For the reaction 2 A+B→3C+D. The reaction rate is written as follows: The reaction rate w.r.t. A=−21dtd[ A] The reaction rate w.r.t. B=−dtd[ B] The reaction rate w.r.t. C=+31dtd[C] The reaction rate w.r.t. D=+dtd[D] Related Theory During the course of the reaction shown below, reactants A and B are consumed while the concentration of product AB increases. The reaction rate can be determined by measuring how fast the concentration of A or B decreases, or by how fast the concentration of AB increases. 
Caution The reaction rate is dependent on the concentration of the reactants as well as the rate constant as well as other factors include temperature and catalysts.