A pair of forward and reverse reactions may occur simultaneously with comparable speeds. For example, A and B react into products P and Q and vice versa (a, b, p, and q are the stoichiometric coefficients):
The reaction rate expression for the above reactions (assuming each one is elementary) can be written as:
where: k is the rate coefficient for the reaction that consumes A and B; k is the rate coefficient for the backwards reaction, which consumes P and Q and produces A and B.
The constants k and k are related to the equilibrium coefficient for the reaction (K) by the following relationship (set v=0 in balance):