Effect of particle size on boiling point in solution

[albertrichardf]

Hello.
Suppose you dissolve something in water. I know that the boiling point should increase because as the water boils the solute's entropy decreases, and the net entropy should increase.
Now suppose, that I dissolve GI chlorides in the water. Each solution of the salt has the concentration, and because they are all from the same group they should react similarly. As I go down the group, the size of the particles increases. My question is as this particle size increases, how does the increase in boiling point change?

My guess is that as solute size increases, the solution's particles have less possible arrangements, so its entropy will decrease. Thus the boiling point will increase with size. Is that correct? And are there any other factors? Also, if I was to conduct a fair test of this, is it more important to keep the number of moles constant or the mass of the solute constant. Finally, is this a trend specific to G1 chlorides, or does it depend on the salt, and on the group?
Thank you for answering my questions. Also, if you know of sources for such data, I would appreciate if you could post them as well.

 

[Ygggdrasil]

Consider the equation for boiling point elevation (see https://en.wikipedia.org/wiki/Boili...tion_for_calculations_at_dilute_concentration). Which terms of the equation (if any) might particle size be expected to affect?

 

[albertrichardf]

Thanks for the link. I would not expect the ebullioscopic constant to change, since that would depend entirely on the water, nor would I expect the Van't Hoff factor to change, since according to wikipedia, it is around 2 for GI chlorides. However, I am a bit confused about the given equation for the molality of the solution.

The molality is defined as the number of moles per unit mass in a solution, but according to the given equation there is the molality of the solution and the molality of the solute. The molality of the solution is equal to the molality of the solute times the Van't Hoff number. But I don't understand what would be the molality of the solute, because there is no mass of solution involved. Unless the molality of the solute is the number of moles per unit mass of solution, and the molality of the solution is the number of moles per unit mass of solution multiplied by the Van't Hoff number. Is that right?

 

[mjc123]

The molality of the solute is the number of moles of solute per unit mass of solvent. The molality of the solution is the number of moles of dissolved species per unit mass of solvent. This is equal to the molality of solute multiplied by the van't Hoff number. For example, if NaCl is completely dissociated into ions, a 1 molal solution of NaCl (solute molality) contains 2 moles of ions per kg water, so the solution molality is 2. (Actually from the article vH no = 1.9, but just for simplicity of illustration.)

 

[albertrichardf]

I see, thank you for the explanation. Thus according to the equation, if the number of moles remains constant, the boiling point increase should not change, but if the mass of solute remains a constant, the boiling point increase should decrease.

 

[mjc123]

Yes, if you're assuming the molar mass increases (and vH no. remains the same).

 

[albertrichardf]

Yes, if you're assuming the molar mass increases (and vH no. remains the same).

The vH should remain close to two for GI Cl salts if I understood the wikipedia article. Thank you for answering