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Homework Help: Enthalpy of saturated vapours?

  1. Nov 25, 2015 #1
    1. The problem statement, all variables and given/known data

    I have the ammonia liquid-vapour tables and I have noticed that the enthalpy of a saturated vapour increases as temperature increases. Can someone please help me to understand why this is the case?

    2. Relevant equations

    H = U + pV

    3. The attempt at a solution

    I assume that at higher temperatures the vapour will have a greater internal energy. Is this why enthalpy increases with temperature?

  2. jcsd
  3. Nov 25, 2015 #2
    It also has a higher PV, since PV=nRT. So, both U and PV are higher.

    Also, this is not the case at all temperatures. The enthalpy of the saturated liquid always increases with temperature, but the heat of vaporization decreases as the critical point is approached. So, at temperatures and pressures approaching the critical, the enthalpy of the saturated vapor actually decreases with temperature (i.e., along the saturation line).
    Last edited: Nov 25, 2015
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