Student A is seeking an explanation for the boiling point elevation of the solvent in a solution with a non-electrolyte. He was looking over a chart that relates the phase equilibrium lines for the melting point, boiling point, and sublimation point of a pure substance and a mixture. He asked Student B,
"Why does the pure liquid - pure gas equilibrium line shift to the right in the graph?"
Student B replied, "Good question. The boiling point of a substance is the temperature at which the vapor pressure of the liquid is equal to the total gas pressure exerted by the molecules above the liquid. This shift in the pure liquid - pure gas equilibrium line must have something to do with the change in the types of intermolecular forces observed as you compare the pure substance against the mixture of the solvent with a nonvolatile substance.
You, interject from the other side of the screen and state correctly.
1 The solute particles bind weakly with the solvent molecules, so that solvent molecules can easily escape, increasing the vapor pressure of
the substance. The temperature required to decrease the vapor pressure of the liquid solvent to the surroundings, under these circumstances, must be lower.
2 The solute particles bind weakly with the solvent molecules, so that solvent molecules cannot easily escape, increasing the vapor pressure of
the substance. The temperature required to increase the vapor pressure of the liquid solvent to the surroundings, under these circumstances, must be lower.
3 The solute particles bind strongly with the solvent molecules, so that solvent molecules cannot easily escape, reducing the vapor pressure of
the substance. The temperature required to increase the vapor pressure of the liquid solvent to the surroundings, under these
circumstances, must be higher
4 There is no relation between the intermolecular forces between the solute and the solvent particles and the boiling point elevation. The colligative property is primarily due to the identity of the solute, not the number of types of interactions with solute particles and the solvent.
Student A nods in understanding and says, "So, in the case of a non-electrolyte solute, the weak interactions between the solute and solvent molecules actually make it easier for the solvent molecules to escape into the gas phase, causing the vapor pressure to increase and the boiling point to elevate. Thanks for clarifying that for me!"
Student B agrees, "Exactly! It's all about the intermolecular forces and how they affect the overall vapor pressure of the solution. I'm glad we could figure that out together."
Overall, the discussion between the students helped clarify the concept of boiling point elevation in a solution with a non-electrolyte solute and the role of intermolecular forces in determining this colligative property.