TBR Phase changes example 7.10

[sps27]

---

Members don't see this ad.

Which of the following statements is TRUE about vapor pressure?

a) The vapor pressure of a liquid decreases with increasing temp
b) The vapor pressure of a liquid is always lower at higher elevations
c) The vapor pressure of a liquid is always higher at higher elevations
d) The vapor pressure of a liquid X is greater than the vapor pressure of a liquid Z, if the normal boiling point of X is lower than the normal boiling point of Z.

The ans given is d). In the explanation provided in TBR it says, """"the temp and enthalpy of vaporization has greater effect on the vapor pressure. A lower atm pressure lowers the boiling point, but it has no effect on the vapor pressure. This eliminates B and C. As the boiling point increases, the ability for a compound to vaporize decreases. This means that as the boiling point increases, less of the vapor is formed and consequently the vapor pressure decreases. Choice D is correct""""

Isn't that contradictory? A decrease in atm pressure should also decrease the vapor pressure if the boiling point goes down. b) is also a correct ans I think.

Here is another thread that talks about it. http://forums.studentdoctor.net/threads/tbr-g-chem-example-7-10-vapor-pressure.843662/

Should I stick with the understanding that vapor pressure also decreases with elevation?

 

[FeinMS]

Elevation has nothing to do with the vapor pressure. You're misunderstanding the concept of boiling point in terms of atmospheric and vapor pressure.

One of the many definitions of boiling point is "the temperature at which atmospheric pressure equals vapor pressure." If you go up high in a mountain, the atmospheric pressure goes down. However, the vapor pressure isn't affected. However, in high elevation, it is easier to boil water because boiling point is decreased. Why is the bp decreased? It's because atmospheric pressure decreased and it takes less increase in vapor pressure to reach the value of atmospheric pressure.

 

[sps27]

Elevation has nothing to do with the vapor pressure. You're misunderstanding the concept of boiling point in terms of atmospheric and vapor pressure.

One of the many definitions of boiling point is "the temperature at which atmospheric pressure equals vapor pressure." If you go up high in a mountain, the atmospheric pressure goes down. However, the vapor pressure isn't affected. However, in high elevation, it is easier to boil water because boiling point is decreased. Why is the bp decreased? It's because atmospheric pressure decreased and it takes less increase in vapor pressure to reach the value of atmospheric pressure.

If boiling happens when Vapor Pressure = Atm pressure, then the same definition does not seem to apply at higher altitudes. I don't know why. I think I just have to accept it coldly that vapor pressure does not change.

 

[FeinMS]

Why do you think the same definition does not seem to apply at higher altitudes? Let's say at sea level, atmospheric pressure is 1 atm. Then when vapor pressure reaches 1 atm, the liquid boils. Let's say at higher altitude, the atmospheric pressure must go down because of "thinning of air" so let's assume that it's 0.8 atm. Then the vapor pressure must reach 0.8atm to boil.

Since increase in temperature can increase vapor pressure, it takes less change in temperature to reach boiling point in higher alt.

 

[sps27]

Why do you think the same definition does not seem to apply at higher altitudes? Let's say at sea level, atmospheric pressure is 1 atm. Then when vapor pressure reaches 1 atm, the liquid boils. Let's say at higher altitude, the atmospheric pressure must go down because of "thinning of air" so let's assume that it's 0.8 atm. Then the vapor pressure must reach 0.8atm to boil.

Since increase in temperature can increase vapor pressure, it takes less change in temperature to reach boiling point in higher alt.

Correct, and that is why option b) is also a correct ans, is it not? Atm pressure goes down, vapor pressure goes down......

 

[phattestlewt]

I remember getting stuck on this question as well. I don't think you should be accepting anything "coldly."
Vapor pressure is an inherent property of a liquid and it's not going to change with altitude. Boiling point might change and that's because the atmospheric pressure changes. If you want to get really detailed you can say that because the atmospheric pressure is not putting as much pressure on the surface of the liquid, there might be a greater vapor (or a greater motion for particles in liquid to evaporate into a gas) but that change is negligible.

Answer D is clearly the best answer here because its most correct.

 

[FeinMS]

Correct, and that is why option b) is also a correct ans, is it not? Atm pressure goes down, vapor pressure goes down......

Vapor pressure does not actually go down. What goes down with altitude is "vapor pressure TO REACH BOILING POINT." Vapor pressure can go up or down depending on the temperature not altitude.

 

[sps27]

Thanks guys, I appreciate the trouble. I will ponder this, hopefully it will come through. Thanks.....