Order of chemical reaction 

Order of chemical reaction is called the sum of the exponents of the reactants concentrations indicated in the rate low.
  Examples:

     
The reaction 
2NO + O2    2NO2      with    
    
υ = k[NO]2[O2]      is third order  2+1=3     

     The reaction  CH3CHO    CH4 + CO    with   

     υ = k[CH3CHO]3/2     is 3/2 order.