An Example of Following Second-Order Kinetics by Simple Laboratory Means
Glsela Schreiber Paedagogische Hochscule Rheinland Hochdahl. Germany
A convenient way of studying reaction kinetics is by incorporating in the reaction mixture a substance, which gives &to a color change as soon as one of the reacting components of the mixture is depleted. Examples are the Landolt reaction,2 in which an excess of iodate is oxidizing a given amount of sulfite, the consumption of which is immediately followed by the appearance of free iodine giving the well known blue color with some added starch. A more recent example is the first-order hydrolysis of tert-butyl chloride, in whkh the consumption of a given amount of alkali by the liberated hydrochloride and the ensningpH change &e marked by the change of color of some added p H indicator. We tried to use a similar procedure while studying the second-order hydrolysis of ethylene bromohydrine in alkaline medium3 CH,Br OHBrCH,OH CHs Starting with excess bromohydrine we added a known amount of hydroxide ions. Adding furthermore a p H inidicator like phenolphthalein or thymolphthalein we measured the time elansing until its decoloration (or color chance): hv- this time theshydyoxideconcentration had fallen in the reaction mixture to a certain low level due to the hvdrolvsis . . of the hromohvdrine. As our results prove, this level does not depend on the initial concentrations of the reactants. In order to interpret the results one sets up the rate equation for second-order kinetics4
I
+
" = ~ C C ~dtH , B , O H-
---
y>o +
_con- dt k z .CSH,B,OH.
COH-
considering CC~H&OH as constant during the course of the reaction.5 By integration one obtains
where co = initial concentrarinn of OH- and c, = cmcentratiun ut OH- at which the indicator chanres - color. Since c, is independent of co and C~C,H,B,OH and since we used always the same amount of hydroxide it follows
Dependence of discoloration time on lhe medium reciprocal concentration of ethylene bromohydrine.
ond-order rate constant: we refrained from this because our scope was to demonstrate reaction kinetics using just ordinary analvtical elassware. a stoo watch. and some chemicals, hut no sophisticated devices.
Experimental A 0.566 M stock solution is prepared by comhining 20 ml of bromohydrine and 5 g of NaBr and adding distilled water to a total volume of 500 ml (the addition of NaBr praduces a fairly constant ionic strength during the reaction).Twenty-five, 30,35, and 40 ml of this stock solution are diluted to 50 ml with a 1%NaBr solution, respeet i d y . Fifty milliliters of the stock solution are used as such. To these 50-ml portions two drops of an alcoholic 0.5%phenolphthalein sohtion and 1ml1 N NaOH are added. Immediatelv after the addition of the hase the solution is well mixed. the stoo watch is started. and the time unril diseolwnrmn of thr mixture 8s measured. 'l'herm
