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As we know enthalpy is defined by the equation $H = E+PV$.

Now to calculate this $PV$ factor for $\Delta H$, Sometime we take $P \Delta V$ and sometime $P_2V_2-P_1V_1$. Why are there so many different approaches?

I am also confused whether $P$ is external pressure or internal pressure.

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  • $\begingroup$ Hello and welcome to Chemistry.SE! I suggest taking the short tour to better familiarize yourself with the site. Regarding you question, it seems a bit unclear what you are asking. I think that giving some examples regarding the different approaches you pointed out would clarify things a bit, and might even lead you to figuring out your question. Best of luck! $\endgroup$
    – airhuff
    Mar 5, 2017 at 18:23

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The derivation for $\Delta H$ is $$H = U + PV$$ $$\Delta H = \Delta U + \Delta (PV)$$

The $\Delta (PV)$ term expands to your $P_2 V_2 - P_1 V_1$ term. However, when the pressure is constant $P_2 = P_1 = P$, and $\Delta (PV)$ term becomes $P\Delta V$. The most common situation when the latter is true is when the system is open to the atmosphere and $P$ is the external pressure. In the former case, $P$ is more likely the internal pressure, which changes during the reaction.

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  • $\begingroup$ If a gas is compressed from state variable p1,v1,t1 to p2,v2,t2 under a constant pressure p3, how will you find change in enthalpy? $\endgroup$
    – user42148
    Mar 6, 2017 at 12:26

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