the computer bulletin board
,
0.0
I
0
5
10
15
20
25
seconds
F~gbre1 Plot of absoroance versus tame for same run as n F gJre 2 lnltlal rate was oolaneo by marklng polnts at the start of the cJNe ano I nmg a lane uslng PSL Calculalor menu
mL BCG Figure 2. Plot of initial rate versus milliliters stock BCG used. Three groups plotted. All runs had same final volume.
Monitoring Oxidation of Bromocresol Green with a Diode Laser: A General Chemistry ISM Personal Science Laboratory (PSL) Experiment John Zimmer, James Reeves, Rebecca Jones, Kirk Cizerle and Dick Ward
University of North Carolina at Wilmington Wilmington, NC 28403 The oxidation of bromocresol green (BCG) has been sugwsted as an alternative kinetic experiment to clock reac;ions (1).The reaction may be carried out in the same fashion as a clock reaction bv notine the time when the color of the reacting BCG solugon changes from blue to green to yellow. However, the absorption curve of the basic form of BCG extends past 700 nm,so the color change of this reaction also may be photometrically monitored using a diode laser (670 nm) light source. The student may experience a more comprehensive view of kinetic studies by simultaneously seeing and monitoring a chemical reaction (2). Articles describiue computer-assisted experiments have appeared in this ~ o & u l ; n recent years (3-5). This work reports a kinetic experiment in which the oxidation of BCG by hypochlorite is monitored using an IBM Personal Science Laboratory (PSL) radiometric probe in conjunction with a diode laser (6). The students used PSL to record the amount of light passing through the reacting solution and converted the signal to absorbance to compare initial rates of absorbance for differing concentrations of BCG and bleach to determine the order of reaction. The reaction for the oxidation of bromocresol green (BCG)in aqueous solution is 2H'
+ blue BCG + NaOCl -t yellow BCG* + NaCl + H20
The rate equation for this reaction is: rate = k[H+F[blueBCGITNaOClP
With pH held constant a t 7.25 (0.1 M phosphate buffer), a new rate constant may be defined ask' = k[H+Ir.By varyA118
Journal of Chemical Education
mL dil. bleach Figure 3. Plot of initial rate versus milliliters NaOCl used. The later values were limited by speed of mixing. All runs had same final volume. ing the relative concentration of BCG and NaOCI, the order of reaction for each species may be determined. The goal of this lab was to determine reaction order, not to determine the value of k'. To accomplish this goal, students obtained initial rates of absorbance versus time and correlated initial rates to powers of relative concentration. Students later encountered the idea of plotting a function of absorbance versus time so as to generate a straight line relationship. This lab was performed by groups of three in a 24 student second-semester General Chemistry laboratory in 1993 and 1994 and was preceded by a lab where students discovered Beer's law and the linear relationship between absorbance and concentration.
Procedure Materials
For stock solutions, commercial bleach (5.25%NaOCl) was diluted 10fold. Bromocresol green was prepared by dissolving 0.30 g of BCG (Fisher) into 1 L 0.1 M phosphate buffer (Fisher), pH 7.25. Experimental Setup
A diode laser (Imatronic Model LDL1751, a 100-mL glass graduated cylinder, a magnetic stirrer (Fisher 14-505-21), and the PSL radiometric probe. A diffuse laser beam passed through the glass cylinder and solution then struck t h e radiometric robe. The orobe was ~ositionedbek n d the focal point of the glass graduated cvlinder that contained the solution and s t i r bar. PSL d a t a was acauired a s light intensity versus time on a laptop computer (&ton 386 Model NB5625). Students added the appropriate volume of colorless buffer to the glass cylinder, aligned the beam, and recorded a 100% transmission value in the Preview mode. Next, a measured volume of stock BCG was added to the glass cylinder. Finally, a measured volume of dilute bleach was rapidly added to the stirring BCG solution in the glass cylinder to start the reaction. Performing Each Run
Data acquisition by the PSL software was initiated when bleach was added and was terminated after the solution turned yellow. Data was saved in PSL as a spreadsheet. Data Analysis
The absorbance was determined by dividing each intensity measurement by the 100% transmission value, taking the log, then multiplying by -1. Next, using the Graph menu in PSL, students fita line to the initial portion of the curve and recorded the slope as the initial rate (of absorbance versus time) as shown in Figure 1. Results Spreadsheet Analysis
In an Excel spreadsheet, students graphed initial rate versus relative concentration. For each group, the relative concentration of either reagent changed from 1to 4. After plotVolume 72 Number 6 June 1995
A119
the computer bulletin board ting the data a s initial slope versus concentration and versus~(concentration),studehts found that the reaction was first order in both BCG and hypochlorite. Results for three (out of four) groups for one section are shown in Figures 2 and 3. Students concluded that a t the highest reactant concentration, the bleach reacted so fast that mixing was the limiting factor. Consequently, they analyzed the first three conceitrations to evaluatereaction order.
