Transcription

UC Berkeley College of ChemistryChemistry 125Physical Chemistry LaboratoryBomb Calorimetry and Heat ofCombustionAuthor:Collaborators:Jonathan MelvilleDavid Gygi and Effie ZhouGraduate Student Instructor:Marieke JagerNovember 7, 2014

1AbstractIn this experiment we used a Parr bomb calorimeter to accurately determine the heatof combustion of a sample of sugar. By carefully controlling the pressure, heat flow,and contents of our bomb, and by using a sample of benzoic acid with known values of 1108000 2000to calibrate, we were able to calculate a value for c Hsucrosereasonably close to the literature value of -1156000cal [1].molcal,molMathematically, a majority ofour uncertainty can be traced back to the calculation of a relatively small Co from a largerCtotal , maintaining the absolute error but greatly increasing the relative error. Most ofthe original error can be traced back to uncertainty in the quality of the fits of the foreand afterdrift, as the original masses of sample and length of fuse wire both contributeonly minimally to the final error. Nevertheless, we received a fairly accurate measurementwith good precision (especially considering how rudimentary our experimental setup iscompared to one you might find at NIST), validating this experiment.2IntroductionCalorimetry is an important field of analytical chemistry which deals accurately measuring heats of reaction and finds application in fields ranging from nutritional analysisto explosive yield tests. The need for increasingly accurate reference measurements andthe limited effects of experimental technique mean that more advanced instrumentationis often the single best way to improve calorimetric accuracy in precision. This bringsus to the ParrTM Model 1108 Oxygen Combustion Vessel (Figure 1 or, as it will becolloquially referred to in this text, the “Parr Bomb”.The Parr Bomb is a bomb calorimeter, a type of constant-volume calorimeter (asopposed to typical styrofoam-cup calorimeters, which are constant-pressure calorimeters,at least in theory). As seen in Figure 2, a bomb calorimeter typically consists of ametal bomb designed to withstand heat and pressure, a large Dewar flask to hold thebomb and a known volume of water, a means of remotely igniting the sample (typicallyelectrically, through the use of a fuse wire), and a means of accurately measuring the1

Figure 1: A ParrTM Model 1108 Oxygen Combustion Vessel , as used in this experiment.temperature of the water. Because UC Berkeley is nothing if not a wealthy and generousinstitution, the Parr bomb used for this experiment came with a number of additionalfrills and features, including a pellet press to compress the sample into a compact formand the ability to fill the bomb with compressed oxygen, both of which ensure thatcomplete and total combustion of the sample occurs. The sealed bomb acts as a closedsystem, and the energy from the adiabatic combustion of a known mass of sample willheat the bomb calorimeter and the water a measurable amount. Through the use of acalibration sample of known combustion value (often benzoic acid[2] , including here), theheat capacity of the calorimeter system can be determined, allowing for the calculationof the heat of combustion of a sample of known mass by the net temperature change andthe heat capacities of the combined water-calorimeter system. To aid in calculation, afuse wire with standardized heat of combustion per unit length can be used1[3] , and asmall quantity of water can be inserted into the bomb in advance (to ensure that watervapor is saturated in the bomb, such that the heat of vaporization of water does not needto be factored in).In addition to its use in calorimetry, the reliability and efficacy of this “oxygen-bomb”method has led to its use as the de facto procedure for measuring heteroatoms like sulfur,1In this experiment, ParrTM 45C10 Fuse Wire with a heat of combustion of -2.3 cal/cm.2

Figure 2: A cutaway diagram of a typical bomb calorimeter.[5]chlorine, arsenic, and many other elements in a broad range of combustible materials,including coal, coke, petroleum, and petroleum products[4] .In this experiment, we measured the heat of combustion of sucrose:cal [1]C12 H22 O11 (s) 12O2 (g) 12CO2 (g) 11H2 O(l), c H 1108365mol 3ProcedureThis experiment proceeded through several discrete steps. Measurement of sample temperature was conducted through an integrated Computer Data Acquisition System knownas LabVIEW. A subroutine to monitor and record the temperature was created in this“programming language”, and interfaced with the instrument. Using the Parr pellet press(previously mentioned), compact samples were created and massed (in that order). Thepellets were placed inside the bomb and measured lengths of fuse wire were threaded intothe bomb, in contact with the samples. This data is compiled in the following table:SampleBenzoic Acid 1Benzoic Acid 2Sucrose 1Sucrose 2 (failed)Mass (g) Fuse length (cm)0.979915.11.029815.10.920013.80.937913.63

