Chemists say something like "amount of heat consumed for a chemical reaction equals the change in enthalpy" but I cannot understand why this is the case.
Here is my argument: Since $H = U +PV$, we have $dH = T dS + V dP + \sum_i \mu_i dN_i$. If we assume that the heat flow is quasistatic so that we can use $dQ=TdS$, and assuming that $P$ is constant during the reaction so that $dP=0$, we have $dH = dQ + \sum_i \mu_i dN_i$.
Apparently we have an additional term $\sum_i \mu_i dN_i$, so that $dH \neq dQ$.
Where am I wrong?