The Thermal Decomposition of Urea An Undergraduate Thermal Analysis Experiment Alfred M. Wynne University of Massachusetts. Amherst, MA 01003 Under the influence of increasing temperature, the decomposition of urea leads to a series of reactions that can he observed using thermogravimetry (TG) and differential scanning calorimetry (DSC). Typical T G and DSC curves are shown in Figures 1and 2, respectively; letters have been included for reference to specific portions. These curves, abetted by the knowledge of the decomposition products that are oossihle. allows the deduction of the seauence of steps involved in the complete decomposition of urea and its products, which occurs in the temperature range ambient to about 480 OC. A thermoanalytical experiment utilizing this decomposition offers five valuable features. (1) Among instrumental analysis experiments it is unusual, if not unique, in that the students must interpret experimental data to draw a conclusion. This is in contrast to experiments in which the student need only prepare a series of standards and use these to learn something about the analyte content of an unknown. While the laboratory report of the latter may include great discussions of theory, instrumentation, sources of error, etc., and may involve extensive andlor rigorous calculations,the student is not apt to be asked to uae the results to solve a oroblem. (2) It may be the undergraduate student's only exposure to thermal analysis, a body of techniques of currently widespread industrial use but of limited academic coveraze. " (3) It provides a vehicle by which the difference between thermal analysis and calorimetry may be made clear. (4) It offersan opportunityto introduce and compare several means of tem~eraturemeasurement. (5) It gives the student an opportunity to apply fundamental stoichiometric principles, an opportunity which often is more challenging than it should be.
A mixture containing 80-85 wt % urea and 5-20 wt % KC1 is used. The relative amounts of the two materials is not important, hut since the urea is responwhlr for the thermal e\.rnti it should rumpose most uf the mixture. The studrnts are told that, under the conditions of this experiment, (1)the KC1 undergoes no chemical reaction and, hence, no weight loss, (2) the only decomposition products of interest are ammonia, cyanic acid, biuret, and cyanuric acid, (3) the
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Tsrnpsroture. Figure 1, Typical TG curve.
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matrrinl that rxists in the sample pan at ahout 255 O C (R in Fia. 1 ) is 3 mixture of KCll and hiuret. 14) the material in the s a k p l r pan at ahout 365-370 " C {C in Fig. 1 1 , where a very s l ~ r lchange ~t of ;lope of the T G curve is observed, is cvanuric acid, formed by the decomposition of hiuret and already just starting to undergo decomposition to only volatile products. and (5) the material in the sample pan a t temperatures greater than approximately 475 "C is pure KCI. The role of the KC1 is twofold. First., it gives the students experience in subtracting a constant weight from the weights of interest on the T G curve. Second. it causes meltine" to occur a t a temperature lower than the melting temperature of Pure urea, illustratina meltine ~ o i ndeoression. t The students are asked to for ihis experiment by writing balanced chemical equations showing the following conversions
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urea urea biuret biuret
biuret
(1)
cyanic acid + other volatile product(s)
(2)
cyanuric acid + volatile product(s)
(3)
volatile product(4
(4)
Experlrnental 'l'hr only mnnipulntions involwd in this experiment are the settmr: uf the rnrtous instrument rc.ntrc,lr and loading the two snmples. h l w uf the ume is spent rvauinc for the curves to be generated. For this reason, it is productive to have the experiment done by small groups of students with an instructor, The waiting periods provide superb opportunities (1) to present and compare methods of temperature measurement, (2) to discuss the theory, applications, and instrumentationof TG andof DSC (as well as of other thermoanalyt-
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+ volatile produet(s)
using only ammonia and cyanic acid as the volatile products. They are also asked to determine the formula weights of all the chemical substances involved. The equations and formula weights will be used to infer the nature of the decomposition reactions.
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Figure 2. Typical DSC curve.
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ical techniques), and (3) tocomment on the features of thecurves as they appear. It is useful to make copies of the curves for each student. Approximately 10-mg samples of a single mixture of reagent grade urea with reagent grade KC1 are used for TG and for DSC. The sample taken for DSC must be accurately weighed (to 0.1 mg) if the AH of urea is to be determined. Because the DSC sample tends to spill out of the sample pan during the heating program, it is desirable that its sample pan be sealed. A pinhole in the lid allows the escape of volatile products so the TG curve and the DSC curve may be used as complements. A temperature rate of 10 ' C h i n gives satisfactory curves and enables both curves to be obtained in a three-hour laboratory period. The curves of Figures 1 and 2 were obtained using, respectively,a Du Font 900 Thermogravimetrie Analyzer and a Perkin-Elmer DSC-1 Differential Scanning Calorimeter under the conditions described in the previous paragraph. The sample weights were 11.12 mg for TG and 11.5 mg for DSC. The purge gas was Np. The DSC plot includes a calibration curve obtained from the melting of an indium sample of known weight, in this ease 17.78 mg, the AH of fusion ofwhichis 28.41 J1g.l The AHofthis transition, then, was 505 mJ. This endothermic peak is obtained under the experimental conditions described. It is informative to allow the liquid indium to cool and to obtain the freezing peak. The students note that the latter is exothermic and of the same area as the melting peak. Because the cooling rate cannot be controlled by the instrument at this low temperature, it is likely that the cooling rate will be lower than the heating rate and, hence, that the freezing peak will differ slightly in shape from the fusion peak. However, they will have the same area and this is also informative. Each student is asked to submit a laboratory report containing: (1) copies of the TG and the DSC curves obtained descriptionof the theory and the instrumentation of TG and of DSC (3) a summary of some important applications of TG and of DSC (4) the fallowing calculations: (a) the wt % urea and the w t 90KC1 in the original sample (b) the weight of biuret formed by the first decompositian step and, then, by the application of eqs 1and 2 the weights of cyanie acid and ammonia formed, noting that the total weieht of volatile oroducts should eanal the observed (2) a brief
tion of biuret (d) afar the transition which produced the peak at about 125 O C (Fin Fia. 2) and the AHof fusion of urea 1(5) the following commentary: (a) a comparison between the temperature of the melting peak (the temperature at which the peak starts to form) and the accepted melting point of urea, with an explanation of any discrepancy or discrepancies (b) a summary of the thermal events that occurred between 100 OC and 480 OC Discussion T G and DSC curves are effected by a number of instrumental and experimental factor^.^ T h e latter, of course, includes the composition of the urea-KC1 sample. T h e T G curves which have been ohtained in this experiment have been reproducible in shape. However, t h e temperature corresponding t o A in Figure 1has varied by as much a s 10 "C and t h a t corresponding t o C has varied by even more than 10 "C. T h e shapes of peaks F, G, and H of Figure 2 are generally reproducible, although the temperatures vary from one exneriment t o another. Peak I of Fieure 2 has shown wide variation in both temperature and shape; in some cases, two overlapping small peaks have been ohtained. In the part of
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Wendlandt, W. W. Thermal Methods of Analysis, 2nd ed.; Wiley:
New York, 1974: p 184.
