ponent sample this time is different for each compound, and thus separation occurs.
Retardation of the solute by the stationary phase is given by eq 3,
n, - n2 enantiomeric purity (%) = -* 100 n, + "2 (peakareas)of individual enantiomers ( n , > n.). The peak areas in this experiment may be obtained by cuttmg out the ehromatogram and weighing the peaks on an analytical halance. In the case of enantiomers peak calibration is not needed. Chromatographic data are summarized in the table. Given the sensitivitv of the HPLC analysis, it is possihle for the first time to document that indeed the enanriomeric purity of the resulved (S,')-(-)-amheis wsm-
Small-Scale Thermochemistry Experiment Henry Brouwer Redeemer College Ancaster. ON, Canada L9G 3N6
This communication describes an inexpensive calorimeter that uses apprmimately %o of the reagents required for the foam coffee cup calorimeter commonly used in general chemistry courses. The calorimeter may be prepared by cutting a piece of 3.8-cm-thick blue Styrofoam1 into blocks measuring approximately 10 by where k' is called the capacity factor. Know15 cm with a saw or sharp knife. Six equally ing the capacity factor, one can calculate the spaced wells can he formed in the block by retention time of a compound, eq 4. heating the bottom of a 16- X 150-mm test Conclusion tube in a burner flame and pressing the hot We believe that this experiment demonend into the foam to the a~orooriate de~th.~ .. t, = tM(l k') (4) strates the importance of ehiral liquid chroBv controlline the temoerature of thetest matography and its place in undergraduate tube and the force w t h w h ~ hit is apphed, organic chemistry. I t is a fast, highly sensivery uniform reaction we119 ran be formed. In a multicomoonent samole the seoaration tive, and accurate analytical method that These wells, with a capacity of about 15 mL of the individualcompounds is uf prime imdoes not require any previous knowledge of each, serve as the calorimeter, while disposportance. T o calculate the relative position additional parameters (e.g., solubility of diable syringes (5 mL or 10 mL) may he used of twoadjacent peaks,une useseq5,wheren sstereomers, absolute specific rotation of for measurine and transferrina ~ o l n t i o n s . ~ A is called the separation factor. enantiomers). I t is being nsed with increasprecision thirmometer ( i 0 . i 'C) is preing frequency on a macroscale (large prepaferred for measurine the temoerature. alration columns) to directlv seoarate and outhough any laboratory thermometer rould rify enantiomers produced in drug d&n be uded (wnh lesr accurate results). The (pharmaceuticals), natural producl synthcthermometer, rupporred loosely at the upsis, and bioterhnology (8). per end in a clamp, is also used to stir the An acceptable degree of separation requires solutions. Lllerature Clled the value of a to he no less than 1.15-1.20. Since the reactioks take very little time, a 1. Roberts. R. M.: Gilben, J. C.: Rodeurald, L. B.: win^ The degree of separation (overlapping), cover is n ~ t u s e dWith . ~ six wells Der block, a grove. A. S. I" ModemExparimanfoiO,ganic Chamisi.e., the actual sharpness of the peaks, is student can perform six reactions before try; Seunders: New York, 1 9 8 5 : ~184. expressed as the ratio of the distance he2. Pavia,D.L.:Lsmpan,G.M.:Kriz,G.S.InInlrodurfion cleanine uo. After rinsine the wells. tissue tween the two peak maxima (At) and the to ormnic Laborntom Terhnioues: paper may be ulied to remove e x e s $ water . . Ssunden: New ~ m k , ' l 9 8 2 p: 460. . mean value of the peak width at base, eq 6, before the next series of experiment*. 3. Mayo, D. W.; Pike, R. M.; Butcher, S. S. Microscale and is called the peak resolution, R. Smcethe heat absorbedorreleased hy the Or@nie Loborutory; wi1ey: New York, 1989. calorimeter and thermometer is not negligi4. Williamson, K. L. Mocroxcole and Microrcole O ~ m n i c Experiments; Heath: Lexinpton. M A . 1989. ble with the small volumes nsed, the heat 5. Pirklc, W. H : Welch, C.J.; HY"", M. H. J. 078. Chem. capacity of the calorimeter must he deter1'383.40.5022. mined. This may be done by adding 5.0 mL 6. Pirk1e.W. H.;Houae,D. W.;Finn,J.M.J.Chromafogr. 1980,192,143. of warm water (e.g., 30 'C) to 5.0 mL of 7. Pirkle. W. H.; Pochapsky, T. C. Chem. Re". 1989, 89, water a t roam temperature, keeping the needle tip below the water level to prevent A good degree of resolution requires the valexcess a g i t a t i ~ nThe . ~ heat capacity of the ue of R to he no leas than 0.8-1.0. calorimeter (e,.~) may he determined from The enantiomeric purity . . can he calculatiipvid ~ h r i m o l o g r o i h y ;Wiley: New York, 197% the final temperature using the equation beChapter 2. ed using eq 7, where n,,nn are moles low: HPLC Analysls of 1-Phenylethylamlne (Derlvatlzed wlth Acetyl Chloride) heat lost hy warm water =
.
