Determination of Ethanol by Distillation

Question

Would a mixture of water/ethanol show a positive or negative deviation from Raoult's law?

Answer 1

Hi Chance P!

We can state Raoult's Law as:

P_i = x_iP_vap

Raoult's law states that the partial vapor pressure of some species is its pure vaptor pressure times its mole fraction present in the liquid phase.

Moreover, it makes assumptions about the gas and solution phases. Specifically, it assumes an ideal gas and an ideal solution (an ideal solution is one in which all the intermolecular forces are of an equal strength). A common way to denote this (particularly in the textbooks) is to say the a-b interactions are of the same strength as the a-a and b-b interactions, wherein a and b represent the different chemical species in solution.

A positive deviation from Raoult's law occurs when the partial vapor pressure observed is greater than what is expected from Raoult's law. This allows us to make an inference about the strength of the intermolecular interactions. Namely, that the a-b intermolecular interactions are weaker than the a-a and b-b interactions?

Why is this the case?

I'm so glad you asked!

If the vapor pressure is greater than what we expected, it means that more liquid in the solution is escaping into the vapor phase than we calculated. This can only happen if the interactions are weaker than expected.

On the other hand, suppose we observe a negative deviation. This would mean that the vapor pressure is less than we expect. This implies that the a-b interactions are stronger than the a-a and b-b interactions. Moreover, this would mean that fewer liquid particles are escaping into the vapor phase above the solution. This would be due to an increased strength of intermolecular forces between the species.

Please let me know what you think or if you have any follow-up questions down below in the comments!

Cheers!