Discussion I n later discussion, students learned that a plot of ln(absorbance) versus time yielded a straight line.
propriate secular determinant involves the student in que&ions of topology and bonding, the expansion of the secular determinant to yield MO enereies and orbital coefficients is always tedious and often to error.-We have used Mathematica to simplify this process. Its speed and utility allow the student to focus more on the derived properties such a s bond order and atomic charge. In addition the availability of this computational engine allows for calculations to be performed on relatively complicated systems such a s 1and 2. Partial Script for 1,3-butadiene
Conclusion A kinetics laboratory was desimed to provide students of obs&ving and moniwith a ~orn~rehensive~ex~erienc~ toring a chemical reaction. Students viewed a reaction that changes color while simultaneously monitoring a changing light intensity with PSL equipment. Students analyzed PSL data to determine the order of the reaction for BCG and for hypochlorite. Each plot of ln(absorbance) versus time was linear, even though the slope varied from run to run.
2
Note (i)
Acknowledgment This material is based upon work supported by National Science Foundation under Grant No. USE-9250828 and the University of North Carolina a t Wilmington.
NRonts[ %==O, x I
Literature Cited
x == -1.61803 l x == -0.618034 1 x == 0.618034 x == 1.61803
1. Piekering, M.; Heiler, D.J Ckem Edw. 1987,M , 81-82
arc2 = arr I. x- -1.61803
Note (iii)
Eigensystem[ arr2 I
Note (iv)
2 Kdlv
r: .r
Chrm Fdrv 1m87AZhCZ57.
1-3x2+x4
Note (ii)
Hiickel Calculations using Mathematica Eamonn F. Healy St. Edward's University Austin, TX 78704 Aleebraic c o m ~ u t e rwoerams such a s MathCad1 a n d ~ a t i e m a t i c a 'have found application in a variety of topics in the undereraduate cumculum. These include, but are not limited to, evaluation of the harmonic oscillator for diatomic molecules (I),calculation of vapor pressures (21, and the graphical display of atomic orbitals (3).This paper describes the use of Mathematica to simplify and elucidate the application of Huckel theory in the calculation of MO energies and orbital coefficients.
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Advantages of Svmbolic ADDli~ations .. The Advanced Organic Course a t St. Edward's, like that a t m a w institutions, begins with a discussion of MO theory and introduces the ~ u c k eapproximation l a s a simple yet effective technique for the calculation of MO coefiicients, orbital energies, atomic charges and a s a basis for the understanding of aromaticity. While setting up the ap-
' MaihCAD; Mathsoft; One Kendall Square. Cambridge. MA
02139.
Mathematica; Wolfram Research, PO Box 6059, Champaign, IL 61826.
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Journal of Chemical Education
Notes (i)The student is required to set up the secular determinant far the rr-electron framework using the standard Hiickel approximations: a = H, p = H a (for all bonded pairs) x = (a- E)1 P (1) (ii) For most svstems the exact solution of the oolvnomial can . be quite complicated and numerical routs are prefemd. The conversion of these four values of x t o n orbital enrrgies is simply: E = a- px (iii) The simplest procedure for finding the coefficients involves taking each value of x in torn and substituting back in to the determinant. (iv) For each value of x a set of eigenvedars is generated corresponding to an eigenvalue of zero, or near zero. For x1.61803 the corresponding eigenvector set is 10.371748, 0.601501, 0.601501, 0.3117481, i.e. the vectors correspondingto an eigenvalue of 0.0000039, This represents a solution of the secular equation for x=-1.61803 : E, = a + 1.61803 P Yn = 0.371701+ 0.6015$2 + 0.6015$3 + 0.3717$4 (2) It is important to note that these coeficients are not normalized and, therefore, need to be divided by d ~ ( ~ ) ~
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