Order of Reaction
Order of Reaction
Let’s consider a reaction,            A   +   B   →     product The rate of reaction of above reaction is given by rate law expression, Rate              where k is constant called rate constant and is rate of reaction when concentration of all reactants is unit molarity (1m). Here m is order of reaction with respect to A, n is order of reaction w.r.t B and (m +n) is overall order of reaction. So, order of a reaction is the sum of the power to which the concentration of reactants is raised in the experimental rate law expression.  Unit of Rate Constant Let’s consider a reaction, A     →      product Rate = k [A]n  The unit of rate constant depends upon the order of reaction.    Order (n) Unit of rate constant 0 1 2 3 mole litre-1Sec-1 Sec-1 litre mole -1Sec-1 litre2 mole -2Sec-1 For a reaction, A    →  product The rate law expression is,  when n = 0 The reaction is said to be a zeroth order reaction and for such reaction the rate of reaction is independent of concentration of reactant. E.g. (i) Enzyme catalysed bio-chemical reaction.       (ii) Decomposition of HI on the surface of gold.       (iii) Reaction between H2 and Cl2 to give HCl in presence of light. When, n =1,            The reaction is said to be a first order reaction and for such reaction the change rate is equal to the change in concentration of reactant.  E.g. (i) Radioactive disintegration (decay)       (ii) Decomposition of H2O2            2H2O2              →          2H2O + O2       (iii) Decomposition of N2 O5            2N2O5             →          2NO2 + O2       (iv) Acid catalyzed hydrolysis of water   CH3COOCH3 + H2O   H+ →   CH3COOH    + CH3OH For 2nd order reaction, there can be two possibilities.            (i) 2A             →           product            Rate = k [A]2            (ii) A + B           →        product For their reaction we can have different possible rate laws.            Rate = k [A]1 [B] Or       Rate = k [A]2 [B]0 Or,      Rate = k [A]0 [B]2 E.g., (i) Base catalyzed hydrolysis of ester C2H5COOCH3 + NaOH           →                     C2H5COONa + CH3OH Methyl propanoate                                         Sudium propionate  (ii) Decomposition of NO2 to NO            2NO2               →           2NO + O2 (iii) Iodination of Acetone            CH3COOCH3 + I2     OH- →      CH2ICOCH3  + HI                                                                         Iodoacetone Â