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Easy definition of enthalpy

  1. Mar 17, 2007 #1
    1. The problem statement, all variables and given/known data
    I am having a bit of trouble grasping the idea and definition of enthalpy. I was wondering if someone could help.

    Am I correct in saying that a change in enthalpy only occurs if there is no temperature change? i.e. during a phase change/chemical change at constant temp?

    Thus since there is no change in temperature, there is no change in kinetic energy. The change in enthalpy is equivalent to a change in potential energy for the molecules involved as they separate/bond.

    For some reason I was confused because I thought that enthalpy was "the total kinetic and potential energy of the system" as defined in my textbook, but if temperature cannot change, by definition kinetic energy cannot change right?

    Can someone let me know whether or not my assumptions about temperature change are correct?
  2. jcsd
  3. Mar 17, 2007 #2
    Let me expand my explanation a little. When calculating the energy change of a system, we use the forumla q=mc[tex]\Delta[/tex]t if there are no phase changes, which makes perfect sense. We only use the formula [tex]\Delta[/tex]H=nH where [tex]\Delta[/tex](H is the change in enthalpy and H is the molar enthalpy value for whatever phase change, and n is the amount of substance in molar units) during a phase change. It seems that a change in enthalpy is a change in potential energy, since the temp does not change in the calculation, but then my textbook says its the total value of the potential and kinetic energy of the molecules in the substance, so I am confused.
    Last edited: Mar 17, 2007
  4. Mar 19, 2007 #3


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    Enthalpy is heat. If heat can change temperature, then a temperature change is observed. If heat is used to change the phase of a material, temperature will not change.
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