[Chanya]

Hi all,
I can’t find that’s why the boiling points of two compounds differ by less than 40oC, they cannot be separated by simple distillation.

Maybe it’s relate with partial pressure and temperature?

[billnotgatez]

Have you learned about Azeotrope
https://en.wikipedia.org/wiki/Azeotrope

quoted below is a small part of the explanation by WIKI

From Wikipedia, the free encyclopedia
An azeotrope (/əˈziːəˌtroʊp/)[1] or a constant heating point mixture is a mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation.[2] This happens when an azeotrope is boiled, the vapour has the same proportions of constituents as the unboiled mixture. Because their composition is unchanged by distillation, azeotropes are also called (especially in older texts) constant boiling point mixtures.

[Chanya]

I understand that if we use temperature tempurature at azeotropic point, pure product could not be obtained even by means of a fractional distillation.
But in this case they don't give composition of mixer [http://classes.kvcc.edu/chm220/CHM220%20Distillation%20Lab.pdf] So, Why they know BP should s differ by less than 40ºC?

[Babcock_Hall]

@OP,

What do you know about fractional distillation?

[Enthalpy]

This is a matter of partial pressure.

That is, the state of a substance isn't binary, liquid below the boiling point and gaseous above. Rather, an equilibrium between a liquid and its vapour can exist at varied temperatures. The equilibrium vapour pressure increases (quickly) with the temperature.

The boiling point is only the temperature for which the equilibrium vapour pressure equals the ambient pressure. Above this temperature, the vapour has enough pressure to push the liquid to the sides to make bubbles. Hence the name "boiling point", not some undefinable "vapour point". And the boiling points depends on the ambient pressure.

So evaporation occurs below the boiling point too, it only produces less vapour pressure than the ambient pressure, and this vapour is diluted in the ambient gas that makes most of the ambient pressure. This happens when oceans too cold to boil inject humidity in the atmosphere. Or when you see vapour (or rather condensing vapour) over a pan before the water boils.

Now if you boil two mixed liquids, the liquid with a higher boiling point contributes to the evaporation too, more so if the temperature is near to its boiling point. So for the vapour to be purer, you need a big difference between the boiling points. 40K is just one arbitrary value to get "enough" difference in the vapour pressures.

This implicitly supposed little interaction between the substances. Counter-example: a solution of ammonia in water doesn't produce the ammonia vapour pressure corresponding to the equilibrium vapour pressure of liquid ammonia.

[Chanya]

Thanks for your helping.