For the mass balances analogy, we have to deal with the rate of reaction (rA = rate of formation of A and if A is reactant, rA < 0). One thing important is to keep in mind that in general (there can be an equivalence in a special circumstance), because in the cases of CSTR and PFR but . That’s why it is defined in the literature where k is the rate coefficient or rate constant, came into formula by thermodynamic consideration (Arrhenius formula). By this way, we can write down the basis mass balance equations: