[Kate]

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.

[Yggdrasil]

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.

[Kate]

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 ?

[Yggdrasil]

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.

[Kate]

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 ?

[Kate]

ANYONE ??

[Yggdrasil]

Shouldn't the vapor pressure increase with temperature?

[Kate]

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 ?

[Yggdrasil]

Does water boil at a lower temperature at sea level or at high altitude (where there is lower atmospheric pressure)?

[Kate]

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).

[Borek]

Does it matter whether you decrease pressure in your system using pump or moving to high altitude?

[Kate]

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 ?

[Borek]

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.

[Kate]

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.

[Borek]

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.