Why does the addition of a nonvolatile solute lower the vapor pressure, but elevate the boiling point.
When a solutions VP = atmospheric pressure, it will boil. Thus, it would have a lower BP since its VP is lower.
These seem to contradict each other.
Ding! The answer lies in the bold statements above. In order to boil, the solutions VP has to equal the atmospheric pressure, which is 1 atm (unless otherwise stated). If you LOWER the vapor pressure (say, from 0.7 atm to 0.4 atm), do you think you are closer or farther from 1 atm? You just lowered the vapor pressure and are now further from the 1 atm mark.
I'm going to copy and paste what I wrote in a similar thread a while ago:
What happens is that the molecules exert a pressure above the liquid as they try to escape into the gas phase. This is the vapor pressure. Now, if you add NaCl, a nonvolatile solute, it will block some of the molecules from escaping at the surface since NaCl will also occupy some room. This lowers the vapor pressure, since you don't have that many molecules crashing up at the surface trying to escape. By LOWERING the vapor pressure, you elevated the boiling point because remember, boiling occurs when the vapor pressure=atmospheric pressure. Think about adding salt to a pot of hot water and pasta. You want to raise the boiling point so your pasta can cook longer/cook well. Here's a silly way to make you understand:
liquid molecules attempting to escape into the gas phase: muahaha, let's become gas molecules! We're creating more vapor pressure by hanging out at the surface and trying to escape into the gas phase. That pasta won't be in there for long!
nonvolatile solute: not so fast! Let me block your way for a while.
liquid molecules attempting to escape into the gas phase: dammit! You just lowered the vapor pressure. We worked so hard! Now this pasta will have to stay in here longer because we have to create more pressure to equal to the atmospheric pressure. We almost had it! Dang.
Another example: think of 20 runners that have to run 5 miles. They can stop running as soon as they hit the 5 mile mark. Now, suppose you drive by some runners who are at the 3 mile mark (they have 2 more miles to go). You stop, stuff them in your car, drive them back to the 1 mile mark and drop the off. Now they have to run 4 more miles instead of 2 more miles to get to the 5 mile mark. Same goes for vapor pressure. When you add a nonvolatile solute, you lower the vapor pressure, but the 1 atmosphere requirement doesn't change. They solution still has to equal its vapor pressure to 1 atm. It just has to work for longer now because you lowered its vapor pressure. So, lowering the vapor pressure does not affect the atmospheric pressure. Hope this helps.
