Vapor Pressure...

[Kikaku21]

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So higher vapor pressure means faster evaporation right? Or do I have this backwards?

 

[DrCurious]

yup

 

[Kikaku21]

yup

Why?

 

[pfh]

i think that when the vapor pressure equals the atmospheric pressure a substance boils/evaporates. So if the vapor pressure is higher the substance will evaporate/boil faster

 

[DrCurious]

Also, VP increases as temperature increases, whihc makes sense, as one approaches the boiling point of a liquid it becomes easier to boil.

 

[Foolins]

You might think you have it backwards because a lower atmospheric pressure means easier evaporation as well. So yeah, you're right about the vapour pressure.

 

[87138]

You might think you have it backwards because a lower atmospheric pressure means easier evaporation as well. So yeah, you're right about the vapour pressure.

I don't believe it's entirely accurate to categorize evaporation and boiling as the same with regards to pressure characteristics.

At least according to Examkrackers:

On evaporation:
"Evaporation occurs when the partial pressure above a liquid is less than the liquid's vapor pressure, but the atmospheric pressure is greater than the vapor pressure. Under these conditions, the liquid evaporates rather than boils."


And according to one of the questions on their website, boiling occurs when the vapor pressure equals the local atmospheric pressure.


Anyone care to elaborate/clarify the real difference here?

 

[googlinggoogler]

I don't believe it's entirely accurate to categorize evaporation and boiling as the same with regards to pressure characteristics.

At least according to Examkrackers:

On evaporation:
"Evaporation occurs when the partial pressure above a liquid is less than the liquid's vapor pressure, but the atmospheric pressure is greater than the vapor pressure. Under these conditions, the liquid evaporates rather than boils."


And according to one of the questions on their website, boiling occurs when the vapor pressure equals the local atmospheric pressure.


Anyone care to elaborate/clarify the real difference here?

hmm, never thought about this that closely before..thanks for bringing it up. These websites might help a little..

http://www.newton.dep.anl.gov/askasci/chem03/chem03054.htm

http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/vappre.html

 

[Foolins]

Evaporation happens constantly, so that's kind of a strange definition. The partial pressure of what (should be lower than vapour pressure)? And the atmospheric pressure should be higher?

 

[googlinggoogler]

Evaporation happens constantly, so that's kind of a strange definition. The partial pressure of what (should be lower than vapour pressure)? And the atmospheric pressure should be higher?

I think when they said vapor pressure, they meant "saturated vapor pressure." So l-->g reaction goes forward if the partial pressure is lower than the saturated vapor pressure. Once it is saturated, it's equilibrium. And once the partial pressure equals the atmospheric pressure, it starts to boil.

 

[Foolins]

it happens MORE when a liquid boils....

 

[ThreeJ]

Evaporation does occur all the time but it is not soley focusing in on the vapor pressure. Within a fluid the fluid molecules contain a certain amount of kenetic energy so once the molecule contains enough kenetic energy that will overcome the surface tension of the fluid allowing that molecule will escape as in evaporation while leaving the fluid behinde at a cooler tmep. this smae method applies to how we cool down when we sweat.

But with vapor pressure as you heat a fluid you raise its vapor pressure because of an increase in kenetic energy of molecules to a point that is greater than the partial pressure above the fluid but lower than the atmospheric pressure above the fluid. So evaporation occurs while boiling does not. Think of a kettle as you heat it up evaporation occurs then it will boil. This is because of the gradual increase in the H2O vapor pressure toward atmospheric pressure.

 

[pezzang]

Evaporation does occur all the time but it is not soley focusing in on the vapor pressure. Within a fluid the fluid molecules contain a certain amount of kenetic energy so once the molecule contains enough kenetic energy that will overcome the surface tension of the fluid allowing that molecule will escape as in evaporation while leaving the fluid behinde at a cooler tmep. this smae method applies to how we cool down when we sweat.

But with vapor pressure as you heat a fluid you raise its vapor pressure because of an increase in kenetic energy of molecules to a point that is greater than the partial pressure above the fluid but lower than the atmospheric pressure above the fluid. So evaporation occurs while boiling does not. Think of a kettle as you heat it up evaporation occurs then it will boil. This is because of the gradual increase in the H2O vapor pressure toward atmospheric pressure.

Thanks, ThreeJ. What's the difference between the two bolded words above (which you wrote; partial pressure above the fluid vs atmospheric pressure above the fluid)?

Also, is evaporation the same as vaporization? How are they different?

Thanks!!! 🙂

 

[MahSpoon]

Also, is evaporation the same as vaporization? How are they different?

Thanks!!! 🙂

Evaporation is one form of vaporization, boiling is the other. The difference between evaporation and boiling is that by definition boiling is when the vapor pressure exerted by the liquid equals the pressure of the atmosphere it is in.

 

[TMP-SMX]

Just remember that temperature is average kinetic energy overall. There are a lot of high energy molecules on the surface that are constantly being vaporized into the gaseous state no matter what the temperature. That's why Ice will evaporate in a freezer... But very slowly. It's also why you can skate on ice. The outer molecules have a higher energy that interact with your skates. They are able to vibrate in a flexible state and therefore you glide with very little friction.

 

[ThreeJ]

Not to be pickey you are right that there is less friction between the skate and the ice but i think vibrational forces are less of a factor. See this is my reasoning, when we glide along the ice our weight allows for a higher pressure between the skate and the ice allowing a small minute amount of ice to melt. This as a result forms a bit of lubrication basically reducing friction to nothing. Unless you meant an increase in vibrational forces that allow the ice to melt then where the liquid would have kenetic energy allowing for us to glide along

 

[TMP-SMX]

Not to be pickey you are right that there is less friction between the skate and the ice but i think vibrational forces are less of a factor. See this is my reasoning, when we glide along the ice our weight allows for a higher pressure between the skate and the ice allowing a small minute amount of ice to melt. This as a result forms a bit of lubrication basically reducing friction to nothing. Unless you meant an increase in vibrational forces that allow the ice to melt then where the liquid would have kenetic energy allowing for us to glide along

That's not exactly correct either. The pressure from the skate is only enough to create a fraction of a degree change. (Not nearly enough to melt it). It's called surface melting, and it seems to be the theory that is currently held to make the most sense. The vibration on the surface/small amount of friction from the skate can melt the ice. So yeah... Doesn't entirely make sense because they really have no idea what is going on either.

http://www.princeton.edu/~lehmann/BadChemistry.html#Skating

Something I found on it. I also remember my high school chemistry teacher talking about it.