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Enthalpy is a State Function

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    359804
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    Our expression for internal energy at constant pressure:

    \[\Delta U=q_P+w=q_P-P\Delta V \nonumber \]

    Rearrange:

    \[q_P=\Delta U+P\Delta V=U_2-U_1+P(V_2-V_1) \nonumber \] \[q_P=\left(U_2+PV_2\right)-\left(U_1+PV_1\right) \nonumber \]

    We can define this term as enthalpy:

    \[H\equiv U+PV \nonumber \]

    This is a new state function.


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