In a closed system, the change in internal energy (ΔU) is equal to the heat transfer (q) minus the work done on or by the system (w), according to the First Law of Thermodynamics: ΔU = q - w.
In a constant pressure process (such as in an open system or a system with negligible pressure changes), the change in enthalpy (ΔH) is equal to the heat transfer at constant pressure. Mathematically, ΔH = q.
If the reaction takes place in a closed system and under constant pressure, the work done (w) is typically zero because the pressure is constant and there is no expansion or compression. Therefore, in this case, the equation for the change in internal energy simplifies to: ΔU = q.
Since the only difference between ΔU and ΔH in this scenario is the notation, the change in internal energy in a closed system would be equal to the change in enthalpy in a constant pressure process.
To summarize, if the same reaction occurs in a closed system and in a constant pressure process, the change in internal energy in the closed system is equal to the change in enthalpy in a constant pressure process.