- #1

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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!