H = U + PV. For reactions at constant T and P: $delta_H$ = $delta_U$ + delta(PV). For ideal gases: delta(PV) = $delta_{ngas}$ * RT. So $delta_H$ = $delta_U$ + $delta_{ngas}$ * RT, where $delta_{ngas}$ = moles of gaseous products - moles of gaseous reactants. If $delta_{ngas}$ = 0 (e.g., H2 + Cl2 -> 2HCl), $delta_H$ = $delta_U$. If $delta_{ngas}$ > 0, $delta_H$ > $delta_U$. If $delta_{ngas}$ < 0, $delta_H$ < $delta_U$. This relationship is frequently tested: given $delta_H$, find $delta_U$ (or vice versa). Always count only GASEOUS species for $delta_{ngas}$. Liquids and solids contribute negligibly to PV change.