Vapor Pressure

[sunshine02]

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Why does vapor pressure not depend on external pressure? I thought it did, but TPR says that it does not.

 

[Czarcasm]

Why does vapor pressure not depend on external pressure? I thought it did, but TPR says that it does not.

The vapor pressure of a solution is primarily dependent on the intermolecular forces within a given solution. The stronger and increased amount of intermolecular interactions, the less likely those molecules are able to 'escape' the liquid phase as free gas molecules. In some instances, we can effect the vapor pressure by introducing solutes such as a salt. Introducing salt to a pure liquid (a colligative property) will increase it's boiling point; this is because the salt increases the entropy of the molecules within the liquid and therefore reduces their tendancy or willingness to escape into the gas phase. We could also influence vapor pressure by increasing heat or temperature; the reason for this is the heat disrupts the intermolecular forces; molecules begin moving more rapidly and thus have enough energy to escape more frequently. All these are the only factors that influence vapor pressure: 1. the intrinsic strength of the intermolecular forces for the molecule itself 2. temperature 3. colligative properties. Because external pressure does not influence any of the properties described above, it does not effect vapor pressure.

 

[pepocho]

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[sunshine02]

adding onto Czarcasm,
The independence of vapor pressure from external pressure is also why a large pot of water and a tiny test tube has the same vapor pressure.

Also, elaborating on properties of solution. The reason salt tends to increase boiling point and decrease freezing point is because the ions in the salt created more powerful bond with the solute (ion-solute bonds tend to be stronger than solute-solute bonds) making it harder for the solution to boil or freeze.

Thank you both for the helpful replies!

But say you decrease external pressure--wouldn't that decrease boiling point and as a result increase the vapor pressure?

 

[pepocho]

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[orangetea]

Okay so I hate vapor pressure it ruined my life for a few days.

But can some one correct me if I am wrong.

Vapor pressure is a colligative property that depends ONLY on solution temperature and intermolecular forces. But atmospheric pressure can have an INDIRECT effect on it through boiling point. Life for example in high altitudes if you lower atm pressure you lowered boiling point which would then increase vapor pressure?

😕

 

[sillyjoe]

Okay so I hate vapor pressure it ruined my life for a few days.

But can some one correct me if I am wrong.

Vapor pressure is a colligative property that depends ONLY on solution temperature and intermolecular forces. But atmospheric pressure can have an INDIRECT effect on it through boiling point. Life for example in high altitudes if you lower atm pressure you lowered boiling point which would then increase vapor pressure?

😕

I posted this on another thread:

Vapor pressure is an intrinsic property of the liquid that can only be adjusted by the temperature.

Whether a molecule is in the liquid or gas phase is dependent on the environment it is in. Lets say Compound X has a vapor pressure of 150 torr at 300 kelvin and that we put compound X into a chamber that is at 760 torr (1 atm) at constant temperature. We would assume that Compound X is in the liquid phase since Pressure(surroundings) > Vapor pressure of Compound X (760 > 150)

However, if we lower the pressure to 130 torr we would expect Compound X to be present as a gas. Pressure(surroundings) < Vapor pressure of Compound X. The vapor pressure is still 150 since the temperature is constant, but it is now greater than the surroundings so it changes phase.

Alternatively, lets say we put Compound X into a chamber allowing for a constant pressure of 300 torr. We would expect Compound X to have a vapor pressure of 150 and therefore be a liquid. However, if he heat Compound X up we can increase Compound X's vapor pressure to exceed the 300 torr of the chamber allowing for Compound X to vaporize into a gas.

I know I am beating a dead horse here...

 

[Czarcasm]

Thank you both for the helpful replies!

But say you decrease external pressure--wouldn't that decrease boiling point and as a result increase the vapor pressure?

If anything, reducing external pressure would allow for a smaller amount of vapor pressure for boiling to occur. Recall again, that boiling occurs when vapor pressure = external pressure. Regardless of how you alter the pressure, you are not physically effecting the molecular energies or inter molecular interactions between the molecules themselves.

I realize exactly what you are saying and this is something I struggle explaining personally. For instance, if you applied some external pressure to a liquid in a container that has vapor above it, some of those vapor (gas) molecules would be forced back into the liquid. Technically, you could argue that the vapor pressure is reduced here due to the external pressure (it just isn't though). That's where things get really iffy for me and so perhaps someone could explain this for you. From experience though, understanding that pressure does not effect vapor pressure has been enough to answer every relevant question I've encountered on that particular concept.