of hydration is for 1 water or 10. I assumed it was the former.


[Edited on 31-3-2021 by RustyShackleford]
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lollerskatez
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[*] posted on 31-3-2021 at 16:28


Sorry for any confusion—I just gave sodium sulfate as a hypothetical, not realizing that its decahydrate form melts at 33 C.

I guess a better example would be magnesium sulfate (the heptahydrate form of which doesn't begin to decompose until 150 C) or calcium sulfate. Also since they're both more commonly used for this purpose.


Quote:

Acetone also has a heat of hydration, and you "lose" that energy.


I was aware of this (I believe it's 4 kJ/mol), but wouldn't that only matter when the water was added to the acetone initially? I guess I should have specified my question better, but assuming that the solution is at 25 C when the desiccant is added, would that still need to be taken into account?


Quote:

You also have the specific heat of the sodium sulfate monohydrate aswell as the enthalpy of vaporization of acetone


Oh, yes, I had forgotten about that. Thanks for pointing that out!

Quote:

Also idk if that value you gave for the heat of hydration is for 1 water or 10. I assumed it was the former


I wasn't completely sure of that either, so I also assumed it was for the former. The only reason I thought of this question was that after adding water to some of these desiccants, the holding vessels got a lot hotter than I had expected. That made me wonder how much solvent would boil off as a result depending on how much water needs to be removed.
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