Ralph L. Dannley and h ester ~riedrnan Case Western Reserve University Cleveland. Ohio 44106
I
A Simple and Inexpensive Constant
1
Temperature Bath
I
Its use in an undergraduate kinetic experiment
For some years our Laboratory Methods and Techniques sequence for undergraduate professional chemistry majors has included an experiment on kinetics which requires quite precise temperature control. Initially, commercial constant temperature baths were used which not only involved a high initial investment, hut also required expensive maintenance and periodic replacement of the electrical controls. For temperatures above 90°C ethylene glycol was used as the liquid hath, and failure of a temperature control unit once led to a dangerous fire. T o eliminate these problems, the constant temperature baths herein discussed were developed and have been found to be eminently satisfactory. They are so inexpensive that a hath can he provided for each student thus eliminating the problems attendant to a communal constant temperature hath, such as crowding, loss of samples, etc. The constant temperature hath for kinetic work (Fig. 1) consists of a 100-ml volumetric flask surrounded by the vapors of a refluxing solvent in an outer jacket. Choice of the proper liquid for reflux permits a wide selection of hath temperatures. The liquids listed in the table are inexpensive and are less toxic than some competitive materials of
equivalent boiling point. The flat bottom of the apparatus permits it to be heated with a stirrer hot plate. The jacketed section of the volumetric flask above the mark is long enough so that addition of a stirring bar to the reaction mixture does not raise the liquid level to an unheated area. A stirring bar or boiling chips should be placed in the outer flask to prevent superheating the refluxing liquid. If the
L--
---=1
Figure 1 (left) Constant temperature bath for kinetic work
Figure 2. (right) T joint (24140).
Volume 53. Number 4, April 1976 / 265
hot plate surface is considerably larger than the base of the hath, it may he necessary to wrap the lower two inches of the side walls of the outer flask with asbestos paper t o prevent superheating by radiant heat. A drying t u b e should be attached to the reflux condenser because in humid weather condensation of atmospheric moisture may produce an azeotrope of the refluxina- liquid and alter the temperature of . the hsth. If a high temperature bath is needed, the jacket can be insulated with asbestos paper or glass wool. I t is also possible to perform low temperature experiments hy condensing a gas such a s a Freon in the outer jacket and attaching a Dry-Ice condenser. In general laboratory use, the temperature variation from the beginning t o the end of a two-hour kinetic run has been found t o he not more than O.l°C. T h e typical undergraduate kinetic experiment listed below has been used successfullv in t h e apparatus and is an .. ex~ensi\,e modification of an experiment given e l ~ e w h e r e . ~ h l ~ n o rchanees 01 this design:' make it suitable for svnuse. ~ L e s einclude incorporation of thetic and a 24140 3 ioint (Fie. 2) which not onlv aives adequate acfitting with concess t o the;eaction;essel h u t also ventional standard taper equipment. Construction with a cylindrically shaped reaction vessel gives a maximum reaction volume for a given iacket size. T h e bottom of the jacke t may be roundeci to fit a heating mantle if desired. In our laboratories these jacketed vessels have been routinely used for reaction mixtures varying in volume from 50-250 ml. The Measurement of the Flrst-Order Hydrolysis Rate Constants of the C, to C6 W-HydroxyalkylChlorides Experimental Procedure Aoonratus: Constant Temoersture Aooaratus (Fie. .. .. - 1). . 10-ml pipet, syringe adapter, small range thermometer (0.1 or 0.2' divisions), syringe, 50-ml buret, 125-ml Erlenmeyer flasks, lead strips, and an ice hath. Chemicals: 3-Chloropropanol, 4-chlorobutanol, 5-chloropentanol, and G-chlorahexanol are supplied in 10-ml dropping bottles from which samples are obtained via weighing by difference. Standard base 10.01 N NaOH). and eresol red indicator solution. and reflux solvents of appropriate bailing point (see the table) are needed. The volumetric flask must he clean. While cleaning the volumetric flask, close the side-arm joint with a standard taper-glass stopper to prevent loss of the reflux liquid. Into the clean, dry 100-ml vnlumetrie flask of the thermostat weigh accurately, by difference, -0.00500 mole of the chlaroalkanol. Attach the reflux condenser to the thermostat, place the thermostat on the stirrer hotplate, and turn on the stirrer and heater to give a full steady reflux.
