For a nonelementary reaction, the reaction equation is described as the sum of all the steps involved. All these steps constitute the reaction mechanism. Each step in the mechanism is an elementary reaction. The rate law of the overall reaction involves the rate determining step (slowest step) in the reaction sequence.
Now look at the overall reaction 2NO(g) + O2(g) ---------> 2NO2(g)
The two steps in the mechanism are
2NO(g) --------->N2O2(g) (fast)
N2O2(g) +O2(g) -------> 2NO2(g) (slow)
Summing all the steps and cancelling out the intermediate N2O2(g), we obtain the reaction equation;
