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Name: pat j bushong
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Question:
It is like this : Sublimation happens all the time (eg: snow, and frost) so what is the latent heat of sublimation? I know from the books that this is supposed to be a triple point deal, but if that e were so then snow and frost would be extremely special! it takes 80 c/g to turn ice into water and 540 c/g to turn water into steam, how many calories per gram does it take to turn water vapor into ice???? (as in frost and snow) this is driving us all mad here in the earth science/physics department - all from a students question!!! please HELP!!! thanks...



Replies:
You release energy in going from vapor to solid, right? So somewhere around 600 calories per gram is released, and it should be a very favorable reaction (especially at low temperatures of course!) The energy released is presumably quickly taken away by the air or whatever else is keeping the forming frost at that low temperature. I assume the actually energy difference depends strongly on temperature (the energy of the vapor is certainly more temperature dependent than the energy of the solid or liquid)...

Arthur Smith


Yours is really a chemistry question, but I will take a stab at it. There is something called Hess's law that says that a net enthalpy change does not depend upon the steps between the initial and final states. Enthalpy is a "state variable". The heat of sublimation is essentially the enthalpy change; it measures the amount of energy required to break up the ionic bonding in an ice crystal, and free up the molecules to make a gas. This energy should therefore be the same as the net energy required to go from solid -> liquid -> gas; the sum of the heats of fusion and vaporization. Sublimation occurs because you are below the triple point; the relevant pressure is the partial pressure of the H2O in the atmosphere. Thanks for the question. Let me know if I got the answer right.

Jack L. Uretsky



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