Page 1 :
¢ Chemical Reactions, , , , © Order of Reaction, , , , Chemical kinetics is the branch of chemistry which deals with, the study of rates of the chemical reactions and their, mechanism. It also describes the con, be altered., , Chemical Reactions, , When one or more substances undergo a change which results, in the formation of anew product, it is called chemical reaction., On the basis of their speeds, chemical reactions are of three, types which are as follows, , (i) Very fast reactions Some reactions (such as ionic reactions), , occur very fast, e.g, precipitation of AgCl from AgNO, and, NaCl., , ditions in which rates can, , (ii) Very slow reactions Some reactions are very slow, e.g., Tusting of iron in the presence of air and moisture., (iii) Moderately slow reactions Some reactions occur at, , moderate speed, e.g. inversion of cane sugar and hydrolysis, of starch., , On the basis of num, types as follows, , (i) Elementary reactions A balanced chemical equation does, Not give a true picture of how a reaction takes place. It is very, rare that the reaction gets completed in one step., , The reactions taking place in one step zare called the, entary reactions,, , ber of steps, chemical reactions are of two, , hemica, inetics, , In this Chapter..., , @ Rate Law and Rate Constant, , e Molecularity of Reaction, , © Half-Life and Pseudo First Order Reaction, , , , , , , , , , (ii) Complex reactions When a sequence of e, reactions (called mechanism) gives us the p, the reactions are called complex reactions., in a complex reaction is called elementary, , reaction. The slowest step is called rate d, step., , Rate of a Chemical Reaction, , Rate of a chemical reaction is the change in mo, , concentration of the species taking part in the el, reaction per unit time., , For the reaction, A> B, , Rate of disappearance of A, — Decrease in concentration of A, = —Ccrease In concentration ofA, , Time taken, Rate of appearance of B, , _ Increase in concentration of B, Time taken, and Rate may be expressed as, nate 7414] _ al, At At, As At ->0, Instantaneous rate = Average rate, , The equations given above represent the ave!, reaction (r,.),, , ; sh, Units of rate =mol L s+ or atms7 (i, , SCANNeU WILT |

Page 2 :
:, , , , , , , (case Term I | Chemistry XII, , cous rate of reaction is the rate of change of, centration of any one of the species involved in the, , jon at a particular instant of time., Fora general reaction, aA+bB—>+cC +dD, —1 MA __ 1 d[B] _1 d{e] _ 1 ad), Daren eS db die dtd, , arat 0, Instaneous rate = Average rate, , Rate Law and Rate Constant, , pate law is the expression in which reaction rate is given in, terms of molar concentration of reactants with each terms, raised to some power, which may or may not be same as, , the stoichiometric coefficient of the reacting species in a, balanced chemical equation. It is also called as rate equation, orrate expression., , Fora general reaction, aA + bB —> cC + dD, , Rate = A]*[B]", where x and y may or may not be equal to, the stoichiometric coefficients (a and b) of the reactants., , — 48 yay (By", , dt, , The above equation is known as differential rate equation,, where k is a proportionality constant called rate constant., Rate law for any reaction cannot be predicted by merely, looking at the balanced chemical equation, i.e. theoretically, but must be determined experimentally., , Characteristics of Rate Constant, , * Rate constant is a measure of the rate of reaction. Greater, is the value of the rate constant, faster is the reaction., , * Each reaction has a definite value of the rate constant at a, Particular temperature and its value for the same reaction, changes with change in temperature., , * Its value does nat depend upon the concentrations of the, reactant., , * The units of rate constant depend upon the order of reaction., , Order of Reaction, , Itis the sum of powers of the concentration of the reactants, inthe rate law expression., Order of a reaction =x+ y; if rate =k{A]*[B]!, , For zero order, first order and second order reaction, value of, nis0,1,2., , craracteristics of Order of Reaction, , ofa reaction is an experimental quantity. It can be zero, , and even a fraction,, , is applicable to elementary as well as complex reactions., complex reaction, order is given by the slowest step., , Fora, , , , , Some enzyme catalysed reactions, reactions which occur on, metal surfaces, decomposition of gaseous ammonia on a hot Pt, , surface, thermal decomposition of HI on gold surface are the, examples of zero order