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Heat of Formation of Water Vapor is Exothermic??

Discussion in 'MCAT Discussions' started by GomerPyle, 08.14.12.

  1. GomerPyle

    GomerPyle

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    From the EK 1001 Chemistry (Q 375), it says that the heat of formation of water vapor is negative (and that it is less negative than the heat of formation of liquid water).

    I do not understand this one bit. I thought that when you are going from liquid to gas, you are putting in energy to weaken the bonds (thus an endothermic process), and an endothermic process means a positive heat of formation, not a negative heat of formation. Also, how would it be less negative than the heat of formation of liquid water (which I thought would be more negative (exothermic) since you are going from gas to liquid)...

    Can anybody help me understand this? From the back of the book, it also says that the heat of formation of water vapor is essentially the heat of combustion of hydrogen gas - a very exothermic process. How are those two even the same? Hellllllp
     
  2. PanRoasted

    PanRoasted

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    You have confused heat of formation for heat of fusion. Heat of formation is the change in enthalpy from forming a compound from its elements. Heat of fusion is the change in enthalpy in going from one state of matter to another. Combustion of hydrogen (meaning, reaction H2 with O2 and heat) is a very exothermic reaction, and is the reaction involved in forming water from its individual elements. If you form water vapor (meaning more of the energy is contained within the vapor rather than released into the environment), your heat of formation is less negative than if you formed liquid water (which contains less energy relative to water vapor).

    Also, wrong section of the forum.
     
  3. GomerPyle

    GomerPyle

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    So liquid water contains less energy, and because it is more exothermic than water vapor, it is more negative than the heat of formation of water vapor? Is this the proper way of looking at it?
     
  4. PanRoasted

    PanRoasted

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    I'm not sure if you have the right idea, your wording is a little vague. But it seems you have understood the general principle... I would personally say the proper way of looking at it would be in the way that I stated, because, well, it's how I understand it. If anyone else would like to chime in or offer a more obvious solution, feel free.
     
  5. GomerPyle

    GomerPyle

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    Okay, I completely understand your post.

    Liquid water has a more negative heat of formation than water vapor because it is more exothermic (more heat is released upon its formation and therefore it contains less energy than water vapor).
     
  6. GomerPyle

    GomerPyle

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    I assume I was thinking too far in depth when I was thinking about bond strength and applying energy. When is it that you think about the bond energy/strength?
     
  7. PanRoasted

    PanRoasted

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    Generally you only consider this if the identity of the two compounds you are comparing are different. In this case, only the phase differed, so bond strengths would be identical for most purposes.

    Also, I think you were considering the intermolecular interactions involved in going from liquid -> vapor, which are not necessarily bonds (I believe hydrogen bonds are only bonds in name). They do have consideration here, but yes, I think you were a bit too focused on the "bond" energies rather than the broader idea that liquid water holds less average kinetic energy than water vapor, in general.
     
  8. GomerPyle

    GomerPyle

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    Oh okay, gotchya. Thank you for the clarification. I appreciate it.
     

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