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Does Condensation of Water Vapor Constitute Heat Transfer?

  1. Jan 1, 2016 #1
    Scenario A: An icicle on the eaves of Uncle Ivan’s dacha in far off Verkhoyansk vaporizes a number of water vapor molecules at an ambient temperature of -80°C. Over the succeeding months, one of these molecules makes its random way to the surface of a cloud droplet over Ouagadougou. Along the way, it undergoes an average of some 109 molecular collisions and consequent changes in kinetic energy of translation per second. Once in Ouagadougou, it condenses onto that cloud droplet, thereby transferring its associated enthalpy of condensation to that surface. The ambient surface temperature of the droplet is some 25°C.

    Scenario B: Aunt Tillie’s teakettle in Tunbridge Wells comes to a rolling boil, producing many water vapor molecules at an ambient temperature of 100°C. Over the succeeding months, one of these molecules makes its merry way to Ouagadougou in its turn. During its journey, it undergoes a similar number of molecular collisions and changes of kinetic energy of translation as Uncle Ivan’s molecule. And, like the latter molecule, it also condenses onto the same cloud droplet, thereby transferring its associated enthalpy of condensation to that same surface. Aunt Tillie’s molecule and Uncle Ivan’s molecule are of the identical isotopic construction and ionic condition. There is no known way of telling the two apart.

    Comment: It is my understanding that classical thermodynamics considers that the transfer of the enthalpy of condensation from Uncle Ivan’s molecule to the cloud droplet does not constitute heat transfer, because the transfer of energy goes from the colder (at its origin) to the warmer. Conversely, the transfer of the enthalpy of condensation from Aunt Tillie’s molecule to the cloud droplet does constitute heat transfer, because the transfer of energy goes from the warmer (at its origin) to the colder.

    Question 1: How then do we consider the transfer of enthalpy of condensation to be a form of heat transfer in its entirety (as in the atmospheric heat budget) when we don’t know the sources of the water vapor?

    Question 2: Let us postulate that the ambient humid air in Ouagadougou is at the same temperature as the surface of the cloud droplet. Net condensation will still take place as long as the ambient vapor pressure exceeds the equilibrium vapor pressure. Since there is no difference in temperature, is this still heat transfer?
     
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  3. Jan 1, 2016 #2

    BvU

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    Hi,
    1. No need to know where the molecules come from in heat transfer. Only changes in averages count. And in general averages over the order of NA molecules, give or take a dozen orders of magnitude.

    2. Yes. Energy is released.

    You may want to recalculate the number of collisions a water molecule makes in a time period of months.
    :smile:
     
  4. Jan 1, 2016 #3
    BvU stated:

    "1. No need to know where the molecules come from in heat transfer. Only changes in averages count. And in general averages over the order of NA molecules, give or take a dozen orders of magnitude."

    That is not my understanding. Heat transfer can take place between quantities of matter having far fewer molecules than Avogadro's Number. It is only necessary (in the classical sense) that the source be warmer than the sink.

    "2. Yes. Energy is released."

    I know energy is released. I said as much. However, not all transfers of thermal energy are classified as heat transfers. Only those from warmer to cooler. In question 2, there is no warmer substance and no cooler substance.

    This is why I prefer the Engineering definition of heat that ignores the temperature of the sink. It is closer to the common sense definition as well. My Climatology students were much more comfortable with this approach. They never really understood the classical thermodynamic concept.
     
  5. Jan 1, 2016 #4

    Bystander

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    Temperature is NOT a conserved property from phase change to phase change.
     
  6. Jan 2, 2016 #5
    Bystander said:

    "Temperature is NOT a conserved property from phase change to phase change"

    Your statement is indisputable. But, then, no one is disputing it. What's your point?
     
  7. Jan 2, 2016 #6
    BvU said:

    "You may want to recalculate the number of collisions a water molecule makes in a time period of months."

    I cannot recalculate it as I never calculated it in the first place. I left it as an indefinite quantity.
     
  8. Jan 9, 2016 #7
    Since my OP's attempt at gentle humor got absolutely no direct response to my questions, I see that I shall have to be blunter. I maintain that the so-called "latent heat" of condensation is not heat in the sense that the term is used in thermodynamics.

    Firstly, this thermal energy does not necessarily travel from a warmer substance to a cooler one in keeping with the classical definition of heat. We don't know (and can't really know in the atmospheric sciences) what temperatures the vaporizing surfaces had at the various times of vaporization.

    Secondly, this transfer of thermal energy does not occur solely because of the difference in temperatures as required by the classical definition. It occurs because the ambient vapor pressure exceeds the equilibrium vapor pressure.

    Consequently, I maintain that the "latent heat of condensation" that is added to the atmosphere's "heat" budget by transfer from the Earth's surface is not heat. It is thermal energy, but it is not heat. In all fairness, I understand that the term "latent heat of condensation" has now been replaced by "enthalpy of condensation". "Latent heat" is no longer considered proper usage. (Pace Black.)

    Many of you will argue that this is a quibble over purely semantic issues, but every teacher has to be concerned with the proper definition of and usage of scientific terms. "Latent heat" is not heat!
     
  9. Jan 10, 2016 #8

    BvU

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    We're having fun here misunderstanding each others' perception of humour :smile: . In my case I try to make it easy for the reader by inserting a :smile: or some other thingy.

    I'm a physicist and to me heat has to do with motion of molecules. Speed of molecules condensing from vapour into liquid is reduced and the kinetic energy is redistributed over other molecules in the liquid. Heat transfer in my book. Bystander points out that if condensation takes place isothermically, something else has to be going on at the same time (cylinder coming down, heat removal :wink: or something).

    PS I'd appreciate the input of @Chestermiller
     
  10. Jan 10, 2016 #9

    jim hardy

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    Isn't heat the work available to a Carnot engine rejecting to absolute zero?

    Water molecules being polar will attract one another like magnets
    heat of vaporization i always envisioned as work to overcome that attraction and push them apart into gaseous state
    steam well away from saturation behaves not too differently from ideal gas
    but as you approach saturation it'll give up that heat of vaporization as the molecules snap back together to liquid state

    oversimplification , but it helps me work out problems ......
    i hope it's at least laughable :smile:
     
  11. Jan 10, 2016 #10
    Certainly in order to vapourize some liquid amount of water requires an amount of heat. Aunt Tilley's kettle or Uncle Ivan's icicle need an energy input from somewhere in order to keep on vapourizing. One can see that quite readily if the stove top element is turned off for the kettle. The water will stop boiling and less and less water molecules become steam.

    For the kettle the heat transfer by conduction from the element into the water goes into the latent heat of vapourization, forming steam. If he element is off, conduction around the side of the kettle into the water from the air supplies the heat so that water can turn into steam.

    The latent heat has to come from somewhere.
     
  12. Jan 10, 2016 #11

    BvU

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    I read up and begin to understand your concern. A lot of stuff in this area was sorted out by R. Leinonen (also this). Must admit that this 'confusion' has never done me any harm whatsoever...
     
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