The bomb was then sealed and tightened. It was flushed twice with 10 atm of O2 gasto purge N2 (to preclude the formation of nitric acid), then filled with 25 atm of O2 gas.The bomb was placed in the calorimeter, which was then filled with precisely 2000. mLof water. The cover was sealed, a stirrer was turned on, and the LabVIEW “program”was initiated. After some waiting some time to establish a baseline level of temperature,the sample was remotely ignited. Once the temperature had equilibrated and anotherbaseline reached, the bomb as removed from the calorimeter and vented, before beingcleaned and dried for the next trial.It is important to note that the second attempted trial of our unknown sample (Sucrose2) failed to ignite and was discarded, and a replacement run was not able to be made inthe interests of time. Raw data for this experiment is included in the Results section,but insufficient data was acquired for proper data workup. As a result, only data fromthe Sucrose 1 trial was used in the calculations that follow.4Results4.1Raw Temperature Data Plots4.2CalculationsSample calculations for the Benzoic Acid 1 sample (Figure 3) are provided. The restof the calculations were performed in Excel and will be provided on request. For thefollowing section, t is used to refer to a time point and T is used to refer to a temperaturepoint.4.2.1Calculation of Tmid and tmidValues for Tinitial and Tfinal of 15.815 C and 18.528 C , respectively, were obtained viainspection. By this method, an estimation of T Tfinal Tinitial 2.713 C . From here, 17.1715 C gives us a rough value of the midpoint temperature. ByTmid Tinitial T2inspection, a value of tmid of 1403 (arbitrary units) was determined.4

Figure 3: A plot of temperature as a function of time for the Benzoic Acid 1 sample.4.2.2Regression fitting to find ΔTBaseline curves were fit for the foredrift and afterdrift curves (Figures 4a and 4b on page 6)and regression curves were taken using a built-in least-squares function (LINEST). Theforedrift equation wasT (x) 0.0000012x 15.80789,and the afterdrift equation wasT (x) 0.00003432x 18.62686.By extrapolating these lines to our calculated value of tmid , a fairly accurate value for T 2.77252 was obtained, since the slopes are very small (hence, there is little variation) and our estimated tmid is very close to the actual midpoint of the curve anyway.5

(a) Foredrift(b) AfterdriftFigure 4: Magnified portions of the Benzoic Acid 1 temperature-time plot, showing fitsfor foredrift and afterdrift.4.2.3Calculation of C and CoIt is already known that the total heat capacity C mCH2 O Co , where CH2 O is thespecific heat capacity of water and Co is the heat capacity of the calorimeter. The purpose6

Figure 5: A plot of temperature as a function of time for the Benzoic Acid 2 sample.of the benzoic acid trials, then, is to use the known heat of combustion to determine Cofrom C and CH2 O , to allow for accurate calculation of the heat output of the unknowngsample (sugar). Using a value of CH2 O 0.999 gcal C , a density of ρH2 O 1 mL , and a knownvolume of 2000. mL of water, mCH2 O 1997.94006 gcal C can be determined. Meanwhile,the total heat capacity C can be determined using the equationC c H 0 m e3, Twhere c H 0 is the heat of combustion of benzoic acid (given as -6318cal),gm is themass of the benzoic acid sample (0.9799 g), e3 is the heat of combustion of the wire(calculable from the heat of combustion of the wire, -2.3calcmand the length of the wire,15.1 cm), and T is the previously calculated temperature rise, 2.77252 C . Combiningall these factors, we can find a value for Co of 277.2553 cal, with a corresponding valueCfor C of 2245.5107 cal. Performing this calculation again for the second benzoic acid trialCallows for calculation of average values of C and Co (tabulated below), which can beused in conjunction with the Sucrose 1 data to provide a value of C from which the heat7

