Qp and Enthalpy

[Shivani Mohapatra 1A]

Hi! When the pressure remains constant, we can replace qp with the change in enthalpy of the reaction in the equation to calculate internal energy. Could anyone explain why we can do this and the conceptual thinking behind it?

[Nhi Pham 3D]

We know that internal energy of a closed system can be changed by two things: heating/cooling the system and doing work on the system. This is reflected in the ΔU equation, which is the sum of the sum of the energy transferred by heating/cooling (q) and the energy transferred by compression/work (w) (ΔU = q + w).

Enthalpy is defined as the amount of heat (energy) that is exchanged/transferred at a constant pressure, so it is pretty much just q, but under conditions of constant pressure. This is why q can be substituted by ΔH when the pressure remains constant (ΔU = ΔH + w). Hope this helps!