How does evaporation work at the molecular level?

[Char. Limit]

Thus causing evaporation.

 

[Char. Limit]

If you mean having a reference on supersonic speeds... well, use the equation for the average (root-mean-square) speed of a water molecule:

$$\nu_{rms}=\sqrt{\frac{3kT}{m}}$$

m is the mass of one water molecule, in g, I think.
T is the temperature of the water molecule, in K.
k is Boltzmann's constant, which I don't have time to look up.

The left side is your molecular speed, measured in m/s. For reference, sound moves in room temperature air at 330 m/s.

 

[andrewbb]

What specifically is vibrating in the atom/molecule? As temperature increases, is the electron shell expanding? If so, then the electrical negativity charge on the atom would increase due to the proton(s) being of less influence. This would cause greater separation of the atoms/molecules at higher temperatures. Therefore the strength of electromagnetic charge is a function of temperature. Now, THAT makes sense in understanding thermal motion.

 

[andrewbb]

If you mean having a reference on supersonic speeds... well, use the equation for the average (root-mean-square) speed of a water molecule:

$$\nu_{rms}=\sqrt{\frac{3kT}{m}}$$

m is the mass of one water molecule, in g, I think.
T is the temperature of the water molecule, in K.
k is Boltzmann's constant, which I don't have time to look up.

The left side is your molecular speed, measured in m/s. For reference, sound moves in room temperature air at 330 m/s.

I think this is a misapplication of that equation. It applies to gases and I'm not sure it means actual velocity as much as potential velocity assuming nothing is in its path.

 

[andrewbb]

Interesting facts about evaporation of water in different fluids:

Butane evaporates water fastest
Helium evaporates water slowest

Butane's shape is a bunch of H's surrounding a few C's, leaving the H's free to attract O in water molecules. For those who still say I'm talking theory, take a look at "Lewis structures".

 

[DaveC426913]

This isn't hypothesizing. This is thinking about the shape and properties of the water molecules and what is occurring as two or more water molecules collide.

No, it is guessing. You do not know what happpens, so you are making it up.


What you are failing to grasp is that liquids comprised of polar molecules and liquids comprised of non-polar molecules behave essentially the same.

 

[Dale]

What is the cause of that motion?

Conservation of energy. Since the temperature is not absolute zero the atoms start out with some initial motion, then by conservation of energy the only way for one molecule to lose KE is to transfer it to another molecule in a collision. This then causes a spread, or distribution, of the number of molecules traveling at any particular speed, but the average energy remains constant. Billions of these collisions happen and, in the steady state, the distribution of the kinetic energies of the individual molecules is given by the Boltzmann distribution.

I'm suggesting it is the electromagnetic charges repelling and attracting each other. Along with shape of the atom/molecule, it creates a rather random spread of velocities and trajectories.

That is a nice suggestion, but the evidence does not back it up. The Boltzmann distribution can be measured in monatomic gasses which have neither a dipole charge nor a non-spherical shape, therefore charge and shape cannot be the reason for the observed distribution of speeds.

 

[Char. Limit]

Butane is a non-polar molecule. Here's another piece of info about butane:

There are two forms of butane, called n-butane, which is "slightly miscible" with water, and 2-methylpropane AKA isobutane, which is "immiscible" with water.

In other words, neither interacts with water much. A good thing to remember is that polar interacts with polar, and non-polar interacts with non-polar.

In other words, butane, which is non-polar (dipole moment = 0 D), won't interact with water, which is polar (dipole moment = 1.85 D).

Sorry.

 

[sponsoredwalk]

Intersting thread, I'm glad you keep asking questions but I think you don't understand some of the factors that are involved.

That explanation of evaporation is good, but doesn't describe the detail at the molecular level.

Water molecules are polar which means they are basically tiny magnets. The two hydrogen atoms sit on one side of the oxygen atom creating a positive charge on one side and negative on the other.

For a single molecule to evaporate it must overcome the cohesive force of the water (hydrogen bond). Hydrogen bonds are essentially magnetism. (The hydrogen is electromagnetically attracted to the oxygen in another molecule.)

You must misunderstand hydrogen bonding.

Oxegen atoms have a high electronegativity with a positive charge coming from it's 8 protons. When it forms a covalent bond with two Hydrogen atoms to form a water molecule, the attraction of all those protons "pulls" the electrons from the Hydrogen close towards it, resulting in a slight negative charge at this end of the water molecule. Conversely, the two Hydrogen molecules respective electrons are slightly displaced away from the single proton that "holds" them resulting in each H atom gaining a slight net positive charge.

This is the basis for Hydrogen bonding, the slightly positive hyrogen of one water molecule is attracted to the negative Oxygen of a neighbouring molecule. However, this hydrogen bond is about one-tenth as strong as a covalent bond (source: https://www.amazon.com/dp/0716798565/?tag=pfamazon01-20 Chap. 2&3). Also, the water molecules are continually breaking and re-forming hydrogen bonds due to their inherent motion. At anyone time a water molecule has around 3-4 hydrogen bonds formed but these quickly dissipate & reform with other molecules. This property is part of the reason why water has such a high boiling point, more heat energy is required to break these bonds.

