Help! I have some basic questions...

[mrman_421]

1. What effect does atmospheric pressure have on evaporation rates? (I know a higher pressure raises the evaporation rate, but I would like some formulas if possible)

 

2. What effect does atmospheric pressure have on sublimation points? (Specifically, in crystals? Again, a formula/equation would be very helpful)

 

Thanks for your help guys!

[5614]

I don't know the context in which you are asking but could using the equation

pV = nRt

help you in this case?

[mrman_421]

If you could clarify the variables, maybe...

 

p = pressure (in atmospheres)

V = volume (in ?)

n = ?

R = ?

T = Temperature?

[mrman_421]

actually, I did a little research and that's not the formula I was looking for... I was wondering about how air pressure affects the evaporation rate of different substances given a constant temperature and volume.

 

Also, a similar formula for sublimation points under the conditions mentioned above would be great too.

[5614]

I don't know one for exactly what you are looking for... just as enquired about pressure and you just said temperature/volume was constant, the equation seems to fit in nicely. You just have to play around with the pressure.

 

Does this:

http://www.newton.dep.anl.gov/askasci/eng99/eng99180.htm

help at all?

[timo]

If I remember that correctly, the ideal gas does not undergo any phase transitions. Therefore you can probably forget about the pV = nRT which 5614 presented you.

I´m really bad at thermodynamics so I´m probably not going to be much of a help, here. But one thing you might want to look in are "phase diagrams". They do not give you a rate of evaporisation (Q1) but at least the give you the points where the phase transitions occur (Q2). Another interesting thing to look up in this context is the "Clapeyron equation" which gives an approximation for the phase boundaries (hopre I remember that correctly ... I did mention that I´m bad at thermodynamics, did I?).

 

Just out of interest: Which crystals do sublimate ?

[Yggdrasil]

1. What effect does atmospheric pressure have on evaporation rates? (I know a higher pressure raises the evaporation rate, but I would like some formulas if possible)

 

I think higher pressure decreases evaporation rates. For example, water boils at a lower temperature at higher altitudes, meaning that it's easier for the water molecules to evaporate because of the lower pressures at high altitudes. Similarly, instead of using heat to promote evaporation (which can sometimes lead to decomposition of your product), chemists sometimes use low pressure to evaporate volitile compounds in a process called vacuum distilation.

 

I believe this is an effect of Le Chatalier's principle. At high pressure, the system will prefer the more dense form (i.e. liquid). This principle also explains why ice melts when pressure is applied; the more dense form of water (liquid) is favored over the less dense form (ice).

 

Similarly for sublimation, high pressures will favor the more dense form (solid) and low pressures will favor the less dense form (gas). Unfortunately, I know no equations for the rates of evaporation and sublimation.

[s pepperchin]

The atmospheric pressure would have an effect on phase changes and PV=nRT would be an equation you could use. If you look in books on thermodynamics you can usually find a three dimensional graph for a substance which relates P,V and T.