Hi! In most textbooks on chemical physics/thermodymanics it is said that under fixed pressure the heat of reaction equals change of enthalpy of the system since [itex]dU = \delta Q - p\cdot dV[/itex], and hence [itex]d(U+pV) = \delta Q[/itex]. But my question is: why they do not write a term [itex]+\mu\cdot dN[/itex] which describes changes in internal energy due to the change of the number of particles, which obviously changes in course of reaction ?! ([itex]\mu[/itex] denotes chemical potential of one of components). If I add it, I get [itex]dU = \delta Q - p \cdot dV + A\cdot d\xi[/itex] (where A stands for reaction affinity), and hence even for p = const: [itex]dH = \delta Q + A\cdot d\xi \neq \delta Q [/itex] ! I cann't believe that so many authors can be wrong. So, where is my mistake? Thank you in advance!