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.