How can I find the "Normal Boiling Point" of something

[RESmonkey]

Specifically, Octane. I don't just want to Wiki it. I want to know how.

Thanks

 

[Suspicious-Teach8788]

i'm glad i passed my polymer class. i learned nothing. i wrote that on the final. somehow i passed.

 

[gururu2]

boiling point is an empirical value dependent on the temperature and pressure. the molar enthalpy of vaporization is merely the heat required to phase change one mole of substance from liquid to gas at constant temperature. if you have the molar enthalpy of vaporization at 298 K, then you know how much heat it takes to vaporize one mole when the surroundings are 298 K. This however says nothing about how much heat was required to reach the boiling point. boiling points must be determined in the laboratory and are documented for reference in indexes such as the CRC or Merck.

 

[5to1baby1in5]

I think you put it in a container at sea level, apply heat, and stick a thermometer in it when it starts to boil.

Otherwise, you could just read a Thermodynamics book.

Seriously, isn't the "molar enthalpy of vaporization" the same as the "heat of vaporization?" AKA How much energy does it take to heat a specific ammount (1 mole) of Octane (C4H10) one Degree Celcius. This would have no relation to the boiling point. It only tells you how much energy you need to vaporize a liquid.

^
Edit: What he said.

 

[jagec]

Originally posted by: gururu2
boiling point is an empirical value dependent on the temperature and pressure. the molar enthalpy of vaporization is merely the heat required to phase change one mole of substance from liquid to gas at constant temperature. if you have the molar enthalpy of vaporization at 298 K, then you know how much heat it takes to vaporize one mole when the surroundings are 298 K. This however says nothing about how much heat was required to reach the boiling point. boiling points must be determined in the laboratory and are documented for reference in indexes such as the CRC or Merck.

No, there is a way to predict based on enthalpy/entropy, IIRC. Obviously measuring it empirically is much more precise, but that's the easy way out.

Unfortunately I can't remember what that method might be...something about vapor pressure maybe...

 

[Fenixgoon]

you'll need an equation relating the vapor pressure and temperature, often given as ln(P) in atm = A/T + B*ln(T) + C

d/dT ln(P) = deltaH/RT^2

deltaH can be solid--> vapor or liquid-->vapor, depending on which vapor pressure equation you have


edit: that's the only way i know offhand, anyway

also, as a clarification to the definition of "boiling point"

boiling point = the temperature at which the vapor pressure is equal to the external (atmospheric, in most cases) pressure.

IE, by raising the pressure, you will increase the boiling point of octane because you need a higher temperature to achieve a larger vapor pressure.

 

[gururu2]

Originally posted by: jagec

Originally posted by: gururu2
boiling point is an empirical value dependent on the temperature and pressure. the molar enthalpy of vaporization is merely the heat required to phase change one mole of substance from liquid to gas at constant temperature. if you have the molar enthalpy of vaporization at 298 K, then you know how much heat it takes to vaporize one mole when the surroundings are 298 K. This however says nothing about how much heat was required to reach the boiling point. boiling points must be determined in the laboratory and are documented for reference in indexes such as the CRC or Merck.

No, there is a way to predict based on enthalpy/entropy, IIRC. Obviously measuring it empirically is much more precise, but that's the easy way out.

Unfortunately I can't remember what that method might be...something about vapor pressure maybe...

those thermodynamic parameters are all based on actual thermal data conducted in the laboratory and universally used as standards under standard conditions. my argument was just that there is no way to obtain the boiling point from knowledge of the molar enthalpy of vaporization. you need to know far more than that mere value.

 

[BurnItDwn]

Forget all teh chemistry mummbo jumbo. Here is the way to do it.
1.) open fridge
2.) grab beer
3.) If necessary grab bottle
4.) grab glass
5.) pour beer
6.) drink beer
7.) repeat steps 2-6 as needed
8.) repeat step 7 as needed
9.) forget chemistry mumbo jumbo