Ad11 A. Jarrar and Ribhi El-Zaru University 01 Jordan ~mman.Jordan
I I
Oxidation of Benzoin by Hexacyanoferrate(III)
(
An advanced kinetic experiment
I
Examination of standard exoerimental ohvsical . . chemistrv
texts reveals that kinetic experimenb involving third-order
kinetics. in aaueous solution are in fact rare. Most of the kinetic experimmts use first- or second-order reactions. I t is therefore a~orooriate to suaaest an exoeriment in .. . which the student can measure a second-order rate constant, and then oroceed easilv to determine an overall third-order .-... . . . ..-. ...
Potassium hexacyanoferrate(II1) is a well recognized oxidizing agent in both acidic1 and alkaline2 media. The reduction of hexacyanoferrate(II1) under such conditions, is assumed to proceed through successive single electron transfer s t e ~ s .The ~ . ~rate of the reaction can easilv he followed colorimetrically hy a uv-visible spectropho~ometer(Sp-8000, Sp-500, or even Sp-20). We here outline the details of a simple experiment, suitable for the general physical chemistry course, which can he performed when the students have learned the basic orincioles acyioin) of kinetics. I t involves the oxidation of benzoin to benzil (a 1.2-diketone) in an alkaline medium. We have found that hexacyanoferrate(II1) oxidizes acyloins, in general, to 1,2-diketones, and we have studied the kinetics of the reaction in some detail a t various conditions.5 The results of those studies showed that the stoichiometrv of the reaction is represented by the equation
(A
PhCOCHOHPh + 2Fe(CN)6"
-+
+ 20HPhCOCOPh 2Fe(CN)e4-+ 2H20 The reaction was found t o be first-order in benzoin, hexacyanoferrate(II1) and hydroxide ions.5 The reaction is carried out in 50% aqueous methanol (VIV), using equivalent amounts of the oxidizing and the reducing agents as presented in the above equation, and a t varying hydroxide ion concentration where hydroxide ion is always in excess. The ionic streneth is kent constant in all the runs by the addition of the required arbount of sodium perchlorate. The reciprocals of the measured ahsorhances (1IA) are plotted versus time, whereupon straight line plots are obtained (fig.), from which Ao and the molar extinction coefficient (t) are obtained. Each s l o ~ is e then multiolied hv c (I-cm cell) to The overall th'ird-order rate constant, get k' in I mole-' 'Mehrotra, U. S., Agrawal, M. C., and Mushran, S. P., J. Phys. Chem., 73,1996 (1969). ZAgrawal,M. C., and Mushran, S. P., J. Phys. Chem., 72, 1497 (1968). G. J.,and Wilson, I. R.,J. Chem. Soe., (Dalton Trans.), 3Bridgw~, 12.1274(1973). 4Agrawal, M. C., Jindal, V. K.,Mushran, S. P., J. Inorg. Nuel. Chern... 32(4). . . . 1257 (1970). . . &Kinetinof oxidatibn of acyloins by hexacyanofemete(III),to be reported soon. Students are encouraged to derive this rate law by proposing a mechanism and assuming steady-state conditions. 7Studentscan work in pairs, and exchange roles. SWhyis such an order of addition preferable? 'The reaction in the last run is fast. and measurements at 2-min intervals are required. The experimentcan be completed in 3 hr, hut if the students are short of time, run 4 can he dropped. 326 1 Jwmal of Chemlcal Education
Solutions for the Experiment Benzoin b)
Solution
Water lmll
NaCl01
NnOH
K1FelCNI6
H20
lmll
I d
ImlI
10.5MI
10.5M) lml)
Benzoin = 5 X 10-'M, 20°C.
anol.
Methanol lmll
F~(CNI~ = ~lo-'M, 50% a w e o u r
MeOH
lml)
meth-
k. is then calculated hv dividine each k' bv the corresoondinahidroxide ion concen;ration. '' The third-order rate can also be calculated from the plot of the second-order rates versus the corresponding hydroxide ion concentrations. In this manner the student can verify the following rate law6 -.dIBenzil1 - -% d[Fe(CN)fi3-1= k [ B ~ ~ ~ ~ ~ ~ ] [ ~ ~ ( c N ) ~ ~ - ] [ o dt dt The energy of activation of the reaction can atso be calculated by repeating the first run (experimental section) a t 25°C.
Experlmenta17 The solutions indicated in the table are prepared. Each pair of solutions corresponding to a particular run (13; 2.6; 3,7; 4.8)are prepared prior to the time of measurements, but the required weights can all be done beforehand. The solutions and solvents can be dis-
pensed from burets or pipets, and should be added in the order shorn in the t a b k s In each run the pair of aalutions is thermostated st 20°C for sufficient time before miring. The two solutions are then mixed, and a clock is started at the same time. The absorbance is then measured, in a 1-emcell, a t intervals of 3-5 min.5
Volume 54. Number 5, May 1977 / 327
