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The activation energy of a reaction is determined by graphical means using experimental data. Bromination of Acetone., E a is the activation energy, and R is the gas constant. Bromination of Acetone in Alkaline Xolution at Low Temperatures.

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Reaction Kinetics. The Bromination of Acetone1REFERENCES Chapter 25 Chapter 17Physical Chemistry, Atkins, 1994Physical Chemistry, Levine, 4th edition 1995PURPOSE:The purpose of this experiment is to determine the rate law and the rate constant for thebromination of acetone. Boldt Software Instruments Garageband. From rate data collected at two or more temperatures, theactivation energy is determined.DISCUSSION:The bromination of acetone in acid solution proceeds according toCH3C(O)CH3 + Br2 H+ → CH3C(O)CH2Br + Br– + H+ [1.]The reaction is catalyzed by hydrogen ion. The rate law is assumed to be of the formrate − d[acetone] − d[Br2 ] k[acetone]p[Br2 ]q[H+ ]r [2.] dt dt = = =where k is the rate constant and [A] represents concentration of A in moles liter-1. Theexponents p, q, and r indicate the order of the reaction with respect to acetone, bromine,and hydrogen ion, respectively.The bromination of acetone is a particularly convenient and interesting reaction to studykinetically. The progress of reaction is readily followed by directly observing thedecrease in bromine concentration spectrophotometrically at a wavelength where noneof the other reagents has significant absorption. Further, the reaction provides aremarkable demonstration of the general rule that it is not possible to predict the ratelaw from just the knowledge of the stoichiometric equation.

As will be confirmed inthis experiment, the reaction is zero order in bromine, i.e., q in equation [2] is zero.This result provides a straightforward application of the method of initial rates whereinthe acetone and acid are present in large excess while the bromine is used in smallconcentrations to limit the extent of reaction. The small amount of bromine iscompletely consumed while the other reactants remain at an essentially constantconcentration.

Since the reaction velocity is independent of the bromine concentrationthe rate is constant until all of the bromine is consumed. Under these conditions rate = − d[Br2 ] = k[acetone]p[H+ ]r [3.] dtand therefore a plot of [Br2] against time is a straight line whose slope is the reactionrate.For the determination of the exponent p it is necessary that the reaction be followed intwo runs in which the initial concentrations of acetone are different while the initialconcentrations of hydrogen ion are not changed from one run to the next. Using1 This experiment has been adapted from Crockford et al., Laboratory Manual ofPhysical Chemistry, 1982. Subscripts I and II to denote the two experiments, we have [acetone]II = u × [acetone]Iand [H+] II = [H+]I.

Then from equation [3], we have rateII = k[acetone]IpI[H+ ]rII = u p[acetone]Ip = u p [4.] rateI k[acetone]Ip[H+ ]Ir [acetone]pIwhich yields ln( rateII rateI ) = p ln( u ) [5.]or p = ln( rateII rateI ) [6.] ln( u )The exponent r is determined from two runs, say I and III, in which [acetone]I =[acetone]III and [H+]III = w × [H+]I. These conditions lead to r = ln( rateIII rateI ) [7.] ln( w )the rate constant is then determined according to equation [2] from the exponents,reaction rate, and the concentration data for which the rate applies.The activation energy E may be estimated from the Arrhenius relationship: k = A exp(–Ea/RT) [8.]if the rate constant is known at several temperatures.