Question about vapor pressure

[socloseithurts]

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can someone please explain the connection between vapor pressure and boiling point? for example, if something has a high boiling point, then it has low vapor pressure, but i'm not sure why. Also, what is the "definition" of vapor pressure?

 

[klutzy1987]

can someone please explain the connection between vapor pressure and boiling point? for example, if something has a high boiling point, then it has low vapor pressure, but i'm not sure why. Also, what is the "definition" of vapor pressure?

Well this is related to intermolcular forces. Something has a high boiling point because there are alot of intermolecular forces holding the liquid together, i.e. H-bonding. If there is alot of force holding molecules together, then it will be very dificult for molecules to escape and form vapor. Both vapor pressure and boiling point rely on the same intermolecular forces.

Any questions ask and ill try and answer.

 

[Rubber Dam Man]

clear and concise. well said... 👍

 

[socloseithurts]

ok, so here's a question: which substance has the greatest vapor pressure at 25C:
acetic acid, ethane, octane, acetone, propanol

 

[glk2101]

you can eliminate all answers except octane and ethane since everything else is polar..
now which one has greater van der waals forces?? since octane is bigger it will, so ethane will have the highest vapor pressure

now i have a question:
how does the vapor pressure of butanol compare with hexanol?? and which has a higher boiling point?

im a little unclear about the diff b/w vp and bp for these two alcohols?

 

[socloseithurts]

you can eliminate all answers except octane and ethane since everything else is polar..
now which one has greater van der waals forces?? since octane is bigger it will, so ethane will have the highest vapor pressure

now i have a question:
how does the vapor pressure of butanol compare with hexanol?? and which has a higher boiling point?

im a little unclear about the diff b/w vp and bp for these two alcohols?

isn't the trend that higher molecular weight means higher boiling point, so a lower vapor pressure? i'm not exactly sure though...

but i actually have a question about what you said earlier: how do you know to eliminate the answer choices that are polar?

 

[glk2101]

i just got a little confused myself about diff b/w vap pressure, bp, solubility concerning alcohols--see my other thread

so im pretty sure that since those three molecules are polar, they will have higher boiling pts and therefore lower vapor pressure than something that is not polar

 

[yakuza]

ok, so here's a question: which substance has the greatest vapor pressure at 25C:
acetic acid, ethane, octane, acetone, propanol

ethane > octane >> acetone >>>>> acetic acid



IMF increases, VP decreases..SO think of it this way

1. ethane has very little LD (london dispersion)
2. octane has ALOT of LD
3. acetone has a C=O which makes it polar and no hydrogen bonding
4. acetic acid has a C=O AND C-OH which makes it polar + HB

 

[yakuza]

sht I forgot propanol.

Propanol goes between acetone and acetic acid

 

[Doa110]

isn't the trend that higher molecular weight means higher boiling point, so a lower vapor pressure? i'm not exactly sure though...

but i actually have a question about what you said earlier: how do you know to eliminate the answer choices that are polar?

-Vapor pressure is inversely proportional to Intermolecular forces and directly proportional to temp. (not BP)
-The Lower the boiling points<....>the weaker the InterMolecular(IM) forces<---> the higher the VP.
-However, if a solvent is not pure and solutes {solids(ex:NaCl or liquids(ex: NH3) with higher boiling points(due to ionic, H-bonding ,..etc) are added to the solvent, then the solution will have higher BP compare to the pure state.

Source: Kaplan white book

 

[mechtel]

Remember that pressure is just force per unit area. You could even draw a free body diagram for a molecule floating above the surface in its phase equilibrium state.

In an equilibrium gas molecule above a substance, there is the force of atmospheric pressure pushing down on the molecule and the vapor pressure pushing up. If vapor pressure is less than Patm, the molecule remains in this equilibrium state. But, when we reach Patm, the substance boils. At lower Patm, there is less force to overcome and the substance boils more easily. Conversely, the higher the vapor pressure, the lower the boiling point.