Order of in the mass action law is the coefficient which is raised to the active concentration of the reactants. It is experimentally determined and can be zero, positive negative or fractional.
The order of the whole reaction is the sum of the order of each reactant which is raised to its power in the rate law.
Thus,
For the reaction:-
2Mg+O₂→2MgO
Rate = k[Mg][O₂]²
Order w.r.t. Mg = 1
Order w.r.t. O₂ = 2
<u>So, order of the overall reaction = 1 + 2 = 3</u>
