Chemical Forums
Specialty Chemistry Forums => Materials and Nanochemistry forum => Topic started by: shvcko99 on May 31, 2022, 02:36:50 AM
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I have put down my schoolbag for many years but now I'd like to revise something.
I remember it's said electrons (or molecules? I forget) will lose from surface of liquid because molecular force at those area are weaker
Does this also happen to solid? Or only to liquid because molecular force between particles in solid are much stronger?
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One process is called evaporation, the other sublimation. Yes, both exist, although evaporation is much faster, and yes, that's because in solids intermolecular forces are stronger.
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One process is called evaporation, the other sublimation. Yes, both exist, although evaporation is much faster, and yes, that's because in solids intermolecular forces are stronger.
Does sublimation always exist in solid but just different materials/chemicals have different rate? Is it because of "surrounding" is an open area so that it cannot reach an equilibrium so that there must be always some atoms/molecules/particles escaping from the surface? Is it conceptually correct?
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Does sublimation always exist in solid but just different materials/chemicals have different rate?
Yes & yes.
Is it because of "surrounding" is an open area so that it cannot reach an equilibrium so that there must be always some atoms/molecules/particles escaping from the surface?
Not exactly. Liquids and solids evaporate all the time, no matter what. But at the same time reverse process (condensation/resublimation) occurs. At some point both processes have the same speed so the net effect is zero. To some extent that's the same behavior you describe, just the logic behind is a bit different.
This is best analyzed in terms of phase diagrams and (partial) pressures of the substances involved.
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Just to point out also that one thing that differentiates solids from liquids is the rate at which the surface is replenished with pristine material (atoms, molecules, whatever). In a solid, the atomic/molecular arrangement is more or less fixed over realistic timescales, whereas in liquids, surface molecules and non-surface molecules are constantly switching position. Liquid molecules are inherently more mobile, and other forces like convection can play a role in mixing. One effect of this is that surface chemistry plays a more important role in the behavior of solids than liquids - chemistry particularly between the solid material and reactive substances like oxygen in the atmosphere. So, with solids you may form thin overcoatings of oxides and sulfides that have very different properties from the underlying material. So while physical processes like evaporation surely occur
in and both solids in liquids, surface chemical transformations (and their effect on the rates of the physical processes) become more complicated and relevant in solids. (Chemistry of course also happens in liquids, but the constant mixing prevents buildup of surface layers. Also reactive substances like oxygen are more readily absorbed into the interior of liquids than the interior of solids.)
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Does sublimation always exist in solid but just different materials/chemicals have different rate?
Yes & yes.
Is it because of "surrounding" is an open area so that it cannot reach an equilibrium so that there must be always some atoms/molecules/particles escaping from the surface?
Not exactly. Liquids and solids evaporate all the time, no matter what. But at the same time reverse process (condensation/resublimation) occurs. At some point both processes have the same speed so the net effect is zero. To some extent that's the same behavior you describe, just the logic behind is a bit different.
This is best analyzed in terms of phase diagrams and (partial) pressures of the substances involved.
I understand that Liquids and solids evaporate all the time, no matter what, it is only because of equilibrium that cancel the net effect. But just wondering why does solid evaporate? I cannot remember exactly but as far as I remember forces between atoms/molecules/particles of solid are so strong that's why they are solid, how can the force from the surrounding strong enough to bring them out (even from the surface)??
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The basic principle is the same for liquids and solids (with plenty on interesting differences): you don't need external force to pull atoms/molecules out. They fluctuate all the time, and energy of each molecule changes all the time around some average value. Sometimes the atom/molecule oscillates/wiggles faster, sometimes slower, sometimes fast enough to jump away.
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I too had believed that the equilibrium vapour pressure of solids is smaller than liquids, since sublimation takes more energy than evaporation, the difference being the melting heat. BUT data speaks against that.
At the "triple point", vapour is at equilibrium with both the solid and the liquid, which means that the solid and the liquid have the same vapour pressure. More: the vapour pressure varies continuously with the temperature around the melting point. Have a look at the brightly colored "Phase diagram"
wikipedia (https://en.wikipedia.org/wiki/Water_(data_page)#Water/steam_equilibrium_properties)
the curve limiting the orange domain of vapour passes smoothly through 0°C.
Which is necessary after all. Ice can be in equilibrium with liquid water, and one can't extract energy from this equilibrium by turbining vapour that would flow from the liquid to the solid, not even temporarily.
Still unclear to me: why the slope of the vapour pressure shows no break at the melting point, since the heat of sublimation exceeds the heat of vaporization.