The change in enthalpy (\Delta H) for a system is related to the change in internal energy (\Delta U) by the equation \Delta H = \Delta U + p\Delta V. For reactions involving gases, assuming ideal gas behaviour (pV = nRT), the change in volume at constant temperature and pressure is due to the change in the number of moles of gaseous substances. Therefore, p\Delta V can be replaced by \Delta ngRT, where \Delta ng is the difference between the number of moles of gaseous products and gaseous reactants. Substituting this gives the fundamental thermodynamic relationship: \Delta H = \Delta U + \Delta ngRT .