While heat excites the molecules and churns them more vigorously, evaporation can happen in two ways:
1. two water molecules align so their oxygens repel from each other.
2. a water molecule hydrogen bonds to a molecule or atom in the air.

Does anyone have an argument to that way of looking at it? We are talking about the same thing, I am merely describing what happens to the individual water molecules. The temperature is a measurement of how vigorously the molecules are moving. If the molecules are moving around more vigorously, their hydrogen bonds are more likely to be broken which allows them to escape into the air.

Again, you assume that hydrogen bonds are like crazy magnetized forces. They are relatively weak but the huge accumulation of them is what gives water such a high boiling point. The molecular motion is too fast to sustain such bonds due to the kinetic energy of motion they have. Also, if you re-read that statement about molecules choosing the shortest path you'll see that most molecules will not interact in a mickey mouse shape and even if they do the natural repulsion of two molecules rebounding after "collision" would barely be influenced compared to the kinetic speeds.

When water freezes it is due to the molecules kinetic energy slowing to the point where more than the standard 3-4 hydrogen bonds can form, but they still break & re-form. To assume that's false is to say that atoms become stagnant when water freezes & that's wrong for so many reasons...

Water molecules are kind of small. Their motion is random, and as you get more and more of them (your water glass holds probably close to 10^25 water molecules) their random motion cancels itself out and tends to 0. In short, each water molecule is moving really fast, but they are all moving in random directions and cancel each other out, motion-wise.

This is the essence of Brownian Motion, [Read Einsteins paper on Brownian motion for more information - but this paper/reasoning is held as a proof for the existence of atoms], there are displacements on either side of a molecule at any particular time and this is "empty" space which is room for another molecule to fly into with kinetic energy to bounce another molecule. All throughout a glass of water they would roughly cancel out but at the top those molecules would fly out. There would be no water molecules of roughly equal kinetic energy to counteract this force. Again, it depends on environmental pressure & the kinetic energy of the particle, [I'll explain why environmental pressure is valid further down].

Furthermore, there wouldn't be as many hydrogen bonds forming with those molecules at the top of a glass of water. You can see that only the bottom half of the circumference of an imagined circular molecule is exposed to those water molecules underneath, the top half is exposed to the air molecules. Well, water molecules are the same.

idk the detail but let's assume half of the amount of hydrogen bonds are formed due to half of the surface area being exposed. Hydrogen bonding is still a weak force & the kinetic energy of even the odd "ice-water molecules" is enough to overcome this. I'll repeat, it's the accumulation of hydrogen bonding throughout a glass of water that gives it it's "strength", individual ones are pretty weak.

On average, the molecules in a glass of water do not have enough heat energy to escape from the liquid, or else the liquid would turn into vapor quickly (see boiling point). When the molecules collide, they transfer energy to each other in varying degrees, based on how they collide. Sometimes the transfer is so one-sided for a molecule near the surface that it ends up with enough energy to escape.

Source: http://en.wikipedia.org/wiki/Evaporation

So, to be clear. If hydrogen bonds were such an important factor they would have to be stronger than they actually are & the fact that roughly half of the normal amount of Hydrogen bonds are capable of forming doesn't help.

Another really substantial claim in this thread which you didn't really address is why hydrophobic substances such as oil evaporate. How could hydrogen bonding explain how substances that share similar electronegativity to that of a water molecule (causing them to cluster when immersed in water & not interact with said water) evaporate due to hydrogen bonds when they simply do not form for these molecules?. You'll see below that atmospheric conditions (by these I mean, what's above the water - be it a vacuum or air) are important & contribute to all liquids that will evaporate.

I believe this is further proof to the molecular kinetic energy being the main factor. As Dave said earlier, Hydrogen bonds are merely a contributing factor... So these electromagnetic forces you keep mentioning are hydrogen bonds, well I hope you can put that idea to rest now. You also never mentioned anything like Van Der Walls forces, but I assume that's because you know they are far too weak to be of significance here.

The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid.[1][2] A liquid in a vacuum environment has a lower boiling point than when the liquid is at atmospheric pressure. A liquid in a high pressure environment has a higher boiling point than when the liquid is at atmospheric pressure. In other words, the boiling point of liquids varies with and depends upon the surrounding environmental pressure.

-----From http://en.wikipedia.org/wiki/Boiling_point

This quote from wikipedia gives insight to some of the claims made on this thread,

Air is irrelevant. You'll observe evaporation even in the vacuum.

&

1. Water is polar, yes. But for the most part, air isn't (99% of air is nonpolar N2 and O2). How can a water molecule attach to a floating air particle when there is nothing to support it?

2. A water molecule is lighter than air, yes. But the only polar component of air that I can think of is... other water molecules. And that's about 1 part in 300. The attached water and "air" molecule would be heavier than the molecular mass of air.

