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XII 04 Kinetics VIII

(iv)Third order reactions of three concentration terms. molecules of the same reactant are involved, the rate expression is given as: A reaction is said to be of third order if its rate is determined by the variation When the cocnentration of all the three reactants is same or three 3AProducts A B + C Products dt On solving this equation, 1 x(2a-x) t 2a2(a-x)2

Characteristics of third order reactions 1. Half life period 1 0.5a (2a-0.5a) 1 0.5ax1.5a 2a2 (0.5a)2 k2a2 0.5a 0.5a 3 = 2a2k Thus, half life is inversely proportional to the square of initial concentration. 2. The change in the unit of concentration changes the numercial value of k. 3. The unit of k is expressed as (mol/litre time-l or litre2 mol-time!

Examples of third order reactions: 1. Reaction between nitric oxide and oxygen. 2NO O2NO 2N0 + Cl2 2NOC 2FeCl3 SnCl2 SnCl4 +2FeCl2 2. Reaction between nitric oxide and chlorine 3. Reduction of FeCl3 by SnCl2

Expression for rate constant for reactions of different orders: Reaction A Product Order 4Rate law eqn. Expression for rate const. Rate -k A Product Rate k[A] 2.303, [Alo 1 2A Products Rate k[A]2 2 2.303 [Blo[A] log A + B Products Rate k[A][B] 2 3A Products Rate = k[A]3

Parallel of competing reactions >The reactions in which a substance reacts or decomposes in more than one way are called parallel or side reactions: dt If k1 k2 then A B main and A C is side reaction Let after a definite interval x mol/litre of B and y mol/litre of C are formed. x k1 y k2

Conc. Time-

Consecutive reactions: This reaction is defined as the reaction which proceeds from reactants to final products through one or more intermediate stages. The overall reaction is a result of several successive or consecutive steps. A B C and so on (i) Decomposition of ethylene oxide: (CH2)20 CH,CHO CH3CHO- CO CH4 k2 Let initially t = 0, [B] = 0 k1 < k2 then maximum concentration of [B] may be calculated as k2/k1-k2 [B]max = [Alola! where, [Ao] = initial concentration of A Time n which B attains maximum concentration may be given as: tmax = 2303, log k2 max k-k

Reversible reactions: The reactions in which the products of chemical change react together to form the original reactants are called reversible reactions. These are also called opposing or counter reactions Let us consider a reversible reactions in which both formed and backward reactions are of first order: 1 A B Initial state t0 conc. at time t Equilibirum conc. xe 1k22.303

Methods of determination of order of reactions: The important methods used are as follows 1. Method of integration (Hit and Trial method) In this method the quantities a, x and t are determined and substituted in the kinetic equations of various order. In equation which gives the most constant value of"k, for a series of time intervals corresponds to order of reaction. 1st order: 2nd order 3rd order 2.303 log -a

3. Half life method A general expression for the half lite (t1/2) aT' and (t1/2)," aT' n-1 n-1 Dividing equation (i) by e.g (ii) (t1/2)1-( (t1/2), = (al, n-1