Heat of reaction

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!