Order of reaction:- Sum of powers to which the concentration terms are raised in rate law expression

Order of reaction = ∝ + β,

            Examples                                            

If 0 → Zero order reaction

1 → Ist order reaction    

2 → Second Order Reaction     

1. Zero order reaction:- Rate is independent of concentration

Rate = K [NH₃]^°                                O.R = 0

Rate = K

2. First order reaction:- Rate depends only upon 1 concentration term.

2 N₂O₅ → 4 NO + 6 O₂

Rate = K [N₂O₅]¹                              O.R = 1

3. Second order reaction:- Rate depends upon 2 concentration terms.

2 NO₂ + F₂ → 2 NO₂F

Rate = K [NO₂]¹ [F₂]¹                      O.R = 1 + 1 = 2

4. Third order reaction:- Rate depends upon 3 concentration terms.

2 NO + O₂ → 2 NO₂

Rate = K [NO₂]² [O₂]¹                     O.R = 2 + 1 = 3

5. Fractional order reaction:- Rate depends upon fractions of concentration terms.

CH₃CHO   CH₄ + CO

Rate = K [CH₃CHO]^1.5                     O.R = 1.5

Units of rate constant [K] for 0, 1, 2, 3 order reactions

Rate = K ([Conc] / Time) = Mol L^-1 Sec^-1

For zero order:- Rate = K [A] ^°

Rate = K = mol lit^-1 sec^-1  

For I^st order:- Rate = K [A]¹

For II^nd order:- Rate = K [A]²

For III^rd order reaction:- Rate = K [A]³

 = mol^-2 lit² sec^-1

Question:- What is the order of reaction for

Rate = k [NO^-₂] [I^-] [H⁺]²

Answer:- If concentration of H⁺ is doubled → 4 times

If concentration of NO^-₂ is Halved → ½ time

If concentration of NO^-₂, I^- and H⁺ are tripled → 81 times

3 [NO^-₂]¹ 3 [I^-]¹ 3 [H⁺]² = 3¹ x 3¹ x 3² = 81 

Question:- If rate of reaction becomes 27 times on increasing the concentration of reactant 3 times, what is the order of reaction.

Answer:- order of reaction is 3                                     flight → 3 [A]³→^O.R

                                                                                    3³ = 3 x 3 x 3 = 27 times  

Question:- What is the effect on rate of reaction, if volume of container is halved rate = K [A]²

Answer:- Rate of reaction is 4 times because concentration ∝ (1/volume)

In Case of gas, units are replaced by atm of place of mol lit^-1. e.g.→ mol lit^-1 sec^-1 = atm sec^-1    

Question:- Calculate the units of K for rate = k [CH³CHO]^3/2