Reference in footnote 1. pp 9 f f and 146 ff. Rossini. F. J. et al.. Eds. Selected Values of Chemical Thermodynamic Properties; U S . Government Printing Office: Washington, DC, 1952; p 595.
the discussion which follows. the results shown in Fiaures 1 and 2 will be used. T h e weight a t D, 1.92 mg, was t h a t of pure KCI. T h e weight a t A, then, included 1.92 mg KC1 and 11.12 - 1.92 = 9.20 mg urea (153.2 pmol). Therefore, the composition of t h e original sample was 17.3 wt % KC1 and 82.7 wt % urea. At B, 4.43 - 1.92 = 2.51 mg biuret was present. This was 24.35 pmol hiuret, which, by the application of the correct statement of eq 1;
indicates the simultaneous formation of 24.35 pmol (0.415 mg) of NH, and the conversion of 2 X 24.35 = 48.70 pmol of urea to these products. T h e amount of urea converted to HCNO and NH, must have been 153.2 - 48.70 = 104.5 pmol (6.28 mg), which, by the application of the correct statement of eq 2,
indicates the simultaneous formation of 104.5 pmol (4.50 mg) of HCNO and 104.5 pmol(1.78 mg) of NH3. T h e student may check his or her work by comparing the sum of the weights of the volatile products formed, 0.415 1.78 = 2.20 mg NH3 and 4.50 mg HCNO, a total of 6.70 mg, with t h e observed weight loss, 6.69 mg. T h e weight a t C is taken t o represent cyanuric acid and KCI, only. Hence the weight of cyanuric acid was 2.67 - 1.92 = 0.75 mg, which corresponds t o 5.8 pmol. T h e correct expression of eq 3,
+
indicates t h a t 8.71 pmol (0.90 mg) of hiuret must have been converted to cyanuric acid and, therefore, 2.51 - 0.90 = 1.61 mg of hiuret must have been transformed directly to volatile products a s shown by the correct statement of eq 4,
H2N-
H.1 -N-
-NHp
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ZHCNO + NH3
The transition which produced the peak a t about 125 "C was the melting of the solution of KC1 in urea. Its AH can h e obtained using the ratio of its area to the area of either the indium melting peak or the indium freezing peak. In this case, the ratio is 4.74; hence, t h e AH of the transition of interest was 2.39 J. Knowing the weight of the sample taken for DSC (11.5 mg in this experiment) and t h e fact t h a t the sample was 82.7 wt % urea (from the T G data), i t i s possible t o calculate the AH of fusion of urea, which, based on these data, is 252 J/g. Generally, results in the range 24E-260 J/g are ohtained. These compare very favorably to t h e reported value of 250.9 Jle.3 T h e melting point ubserved in this experiment is 113 "C, aonroximatel~20°C lower than 132.ti°C, thenccepted meltin; point of irea3, and more than 660 "C lowe; than the accepted melting point of KCI, 776 "C. It isexpected t h a t the student will recognize t h a t this manifests freezing or melting point depression. A summary of the thermal events t h a t occurred in this Volume 64
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experiment should follow readily. The important phenomena have already been noted, except for the existence of the peaks a t G and H. These show that what appears as asinglestep weight loss on the TG curve actually involves a t least two reactions. I t is likely that one of these arises from the formation of biuret and the other arises from the formation of cyanic acid, although it is not known which is which. Comment I t is recommended that the students obtain the TG curve
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before obtaining the DSC curve. First, weight changes seem more meaningful to students than do enthalpy changes. Second, after inspecting the TG curve, most students believe that the first DSC peak will be observed at temperatures greater than 150 "C or so. The start of the formation of the ~ e a akt F oroduces surnrise and. often. the immediate conclusivn that the instrument is malfunctioning. When the w a k hns heen comoletelv formed. they realize that it is real and see at first hind an example o? the complementary relationship of TG and DSC.