1) Heat distilled water (-150 ml) on a hat plate to within 1' of
the proper temperature. 2) Add the hot water to the 100-mlmark of the volumetric flask, thus making a solution about 0.05 M in chloroalkanol. 3) Immediately add a stirring bar to the volumetric flask (this will raise the level of the water above the mark) and start the timer
'A modification of this kinetic equipment was reported by Dannley, R. L., and Jalics, G., J . 0%. Chem., 30,3848 (1965). This apparatus is now commerieally available from Ace Glass, Inc., Vineland, N.J., Catalogue No. 9455-10. 21saacs, N. S., "Experiments in Physical Organic Chemistry," The MacMillan Co., Callier-MacMillan Limited, London, 1969, p. 77. Acknowledgment is given to J. E. Norlander, G. R. McMillan, and others for suggested modifications. T h e synthetic use of this equipment was described hy Logullo, F. M., Ph.D. Thesis, Case Institute of Technology, 1965. This equipment is now eammerically available fram Ace Glass. Inc., Vineland, N.J., Catalogue No. 9455-20.
266 1 Journal of Chemical Education
Suggested Reflux Liquids for the Constant Temperature Bath Liquid (or gar)
. ... ..... . ...
BP
loci
...
Dichlorodifluoromethane (Freon 12) 1.2-Difluoroethane (Genetron 152A) 1.1-oitluaro-I-chloroethane (Genetron 1428) Ice bath preferred 1.2-~icnlorotetrafluoroethane (Freon 1141 Dichloroflu~lomethane(Genetron 211 (Freon 11) Trichloroflu~r~methane Dlethyl ether Methylene chloride Acetone chloroform Methanol n - P r o W bromide ~ethyicnloroform Cyclohexane Ethvlene olvcol dirnethvl ether ~l&thyl>irbonate ~ e t h v isor)roo~i l ketone
.
-29.8 -24.7 -9.2
0 3.6 8.7 23.8 34.6
64.7
70.8
90 95
4) At the indicated time intervals, remove 10-ml samples using a clean, dry 10-ml pipet fitted with a syringe while noting mentally the elapsed time. Eject the sample into a clean 125ml Erlenmeyer flask surrounded and precooled in an ice bath (lead strips wrapped around the Erlenmeyers will weight them to prevent floating or tipping). Swirl the sample briefly to ensure rapid chilling, and record the time the sample was taken. Continuing to work fairly rapidly, add -10 ml of distilled water and a drop or two of cresol red indicator solution (0.1 wt % in 80% aleahol-20%water), and titrate from yellow to a permanent purplered paint with standardized 0.01 N sodium hydroxide solution fram a 50-ml buret. Record the concentration and the required volume of base. Conduct a similar titration on distilled water alone, to determine the volume of base necessary to effect the indicator change per se (i.e., an indicator blank titration). This volume will then be subtracted from the volume of base used in each working titration. Repeat a minimum of six times at intervals of -20 min the process of sample withdrawal, time observation, thermal quenching, time recording, and titration. Record the temperature near the start of the kinetic run, about the middle, and at the end of the run using the special (graduations 0.1 or 0.2') thermometer. The refluxing liquids do not boil exactly at the temperatures listed, and there is some loss of heat in transfer through the wall of the reaction vessel. Immerse the thermometer directly in the reaction mixture while taking a thermometer reading. Take the average as the run temperature. In our laboratories we allow the students to work in pairs. Each pair is assigned duplicate kinetic runs a t three different temperatures with either 3-chloropropanol, 5chloropentanol, or 6-chlorohexanol, T h e suggested approximate reaction temperatures for the 0.05 M chloroalkanol kinetic runs are: 3-chloropropanol, 90, 95, 100°C; 5-chloropentanol, 70, 80, 90°C; and 6chlorohexanol.. 80.90. . . 100°C. T h e samnline . intervals are 20 min each. For 4-chlorohutanol the temperatures of 60, 70, and 80°C uermit samuline . intervals of 12.6. . . and 3 min. respectively. They plot their data to obtain E. and log A and then use a computer program to get AH$and A S . In addition, each pair is assigned a duplicate run with 4-chlorobutanol a t one temperature. T h e chlorohutanol data from the student pairs is pooled t o calculate the reaction parameters. T h e 4chlorobutanol is treated in this fashion because its kinetic runs are so much faster than the others t h a t the time required is much less. T h e combined data are posted so t h a t students can compare the effect of structure and chemical reactivity.
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