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Thermochemistry (Enthalpy) queery!

  1. Jan 23, 2010 #1
    I know that: Change in Internal Energy = Heat change - (pressure X change in volume)

    and at constant volume PV is negligable.

    But i heard that ENTHALPY is like internal energy but at constant PRESSURE, so PV becomes negligable and enthalpy = heat change - (PV) PLUS (PV) = heat change.

    How come at constant pressure, we just use enthalpy (heat change)??

    I hope you understood the question, help is very much appreciated!!! thankyou!!!
  2. jcsd
  3. Jan 26, 2010 #2
    Enthalphy and internal energy are two similar yet different things. Enthalpy is defined as the sum of internal energy and work due to expansion, that si:
    H = U + pV
    We usually use enthalpy because it is easier to keep pressure constant than to keep volume constant. Its just a matter of convenience.
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