Vapor pressure as a function of...? 2

[tsurikov]

Hello all,

I'm having a hard time understanding something about the concept of vapor pressure. I've heard everywhere that VP is a function of temperature only. So let's say I have a liquid in a container and I know its vapor pressure plot - VP versus temperature. Is this info relative to any specific PRESSURE? Let's say I have VP = 40 mbar, and the container is at 1 bar. Now I raise the pressure in my container to 5 bar - same temperature. Is the VP still 40 mbar, or has it also gone up by a factor of 5? In other words, is VP data universal, or is it always relative to 1 bar?

I feel this is a simple question, but I can't convince myself one way or another... This isn't my field, so I appreciate your patience! Thanks for any advice!

 

[phex]

vapor pressure is a property of the substance, and - as you correctly mentioned - just a function of temperature.
a short explanation.

the vapor pressure of water at 100 degC is, as we all know, about 1 atm. thats why water changes its state of aggregation from liquid to vapor at 100 degC and 1 atm (it is boiling).

so vapor pressure has really nothing to do with the ambient pressure. if ambient pressure is higher than vapor prerssure, the liquid doesn't boil, if ambient and vapor pressure are equal, it boils, and if ambient pressure is lower than vapor pressure, you will have a hard time keeping it liquid.

pressure is a state variable, vapor pressure is a material property.

hope this rambling around i produced does help you a little =)
chris

 

[25362]

Allow me to amplify (and possibly complicate?) a bit phex's explanation:

1. Vapour pressure does not depend on the amount of liquid present.
2. Vapour pressures are also exerted by solids. For example, p-dichlorobenzene, a component of mothballs, sublimes and creates a vapour pressure of 0.4 Torr at 20 deg. C.
3. As an ongoing process in dynamic equilibrium (a condition in which a forward process and its reverse occur simultaneously at equal rates) at a given temperature, the vapour above a liquid is condensing as fast as the liquid is vaporizing. The concentration of the molecules in the vapour, and hence its pressure, remains constant and we say that the vapour and liquid are in dynamic equilibrium.
4. This has nothing to do with the pressures developed by a packed (no vapour space) liquid when heated. These pressures depend on the liquid's cubic thermal expansion and isothermal compressibility.
5. Vapour pressures are low when the intermolecular attracting forces in the liquid are strong. This characteristic is shown strikingly by dimethyl ether and ethanol. Both have the same molecular formula C₂H₆O. However, ethanol molecules have an -OH group and can link together by forming hydrogen bonds. Ether molecules cannot. As a result ethanol is a liquid at room temperature whereas dimethyl ether is a gas.
6. The normal boiling point of a liquid is the temperature at which its vapour pressure is equal to 1 atm.
7. As phex clearly says, boiling temperature is higher when the external pressure is increased as in a pressure cooker, and it is lower when the external pressure is lower. Water boils at 71 deg C at the top of Mt. Everest where the pressure is only 240 Torr.

 

[TD2K]

As Prex has said, vapor pressure is a property of the fluid in question. Change its composition and its vapor pressure for the same temperature will change. Change its temperature and the vapor pressure will change.

Tsurikov, for your example, if you ONLY had the liquid in question inside the vessel, the pressure would read 40 mbar (let's assume here that all pressures are absolute to keep things simple). IF the pressure is 1 bar, that means that the part of the total pressure comes from the liquid and the rest from some other material to give a total pressure of 1 bar.

The pressures are additive in the sense that if you had the vessel originally containing only the liquid, the pressure in the vessel would be 40 mbar (the liquid's vapor pressure). If you added another gas to the vessel to bring the total pressure up to 1 bar, you'd have the vapor pressure of the liquid of 40 mbar plus a gas pressure of 960 mbar. If you increased the total pressure using the same gas to 5 bar, you'd have the same vapor pressure, 40 mbar, plug 4960 mbar from the gas. Now, if you increased the pressure to 5 bar from 1 bar with another gas, you'd have 40 mbar of the total pressure coming from the liquid, 960 mbar from the first gas and 4 bar from the second gas.

Next class will be on the vapor pressure of miscible and non-miscible liquids given by Professor Quark

 

[phex]

professor, professor, i need to go to the toilet, please wait for me

ah, we're much to sober here, sometimes. just to bring a little laugh ;p

chris

 

[tsurikov]

Thanks, everybody - those are great explanations. Just what I needed! I really appreciate it!

Cheers,
--M. Tsurikov

 

[quark]

Am I being pulled at my leg? (edit)LMAO(edit):-(

Note: TD2K! If I was not subjected in your post, just be careful. I copyrighted my handle, lol

By the way what is vapor pressure? lol

Best Regards,

 

[chela]

I would like to add my 2 cents to the discussion. Normally vapour pressures are expressed in absolute pressure units( though nobody speifically mentions this in a Lab report, Or in a discussion for example) and Actual field pressure measurements are in gauge pressures.This has led many times to serious confusion regarding the nature of product whose temperature and saturation vapour pressures are measured in the field but did not match with the known(??) vapour pressure, which invariably was assumed to be in gauge pressure.Imagine the sorry plight of the Pump engineer who invariably adds the atmospheric pressure to the liquid's vapour pressure in NPSH calculations!

 

[Flareman]

tsurikov
If you can get your hands on a copy of Chemical Engineer's Handbook (mcGraw Hill : Perry and Chilton) there's a section in Chapter 3 dealing with vapor pressure prediction if you have any known values.
Otherwise, look up Antoine's Equation on the web. there are several links at Google et al.

David