reaction., , Hydrogenation of ethene, all natural and artificial, radioactive decay of unstable nuclei, decomposition of, N20; and N,Oare the examples of first order reaction., Units of Rate Constant, , The units of rate constant of different orders are different., This can be easily illustrated below, , For a general reaction,, aA+bB—+cC +dD, Rate =k[A]*[B]”, where, x + y=n=order of the reaction, + 4, Rate:, (A (BY, =(concentration)'~" time, , _ concentration x 1, time (concentration)”, , (where, [A] =[B]), , Considering SI units of concentration, mol L-‘and time s, the, , units of k=(molL')+* s~. Thus, the units of k for different, order of reaction are given below, , -1, , Units of rate constants, , , , , , Reaction Reset Se Cuca, solutions reactions, , Zero order mol L's atm s*, , First order st =, , Second order mol'L st atm™ s, , nth order (mol L's? (atm st, , , , Molecularity of a Reaction, , It is the number of the reacting species taking part in an, elementary reaction, which must colloide simultaneously in, order to bring about a chemical reaction.e.g., , Unimolecular reaction, NH,NO,—>N, +2H,O, Bimolecular reaction, 2HI—-> H, +1,, ‘Trimolecular reaction, 2NO-+O,—+2NO,, , Note The probability that more than three molecules can collide and, , react simultaneously is very small. Thus, reactions with the, molecularity three are very rare and slow to proceed,, , Characteristics of Molecularity of a Reaction, + Molecularity cannot be zero or a non-integer., + Itis applicable only for elementary reactions., , + Incase of complex reaction, molecularity of the slowest, step is same as the overall order of the reaction,, , Scanned with CamScan

Page 3 :
peer, , eran, , 4, , CBSE Term Il | Chemistry xy, , Difference between Molecularity and Order of Reaction, , Molecularity, , ‘The number of reacting species which collide simultaneously in. The sum of powers of the concentrati, order to carry out a chemical reaction is called molecularity of a law expression is, , reaction., , Molecularity is always a whole number value (except zero),, ie.1,2,3., , Molecularity is a theoretical concept., , , , reactions, it has no meaning., , Order of Reaction, , ions of the reactants in the rate, , called the order of that chemical reaction,, , Order of reaction can be a whole number or even a fractional value,, , Order of reaction is determined experimentally,, Molecularity is applicable to elementary reactions. For complex Order of reaction is applicable to elementai, reactions., , ry as well as complex, , , , For simple reactions, molecularity can be obtained from the, stoichiometry of the equations., , For simple reactions, order of reaction may not be e:, , , , , , , , , , , , , , tual to the number, of molecules of the reactants as seen from the dered na, Molecularity ofa reaction cannot be zero. Order of a reaction can be zero., Differential and Integrated Rate Equations, The differential and integrated rate equations for zero and first order reactions are tabulated below, Reaction Order Pee Integral equation UME OF ty. Plot, () A—> Product dx _ Red) emo st 2, © = initial dt t ok t 1 1, concentration of 9 x oe he, reactants g, —. (ax) ex, @ ASP 1 ae yp 2803, ag! 698 z, ee ag oe k T i, log ox he, (ex) ot, = OAS Se, , , , Half-life of a Reaction, , The time in which the concentration of a reactant is reduced, to one-half of its initial concentration is called the half-life of a, reaction. It is represented as t/.., , + For the first order reaction,, , , , , , Fa In [Alo eons logt Alo, t DRI t {R\, R, At tio, [R ait, Therefore, k= one log LRlo, tie [R]o/2, or fea og 2a oe x 0,301, 12 hie, 0.693, or tye = I, , Thus, for first order reaction, t)/. is independent of [R]y, and hence, constant., , * The half-life of a reaction with nth order is given as, , 1, Lo, 1/2 [RI, , Therefore, for zero order reaction t 2 &[R]q. For first order, reaction, t))) is independent of[Ry].—&, , oly and so on., , [Ro], Pseudo First Order Reaction, , The reaction which is not actually of first order but behave, so due to altered conditions is called pseudo first order, reaction, In such a chemical reaction, between two, , substances, one reactant is present in large or excess, amounts.e.g., , + Acid hydrolysis of ethyl acetate, CH,CO0C,H, +H,0 ~>CH,COOH+C,H50H, , Rate =k’ [CH,COOC, Hs JH 0), but[H1;0]eanbetaken, as constant., , *, Rate =k[(CH ,COOC,H, }, where, k =k’ [H30], * Inyersion of cane sugar, , For second order reaction ty) 2, , , , , , CyyH:01, +H,0 yO gHlj205 +Ceth20o, Rate =k{C gH 011], , , , oCanned WIL Gal