So, how could a weak polar bond form between the odd single water molecule in air and a water molecule in a glass moving with a good bit of energy? http://www.youtube.com/watch?v=zz4KbvF_X-0&feature=SeriesPlayList&p=166048DD75B05C0D This video gives you an idea of the energies the molecules have/need at specific temeratures. For a particle in the water to be "pulled" out by a single hydrogen bond with a H20 molecule in the air the force of attraction would have to be greater than the few Hydrogen bonds formed underneath this surface water molecule in the glass pulling it down, (if that's how evaporation occurred)...

I believe air pressure matters as the molecules of air above the glass would provide a repellant force down on these water molecules on the top of the glass flying up into them. Wouldn't that account for the temperature needing to be higher for these molecules to escape?

Wouldn't the water molecules require higher heat energy in order to gain a higher kinetic energy to overcome the downward force caused by the air above & the natural tendency for the water molecules to remain in the glass?

Doesn't that also show is why the temperature required for evaporation to occur in vacuo is lower?

What specifically is vibrating in the atom/molecule? As temperature increases, is the electron shell expanding? If so, then the electrical negativity charge on the atom would increase due to the proton(s) being of less influence. This would cause greater separation of the atoms/molecules at higher temperatures. Therefore the strength of electromagnetic charge is a function of temperature. Now, THAT makes sense in understanding thermal motion.

Do you understand electron shells, nevermind expanding ones...? If an electron shell were to expand wouldn't that mean the electrons would have to invent enough energy to overcome the attraction of the protons in it's nucleus, or to overcome the attraction of another atoms protons in a covalent bond? I'm not sure, hopefully somebody could answer that one for me...

Actually, the orientation is pretty much irrelvant. If two otherwise isolated water molecules collide with low enough kinetic energy to form a hydrogen bond then they will simply rotate into the minimum energy orientation regardless of their initial orientation. They are not constrained to always maintain a certain orientation.

Also, the dipole field of a water atom is electrostatic, not magnetic. If it were magnetic then water would jump onto a magnet.

That is a very interesting thing I didn't know, this minimum energy orientation thing reminds me of Snell's Law, the snippets I could unerstand from Introductory chapters on Lagrangian Dynamics & stuff Feynman wrote about in the sum over histories stuff (I vaguely recollect from Hawking book, or somewhere like that :tongue2:). Basically choosing the shortest & easiest path. I hope to find out more about this in the future.

This is the way I see the process, if I've made an error please try to correct me constructively, and with a source if possible.

 

[andrewbb]

Thanks for that as it accurately describes what we are discussing. I am aware and agree with all you said.

Can you describe what's happening to a single molecule when you say "kinetic energy"? I know its relationship to temperature, but what is physically happening to that molecule? If you say "vibrating", what specifically is vibrating and why does temperature make it vibrate?

 

[sponsoredwalk]

http://www.sumanasinc.com/webcontent/animations/content/propertiesofwater/water.html

Watch this animation. Specifically, when you see the ice structure and the atoms jiggling that is caused by the inherent kinetic energy of the molecules.

You understand what the word "Kinetic" means right? It means motion.

When you lift a football up off the ground, you are raising the footballs Potential Energy.

When you let go of the football it starts to fall down and increase in speed, releasing this Potential Energy in the form of Kinetic Energy, the energy of motion. Get a good introductory physics text to find out more of the details about it.

Here is a 30 minute lecture that you'd appreciate if you know a bit of math, but not essential to get the idea & enjoy the presentation.

http://video.google.com/videoplay?docid=-1865978801687874664#docid=-5313203621222966483

Also, the 14th lecture is worthwhile after for more or the math - good animations, (actually the whole 52 part series is worthwhile!).

Also, http://en.wikipedia.org/wiki/Temperature

Watch the two animations here and you'll see it perfectly. it is the exact same in water.
As for your last question, why does this temperature make the atoms move, you've phrazed that question wrong - at least in the way I understand this concept so far.

My understanding tells me that there is no such thing as a stationary atom, they are in perpetual motion. The temperature doesn't make the atoms move faster it merely records the fact that they are moving faster due to an increase in some form of energy.

I really can't be sure of this last part though & would appreciate somebody to clarify & expand upon that. Otherwise, I'll get back to you after all the thermodynamics texs I plan to study lol...

 

[andrewbb]

That's the thing. No one really has an explanation for WHY the atom is vibrating. They just call it "kinetic energy" and say it increases with temperature.


I think vibration of atoms is caused by increased electronegativity of the atoms/molecules. As temperature increases, the molecule's electronegativity increases. This weakens hydrogen bonds and repels molecules at a more vigorous rate. This seems obvious to me, but I have not seen anyone explain as such.

Now.. .why does the electronegativity increase with temperature? I think that's due to the proton's positive charges becoming of less effect. Possibly due to the electrons either becoming more active OR the electrons are moving away from the nucleus as temperature increases. I'd like to know the answer to this.

 

[DaveC426913]

No one really has an explanation for...

And you are sure of this because why?

I think...

You don't see a problem with this?

 

[Char. Limit]

I'm pretty sure, or so my Organic Chemistry textbook says, that what is vibrating in a molecule is the bonds themselves. They expand and contract, the angle of the bonds changes, and I think there's something else too.

In answer to your question to what vibrates in the molecule.