XCVIII. Experiments Involving Thermal Methods of Analysis for Undergraduate Chemistry Laboratories Charles M. Earnest Norlheast Louisiana University, Monroe, Louisiana 71209 Due to the increasing number of applications of thermoanalytical techniques in the modern world, and the corresponding dearth of academic activity in teaching these techniques, some experiments whieh may he incorporated into the undergraduate teaching laboratory are offered here. Since the micraanalytical techniques of differential scanning calorimetry (DSC) and the closely related technique of differential thermal analysis (DTA) may be applied t o a variety of problems dealing with both organic and inorganic compounds (polymers, metals, metal alloys, coordination compounds, liquid crvstals. .. . etc.1..the", mav , he incornorated into either the phw~cnl,nnalytiral, im~r:snir, or ormnir tmchiny l n h m t o r ~ e s . 'I'hermnl m r t h d r uf ;analyrii have irecmw important tools far the coal, tobacco, pharmwcutical, mrtal. polvmcr, and ruhhrr indwtrirs. The prcirnt ratr "1 itnwduct~oni l f the trrhniaursot I X C . 1JT.2. ~ h ~ r r n n l c m v i metric analysis (TGA), and other t h e r moanalvtical technioues into the academic tenrhmg lnlxxatmy i; are3tly laogtng their incrr.>amg n,ler in the lriol X A H ~ (1) Ah W" where AH, is the heat of transition or fusion sought in calories per gram, A. is the area of the transition or fusion endotherm arising from a sample of mass W , (grams), Ah is the area of the fusion endotherm caused by Wh grams of standard indium, and AHh is t h e heat of fusion of indlum metal (6.8 callg). An additional requirement for the student is the calculation of the increase in entropy, AS, for each of the transitions and fusions in the two thermosrams ohtained. This is easilv calculated for isothermal phase changes b; the expression AH AS=(2)
T
where AH is in calories per mole and T is in degrees Kelvin. In concluding the erperiment, the student is asked t o complete the form shown in Table 2. T h e temperatures of the transitions and of the fusion for n-dotriacontam are 6 4 3 , 6 5 5 , and 69.3"C, respectively. T h e same transitions occur a t 72.1, 73.8, and 7 5 . 9 T for n C:ibH71. T h e AH for the rotational (orthorhombic to hexagonal) transitions are around 9 kcallmol for n-CsnHss and 9.7 kcallmol for
(Continued on page A336)
Volume 55, Number 9, September 1978 / A335
Chemical instrumentation Table 2. Data Tabulation for nParaffin Thermograrns *Cdse
n-CsHn
r,B r2 rm
AH,B AH2 A Hm
as,= AS2 Asm
-
-
-
'If observed Subscript rn indicates lusion
The heat of fusion ( A H , ) for nl8 kcal'mol and is 21.1 kcall mol for n-Cz6H74. The student should be required to discuss the meaning of both the magnitude and sign of all thermodynamic values reported in table, A,, additional assignment for the student is to give a value for AG (change in Gibbs free energy) for each reversible phase change occurring in the ex. periment. An alternate to this experiment is thestudy of the phases of liquid crystals such as those by cho'estery' myristate and other cholesterol esters. n-CnsH74.
C92H66is
(To be concludes.)
A336 / Journal of Chemical Education
Cited I l l Earnest. C. M.. and Bruscatn, F.N., "A Survey of the lnrtrumentation Capshilities of Colleger and Univeraitios in lauiiiana and Border States Which Do Not Offer the PhD in Chemistry," Presented at the ~ 2 n d~ n n u a ~l e e t i n gofthe 1.ouisiana ~ c a d e m yof Seienees. Whrunry, 1978. 12) Willard, H. H., ~ e r r i t tL, L., and Dean, J. A.. "1"strumental MethodrofAnalysir,.'bth Ed.,VanNos~ tmnd. New York, 1971. 131 Ewinz, G. W., "Instrumental Methods of Chemical Anslysis."4thEd.,MeGrsw-Hill. New York. 1975. 141 Rohinsun, J. W.,"llndergrduato Instrumental Analy~ir."2nd Ed.. Marcel Dekke~.NewYerh. 1970. IS1 Rnuer. H. H., Christian, G. D., and O'Heilly, J. E., "Instrumental Anslysir? Allw and Uacun, Bortm, 1978. (61 Wmdlandt, W. W.. "Thermal M e h d r of Analysis," Wilhy-lntorrcience. New York. 1961. 17) G a n . P. D.,"Thermnanalytieal Methud. ofAnallr;is," AeadcmkPrers,New Yurk, 1965. 181 McKenzie, R. C.. (Editor). "Differential Thermal Analysis? Academic PIPSI,Landnn, 1970. 19) Wendlandt, W. W.. J. CHEM. EDUC. 49. A628 119721. (10) Wendlandt, W. W.,in"Chemicsl Inrtrumentation-11." [ ~ d i t u r :~ w i n p G. , w.1, ~ m e them r SO