+
. .
" .
heat gained by cold water
Enamiomeric Purity (%)
K
OL
R
-
-
Sample
Configuration
Peak Areas ( % )
Resolved (W-bamhe Resolved (RH+)-amlne Racemic
S
100
100
4.9
R
84
68
1.43
16
4.3 4.9
1.14
S
R
49
0
4.1
1.15
1.37
+ heatgained by calorimeter
(continued on page A1811
A178
Journal of Chemical Education
the micro~cale loboratory T y p i c a l values for t h e heat capacity of the thermometer a n d well are about 6 J/K. T h e table below lists the conditions for some reactions t h a t may h e studied readily using this apparatus. Thermocheml6try Reactlorn Reaction
Reaoents
Neutralization
5.0 mL NaOH (1.0 MI r 5.0 mL HCi ( I 0 MI Or 5.0 m- HOAC (1.0 M) 10.0 mL HCI (1.0 M) Mg (0.010 g) 10.0 mL H20 0.5 g compound (e.g. NH4N0.. NHACI.LiBr. NaNO,. NaOH)
Redox Dissolution
+
+
For calculating t h e enthalpy changes above, it i s assumed t h a t the heat canacitv . . and density of the solutions will be the same ar water (4.18 J/li g and 1.0 g ml., respectively,. T h e heat of reartron i d determined using t h e equation
- 4reaetion = Pso1ution + 9ce1urimeter Photograph showing apparatus used in thermoChemistry experiment: Styrofoam block with reaction wells, syringes, and thermometer.
= m,,m A T h + ic.,
A T,,I
Since t h e time required t o perform the exp e r i m e n t i s m u c h less than w i t h conventiona l experiments, students can b e given a
greater variety of reactions to study, a n d the experiment itself can also he more open ended. For examnle. rather t h a n nrovidine a "rocedure t o detdrmine t h e enthalnv o f , d m r i o n of copper sulfate, studenw could be asked to devise their own method alter havi n g done some enthalpies of solution. A more detailed description o f t h e experiment for students is available u p o n request.
.~~~~~~~~~ ~~~~~~~
~
~
.. b-
'
Small scrap pieces are often available from construction sites. White foamboard was found to be too porous for this application. Smailerwells can be made using a smailertest tube or a glass rod. These would permit use of ~mallervoiumesof reactants but may lead to higher errors. SNdents must hold the syringe properlygrasping the syringe by the barrel may affect the temperature of the contents. Since the heat loss to 'hsurroundings is slow and the temperature Is measured only to the nearest 0.1 'C. the maximum lor minimum). temoera. ture reached was recorded as the final temperature, rather than graphically determining the tern perature at the time of readion. 5 T insure ~ that the syringe and warm water are at the same temperature, the syringe may be iell for a few minutes in a water bath set lo the desired temmrature.
Volume 68
Number 7
July 1991
A181
