Relationship between ΔH and ΔU for chemical reactions

At constant pressure, $\Delta H$ and $\Delta U$ are related as

i. For reactions involving solids and liquids, $\Delta V$ usually is very small (solids or liquids do not show a volume change with a change of pressure), and $\Delta H = \Delta U$.

ii. For reactions involving gases, $\Delta V$ cannot be neglected:

where $V_1$ is the volume of the gas-phase reactants and $V_2$ that of the gaseous products.

We assume the reactants and products behave ideally. Applying the ideal gas equation, $PV = nRT$: when $n_1$ moles of gaseous reactants produce $n_2$ moles of gaseous products, the ideal gas equation gives

Substitution of Eq. (4.26) into Eq. (4.25) yields …