(a) Foredrift(b) AfterdriftFigure 6: Magnified portions of the Benzoic Acid 2 temperature-time plot, showing fitsfor foredrift and afterdrift.of combustion can be determined through the exact same process in reverse, and thedifference between energy and enthalpy can be accounted for by factoring in the effects8

Figure 7: A plot of temperature as a function of time for the Sucrose 1 sample.of the (negligible in practice) P V term, using the following equation: H U (P V ),where (P V ) (n2 n1 )RT.4.2.4Tabulated results and uncertaintiesVariableC1C2CavgCo1Co1Coavg c Usucrose c Hsucrose c Hsucrose(lit.)% errorValue2245.5107 calC2275.1953 calC2260.3530 calCcal247.57063 C277.25529 calC262.41296 calCcal-1187000 molcal-1186000 molcal-1108365 mol7.03%9Error (abs.) Error 2083-24700.002085————

(a) Foredrift(b) AfterdriftFigure 8: Magnified portions of the Sucrose 1 temperature-time plot, showing fits forforedrift and afterdrift.Detailed error ranges and propagation calculations are available via Excel spreadsheeton request.10

Figure 9: A plot of temperature as a function of time for the failed Sucrose 2 sample.5DiscussionFrom comparing the contribution of the Δ(PV) term (equal to the difference between thecal terms, approximately 1000 moland c Usucrose c Hsucrose), we note that the magnitude ofcal) is greater than the entire Δ(PV) term.the uncertainty (2470 molSimilarly, the error produced by a lack of knowledge of the specific heat of the sampleis dwarfed by other sources of uncertainty in the the temperature fits to find ΔT. Werethe uncertainty in specific heat the only form of error in the sample, our error would besome hundred times smaller than our actual calculated error.The HT 1 term pertains to the initial temperature, as we circumvented the finalequilibration of Tf to Ti by taking into account the U mc T term. In doing so, weapproximate HT1 c H (standard state.)The temperature display in the LabVIEW program does not read 23 C before ignitionbecause we did not provide a reference temperature input (typically 0 C ) to LabVIEW.This ends up not mattering, because only the relative temperature difference makes adifference in the final result.11

6ConclusionIn this experiment, we used LabVIEW in conjunction with a bomb calorimeter, andsuccessfully calibrated and found the heat of combustion of a sample of sugar, while accounting for externalities like the vaporization of water or the PV-work that separatesenthalpy from entropy. Our calculated values of the heat of combustion of our unknownsample, though not perfect, are far from bad, with a respectable 7% error from literaturevalues. Considering that analytical facilities like NIST use far more sophisticated instruments with far stricter controls, and that even then experimenters are unable to reacha perfect consensus (judging by the four different listed heats of combustion[1] ), our result is understandable and adequate. Understanding how bomb calorimetry (genericallyconstant-volume calorimetry) is different from standard constant-pressure calorimetrymethods is key to realizing why bomb calorimetry is the method of choice for accuratemeasurement of reaction energies ranging from the breakdown of food to the detonation ofexplosives. In addition, the use of a high-pressure oxygen bomb is not limited to calorimetry, and is useful in elemental analysis. Truly, this lab is a wonder of multidisciplinarytechnique with a broad range of applicability across various fields.References[1] Sucrose – the NIST WebBook. National Institute of Standards and D C57501&Mask 2[2] Suga, H.; Seki, S. An Automatic Adiabatic Calorimeter for Low Temperatures. TheHeat Capacity of Standard Benzoic Acid. Bull. Chem. Soc. Jpn. 1965, 38, 1000-1006.[3] Peng, P.; Caster, A.; Anderson, M.; Switz, N.; Brittman, S.; Chemistry 125 LabManual, Fall 2013 ed.; University of California, Berkeley: Berkeley, 2013.[4] del-1108/121108.

[5] halpy/13%20Enthalpyc.htm[6] Garland, C.W.; Nibler, J. W.; Shoemaker, D.P. Experiments in Physical Chemistry,7th ed.; McGraw-Hill: New York, 200213