Order Of A Reaction

ORDER OF A REACTION The dependence of the reaction rates on concentration may be expressed in terms of order of a reaction. For calculating order of a reaction we must have to do experiments to find the dependence of a reaction rate on the concentration of each individual reactant at a given temperature. The order of a reaction can be defined as: the sum of the powers to which the concentration terms are raised in the rate law expression. Take a general equation aA +bB +cC → products Rate law is given by Rate= -dx/dt = k [A]p [B]q [C]r Then, the order of the reaction, n, is: n = p + q + r Where p,q and r are the order with respect to individual reactants and overall order of a reaction is sum of these components, i.e.,p+q+r. When = 1, the reaction is first order, if 2 the reaction is said to be second order reaction and so on. Some examples of reaction of different orders a) Reactions of first order i) Decomposition of nitrogen penta-oxide (N2O5) N2O5 (g) → 2NO2 (9) +1/2 O2 (g) Rate = k [N2O5] ii) Decomposition of ammonium nitrite in aqueous solution NH4NO2 →

N2 +2H2O

Rate = k [NH4 NO2]

iii) Decomposition of H2O2 in the presence of I- ions

H2O2 → H2O + ½ O2 Rate = k [H2O2]

b) Reactions second order of i) Decomposition of nitrogen peroxide 2NO2 → 2 NO +O2 Rate =k [NO] 2 Order of reaction=2 ii) Reaction between H2 and I2to give HI H2+I2

2 HI

Rate = k [H2][I2] Order of reaction=2 c) Reactions of Third order Reactions i) Reaction between nitric oxide and oxygen 2NO + O2 → 2NO2 Rate =k [NO] 2 [O2] Order of reaction=2 ii) Reaction between nitric oxide and Cl2

2NO + Cl2 → 2NOCl Rate = k [NO] 2 [Cl2] Order of reaction =2 d) Reactions of fractional order i) Decomposition of acetaldehyde is a fractional order reaction ii) CH3CHO → CH4 + CO Rate =k [CH3CHO] 3/2, Order = 3/2 or 1.5 ii) The reaction between hydrogen and bromine to form hydrogen bromide is fractional order reaction. H2

(g)

+ Br2

(g)

→2HBr

Rate = k [H2][Br2] ½ Order of reaction =1+1/2 =3/2 e) Zero order reaction A zero order reaction is that in which rate of a reaction is independent of the concentration of the reactant species. i) Decomposition of ammonia is zero order reaction 2NH3 (g) → (g) N2 +3 H2 It has been observed that the rate of a reaction is independent of the Concentration of ammonia, i.e.,

Rate = -dx/dt = k [NH3] 0 or rate = k Order of such type of reactions is =0