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1|Page, , Chemistry Notes for class 12 Chapter 4, Chemical Kinetics, , The branch of chemistry, which deals with the rate of chemical reactions. the factors affecting, the rate of reactions and the mechanism of the reaction. is called chemical kinetics., , Chemical Reactions on the Basis of Rate of Reaction, , 1. Fast/instantaneous reactions Chemical reaction which completes in less than Ips (10°, '? s) time, IS known as fast reaction. It IS practically impossible to measure the speed of, such reactions, e.g., ionic reactions. organic substitution reactions., , 2. Slow reactions Chemical re actions which completes in a long time from some minutes, to some years are called slow reactions. e.g., rusting of iron. transformation of diamond, etc., , 3. Moderately slow reactions Chemical reactions which are intermediate between slow, and fast reactions are called moderately slow reactions., , Rate of Reaction, , Rate of a chemical reaction IS the change in the concentration of any one of the reactants or, products per unit time. It is expressed in mol L” s* or Ms" or atm time” units., , Rate of reaction, = (decrease/increase in the concentration of reactant/product/time taken), , This rate of reaction is known as average rate of reaction (Tay).(f,y can be calculated by dividing, the concentration difference by the time interval)., , For a chemical reaction,, , Scanned with Cases
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2|Page, , For a chemical reaction,, aA+bB — cC+dD, 5 1-A[A] 1 AfB), AY te of reaction (r,,) =———— =~—-=+, verage rai (ray) . . A, = AIC) _ 1 41D), ce At di At, Rate of disappearance of A =~ S{4), Rate of disappearance of B=~ “181, Rate of appearance of C “4, Rate of appearance of D = 412), At \, , Instantaneous Rate of Reaction, , Rate of a chemical reaction at a particular moment of time, is known as instantaneous rate of, reaction., , For reaction,, , For reaction, R— P, ring = — “1B or Al asad, d{R]_ d[P, , Methods for measuring reaction rate (i) pH measurement, (ii) change in optical activity, (iii), change in pressure, (iv) change in conductance., , Slowest step of a reaction was called rate determining step by van’t Hoff., Factors Affecting Rate of Reaction, , 1. Nature and concentration of reactant, 2. Temperature, , Scanned with Ca
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3|Page, , 3. Surface area of reactant, 4. Radiations and catalyst, 5. Pressure of gas, Rate Law Expressions, According to the law of mass action,, For a chemical reaction,, aA + bB — Products, Rate a [A}’ [B]’ = k[A} [B]?, , But experimentally, it is observed that the rate of reaction is found to depend upon ‘a’, concentration terms of A and ‘f’ concentration terms of B Then,, , Rate a [A]* [B]* = k[A]* (By?, , where, [A] and [B] molar concentrations of A and B respectively and k is the velocity constant, or rate constant. The above expression is known as rate law., , Rate Constant, In the above expression, k is called rate constant or velocity constant., , Rate constant may be defined as the specific rate of reaction when the molar concentrations of, the reactants is taken to be unity, i.e.,, , Rate =k, if [A] = [B]=1, , Units of rate constant or specific reaction rate for a nth order reaction is given as, K =(1/Time) x (1/[Conc.}°~!), , Characteristics of rate constant, , Greater the value of rate constant, faster is the reaction., , Each reaction has a particular value of rate constant at a particular temperature., , The value of rate constant for the same reaction changes with temperature., , The value of rate constant for a reaction does’t depend upon the concentration of the, reactants,, , Feber, , Scanned with Cases
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4|Page, , , , , , ‘\the molar concentrations inthe rate law, equation., , Order of reaction Is the: sim ~of' the, concentration terms on which rate “of, , Moleculatity of a reaction ts the number of, atoms, ions-or molecules that must collide _, , reaction actually, depends, a ‘Jwith:-one anotherto. form, products, ita., O't chemical reaction. :, hie seo detined'es surh ofthe epost . ., , , , 2. |(t can be fractional as well as zero., , , , 3. | It is an experimentally determined term., , , , 4. |Order of reaction is applicable to, , elementary as well as complex reactions, |, , , , Negative order reaction Is also possible,, eg,, , 20, —» 30, Rate = MOJ10,1", Order w.rt to Op is -1, , , , Types of reactions depending upon orders, , Types of reactions depending upon, motlecularity, , , , , , , , () 2H —+ H+ h, , (Il) Alkaline hydrolysis of ester, (saponification), , (i) Zero order reaction (i) Unimolecular reaction,, WHale) + Ce) > 2HeI NO) —> 2NO,(e), (iy 2NH, PEs Np + 3H, ;, Gi) First order reaction (il) Bimolecular reactions, ., (HO, —> H,0+ 10, 2h) —> Hole) + bee), 2 (iii) Termolecular reactions,, (Il) Radioactive disintegration 2NO(g) + 02(g) —+ 2NO0,(g), (Il) Inversion of cane sugar. ., (iii) Second order reaction, , , , (iv) Third order reaction, 2N04 0, —+ 2NO,
