What is the internal energy at constant pressure?
If you are working with an ideal gas mixture, for which the internal energy of the reactants and products is not a function of pressure, the change in internal energy in going from reactants to products at constant temperature and volume is the same as the change in internal energy in going from reactants to products at constant temperature and pressure.  With regard to the changes in enthalpy, the same is true.  If the number of moles of reactants and products is the same, then the change in enthalpy for each of these processes will be the same as the change in internal energy.  However, if the number of moles changes, the change in enthalpy will differ from the change in internal energy in proportion to the change in the number of moles.