Confused about Colligative properties/ Boiling point

[jillzhou]

---

Members don't see this ad.

One of the questions in the AAMC practice test 5 had a question on Boiling point where they said the more ions dissociated from solution, the higher the BP (known as BP elevation). If I'm not mistaken, in Orgo we learned that as molecular weight increases so should BP? So why the contradiction in Genchem? 😕

 

[Gauss44]

One of the questions in the AAMC practice test 5 had a question on Boiling point where they said the more ions dissociated from solution, the higher the BP (known as BP elevation). If I'm not mistaken, in Orgo we learned that as molecular weight increases so should BP? So why the contradiction in Genchem? 😕

Change in boiling point = kb*m*i

Kb=a constant
m=molality
i="number of particles into which a single solute particle will dissociate when added to solution" (Cited from ExamKrackers MCAT Chemistry, 7th ed., p. 88)

 

[asf503]

One of the questions in the AAMC practice test 5 had a question on Boiling point where they said the more ions dissociated from solution, the higher the BP (known as BP elevation). If I'm not mistaken, in Orgo we learned that as molecular weight increases so should BP? So why the contradiction in Genchem? 😕

I think you're confusing dissociated ions in solution and pure compounds.

For ions in solution, the ions effectively interfere with the surface area of a pure solvent. For example, in pure water, at the surface between water and air, water can freely just gain more energy and float up and off as a gas.

If you add NaCl to this, you get Na+ and Cl- ions in the water that then take up some of this surface area. Now instead of the water molecules just being able to float up and out, some of the water molecules are blocked. Thus, to compensate more heat must be added, so boiling point increases.

For pure compounds, we're talking about how much energy you need to put into a sample to make it boil off. If the molecules are heavy, they take more energy to boil off. Thus, increasing molecular weight increases boiling point. There's no solution here, just pure compound.

Did that help? Or am I just confusing you more?

 

[mehc012]

I think you're confusing dissociated ions in solution and pure compounds.

For ions in solution, the ions effectively interfere with the surface area of a pure solvent. For example, in pure water, at the surface between water and air, water can freely just gain more energy and float up and off as a gas.

If you add NaCl to this, you get Na+ and Cl- ions in the water that then take up some of this surface area. Now instead of the water molecules just being able to float up and out, some of the water molecules are blocked. Thus, to compensate more heat must be added, so boiling point increases.

For pure compounds, we're talking about how much energy you need to put into a sample to make it boil off. If the molecules are heavy, they take more energy to boil off. Thus, increasing molecular weight increases boiling point. There's no solution here, just pure compound.

Did that help? Or am I just confusing you more?

While the surface area image is very memorable, it's important to remember that that is an oversimplification of how BP elevation actually works (otherwise it would be dependent on the size of the solute particle, not simply the concentration).

A more accurate description would be that the solute 'dilutes' the solvent, lowering its vapor pressure. The solute generally does not contribute to the vapor pressure (once you start discussing volatile solutes it gets a bit more complex), so the total final vapor pressure of the liquid is lower than for pure solvent. Boiling occurs when vapor pressure = pressure above the liquid. Since the vapor pressure of the dilute (contains solute molecules) solvent is lower at a given T than that of the pure solvent, it will not be able to boil at the regular BP, and will require higher temperatures to do so.