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Topic: Molar enthalpy of vaporization  (Read 18855 times)

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Offline Kate

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Molar enthalpy of vaporization
« on: March 12, 2008, 01:37:44 PM »
Hello.  :)
The other day in class I did an experiment with the purpose of determining the molar enthalpy of vaporization (or heat) of ethanol based on the variation of its boiling point with the exterior pressure.
Basically, I need to calculate the molar enthalpy of heat of ethanol considering first the ideal vapor phase and then the real behaviour of the vapor phase. I'm not sure if anyone is familiar with this but, anyway, I was wondering if someone could shed me some light on this as I have no idea where to even start.
Thanks.

Offline Yggdrasil

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Re: Molar enthalpy of vaporization
« Reply #1 on: March 12, 2008, 01:53:48 PM »
Sounds like reading about the Clausius-Clapeyron equation in your chemistry text or on wikipedia (http://en.wikipedia.org/wiki/Clausius-Clapeyron_relation) would be a good starting point.

Offline Kate

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Re: Molar enthalpy of vaporization
« Reply #2 on: March 12, 2008, 02:35:01 PM »
So, if I want to calculate the molar enthalpy of heat of ethanol considering the ideal vapor phase I have to use this equation...

...right ?

And in order to calculate the molar enthalpy of heat of ethanol considering the real behaviour of the vapor phase then I have to use this equation...


...right ? Or wrong ?

Offline Yggdrasil

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Re: Molar enthalpy of vaporization
« Reply #3 on: March 12, 2008, 03:20:43 PM »
Both are equations that you posted are equivalent statements of the same equation.  I'm not quite sure what they mean by ideal and real vapor pressure data.

Offline Kate

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Re: Molar enthalpy of vaporization
« Reply #4 on: March 12, 2008, 04:02:14 PM »
Here are the values that I have:


I used the second equation and the value I got for enthalpy was negative. Is this normal ? I'm assuming not. So what am I doing wrong ?
« Last Edit: March 12, 2008, 05:25:52 PM by Kate »

Offline Kate

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Re: Molar enthalpy of vaporization
« Reply #5 on: March 13, 2008, 03:36:14 PM »
ANYONE ??

Offline Yggdrasil

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Re: Molar enthalpy of vaporization
« Reply #6 on: March 13, 2008, 05:28:06 PM »
Shouldn't the vapor pressure increase with temperature?

Offline Kate

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Re: Molar enthalpy of vaporization
« Reply #7 on: March 13, 2008, 06:07:37 PM »
According to my teacher (he was there when I did the experiment) no, it shouldn't. As you increase the pressure, the temperature (bp) starts lowering. What's your opinion ?

Offline Yggdrasil

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Re: Molar enthalpy of vaporization
« Reply #8 on: March 13, 2008, 09:33:13 PM »
Does water boil at a lower temperature at sea level or at high altitude (where there is lower atmospheric pressure)?

Offline Kate

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Re: Molar enthalpy of vaporization
« Reply #9 on: March 14, 2008, 04:40:09 AM »
The water boils at a lower temperature at higher altitudes. But the pressure values above don't correspond to the atmospheric pressure. The pressure values that I posted are the ones that I read using a manometer which was attached to some kind of vaccum machine (not sure how to say this).
« Last Edit: March 14, 2008, 04:59:58 AM by Kate »

Offline Borek

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Re: Molar enthalpy of vaporization
« Reply #10 on: March 14, 2008, 05:09:36 AM »
Does it matter whether you decrease pressure in your system using pump or moving to high altitude?
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Offline Kate

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Re: Molar enthalpy of vaporization
« Reply #11 on: March 14, 2008, 05:21:38 AM »
Does it matter whether you decrease pressure in your system using pump or moving to high altitude?
Oh, so the pump is used in order to decrease the pressure in the system. So, in order to calculate the pressure values to be used in the Clapeyron equation that correspond to the temperatures (bp) that I posted above, I must subtract to the atmospheric pressure the pressure values that I already know (that I read in the manometer). Is this it ?

Offline Borek

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Re: Molar enthalpy of vaporization
« Reply #12 on: March 14, 2008, 06:11:33 AM »
I haven't seen your experimental setup, so I can't be sure what you have measured - but I doubt. I would expect bp to go down together with the pressure.
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Offline Kate

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Re: Molar enthalpy of vaporization
« Reply #13 on: March 16, 2008, 07:46:45 PM »
Thanks to both of you for helping me out. I have figured out what was wrong already. Anyway, apart from the Clapeyron equation and Trouton's rule, is there any other way to determine ethanol's molar enthalpy of heat ? Any other equations that can be used ? Thanks again.

Offline Borek

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Re: Molar enthalpy of vaporization
« Reply #14 on: March 16, 2008, 07:54:05 PM »
Thanks to both of you for helping me out. I have figured out what was wrong already.

Can you tell us? It can be usefull for someone one day.

Any other equations that can be used ?

Equations I don't know. Handbook I have ;)

10.33 kcal/mol.
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