# How do I evaluate the PV term when calculating a change in enthalpy?

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.

• 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! Mar 5, 2017 at 18:23

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.