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First Law of Thermodynamics

  1. Feb 18, 2010 #1
    Concerning the first law of thermodynamics:

    dE = q - w
    dE: change in internal energy of a system
    q: heat added/removed from system
    w: work on/by the system

    What is the difference between q (heat added/removed from the system) and H (enthalpy, or heat flow)? Are they analogous to each other?
  2. jcsd
  3. Feb 18, 2010 #2
    First off, enthalpy is a state quantity and heat is not. For instance, you can talk about how much enthalpy a system has, but you can't talk about how much heat it has.

    You could, however, compare a change in enthalpy to the heat added to a system. Enthalpy is defined as:
    [tex]H = U + PV[/tex]
    Where P is the pressure of the environment and V is the volume of the system. If you have a change in enthalpy at constant pressure, then:
    [tex]{\Delta}H = {\Delta}U + P{\Delta}V= Q-W+P{\Delta}V[/tex] (using the convention you chose where W is the work done by the system.

    As long as the only work you do on the gas is compression/expansion, [tex]W=P{\Delta}V[/tex] so the change in enthalpy is equal to the heat added to the system. So in a sense enthalpy is a way of letting you ignore compression-expansion work during a given process.
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