Experiment 1 - Bomb Calorimetry 46g2f

EXPERIMENT #1 Bomb Calorimetry

I. Objective 

To be able to calibrate the bomb calorimeter by determining its heat capacity.

II. Theory Calorimetric measurements involve the use of various temperature and energy units. In order to avoid errors and confusion in the interpretation of these data, their relationships should be well understood. Calorimetry is the science of measuring quantities of heat, as distinct from “temperature”. It is an important field of analytical chemistry which deals accurately with measuring heats of reaction and finds application in fields ranging from nutritional analysis to explosive yield tests (Melville, 2014). The instruments used for such measurements are known as calorimeters. A calorimeter can be a simple container with good insulated walls to prevent heat exchange with the environment. There are different types of calorimeter that are widely used namely: (a) Oxygen bomb calorimeter; (b) Cup calorimeter; (c) flame calorimeter; (d) Solution calorimeter; and (e) calorimeters used for kinetic studies. But among these types, the most common is the oxygen bomb calorimeter. As shown in Figure 1 below, four essential parts are required in any bomb calorimeter: (1) a bomb or vessel in which the combustible charges can be burned, (2) a bucket or container for holding the bomb in a measured quantity of water, together with a stirring mechanism, (3) an insulating jacket to protect the bucket from transient thermal stresses during the combustion process, and (4) a thermometer or other sensor for measuring temperature changes within the bucket (Parr Instrument Company, 2009).

Figure 1. Bomb Calorimeter (Polik, 2016)

According to the first law of thermodynamics, a change in internal energy depends on heat transfer between the system and the surroundings (qsyst < 0 and qsurr > 0) and work done by/on the system (w): ∆U =qsyst +w (1) (Eloranta, 2010) Recalling that both q and w indicate changes in properties even though we have not written ∆ in front of them. If we assume that only pressure (Pext) - volume (V) work is done and the volume is constant (i.e., ∆V = 0), we

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have: w = (−Pext)(∆V) = 0

(2) (Eloranta, 2010))

Note that Pext denotes the external pressure against which the system does work. If the process is reversible, the internal pressure (P) and Pext are identical. The first law at constant volume now becomes: ∆U = qsystem

(3) (Eloranta, 2010)

In an adiabatic bomb calorimetric experiment, changes in the water bath temperature (∆T) are measured. If the heat capacity Ccal of the calorimeter (surroundings) is known, thus the amount of the heat released by the bomb (i.e., chemical combustion reaction) is given by: −qsystem = qsurr = Ccal x ∆Tsurr

(4) (Eloranta, 2010)

Hence, ∆U is the quantity that an adiabatic bomb calorimeter determines directly through the measurement of the heat absorbed by the surroundings (qsurr). For the heat released in combustion Qcomb, it can be calculated from equation (5) wherein the heats of combustion (∆Hc) for the benzoic acid (BA) sample and the fuse wire (FW) are added given by Leger and Teng (1999): Qcomb = ∆Hc BA + ∆Hc FW

(5) (Leger and Teng, 1999)

Qcalorimeter and Quniverse refer to the heat gained by the calorimeter and the universe, respectively. Since Quniverse is equal to zero, we have: Qcombustion = -Qcalorimeter

(6) (Leger and Teng, 1999)

The calorific value (heat of combustion) of a sample may be broadly defined as the number of heat units liberated by a unit mass of a sample when burned with oxygen in an enclosure of constant volume. In this reaction, the sample and the oxygen are initially at the same temperature and the products of combustion are cooled to within a few degrees of the initial temperature; also the water vapor formed by the combustion is condensed to the liquid state. A more exact definition would specify the temperature at which the reaction begins and ends. However, the change in the heat of combustion with possible variations in the initial temperature is so small that this specification is not necessary. Also, the initial and final temperatures are not the same – differing by the amount of temperature rise in the calorimeter – but the effect of this difference is small and usually it is neglected. Thus the term calorific value (or heat of combustion) as measured in a bomb calorimeter denotes the heat liberated by the combustion of all carbon and hydrogen with oxygen to form carbon dioxide and water, including the heat liberated by the oxidation of other elements such as sulfur which may be present in the sample (Parr Instrument Company, 2009).

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Many experiments conducted in bomb calorimetry include the calibration of the bomb calorimeter by measuring its heat capacity. Once the heat capacity of the calorimeter is determined, heats of reaction of other substances can now also be determined. Heats of combustion as determined in an oxygen bomb calorimeter, are measured by a substitution procedure in which the heat obtained from the sample is compared with the heat obtained from combustion of a similar amount of benzoic acid or other standardizing material whose calorific value is known. In principle, this involves comparing the corrected temperature rise of the calorimeter in an experiment in which a known quantity of energy is supplied to it, with that produced in another experiment by combustion in the bomb of a weighed sample of the given material. This method of using the corrected temperature rise of the calorimeter to compare an unknown with a known quantity of energy eliminates some systematic errors. These measurements are obtained by burning a representative sample in a high- pressure oxygen atmosphere within a metal pressure vessel or “bomb”. Experiments conducted involving these measurements are often called Calibration experiments. The energy released by this combustion is absorbed within the calorimeter and the resulting temperature change within the absorbing medium is noted. The heat of combustion of the sample is then calculated by multiplying the temperature rise in the calorimeter by a previously determined energy equivalent or heat capacity determined from previous tests with a standardizing material. Corrections must be applied to adjust these values for any heat transfer occurring in the calorimeter, as well as for any side reactions which are unique to the bomb combustion process (Ginnings, 1970). One of the corrections that needs to be addressed is the calorimeter’s nonadiabaticity. A bomb calorimeter is only approximately adiabatic. In reality, there is a small heat leak through the dewar and the stirrer does work on the calorimeter (wcalorimeter is not equal to zero). Noadiabaticity is corrected for with an empirical radiative correction, RC. RC can be calculated by using the equation provided by: (RC)

=

5(𝑇18− 𝑇12 )+(𝑇6 −𝑇0 ) 6

Figure 2. Radiative Correction RC (Ulm University, n.d)

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(7)

III. Materials and Methods

A. Preparation of the Benzoic acid sample An approximately 1.5 g sample of benzoic acid was weighed using analytical balance and was firmly pelletized using the sample pelletizer as shown in Figure 2 of the Appendix. One pelletized sample was prepared for every trials. In the experiment, there were 2 trials conducted.

B. Filling the calorimeter with water Distilled water was placed in a plastic ice wrapper and was cooled in the refrigerator at the lab to prepare the cold water needed for the experiment. The temperatures of the warm and cold water were then measured using the thermometer and via energy balances, the required masses of cold and warm water were determined. In calculating the required masses, the final temperature must be at 25 degrees celsius and that the calorimeter water must be 3000 mL. After calculating the mass of the cold and warm water, they were measured using a graduated cylinder and were immediately mixed together in a large container and was placed in the calorimeter.

C. Attachment of the Platinum fuse A 7 cm length of fuse was cut using a pair of scissors and was weighed using the analytical balance. In the experiment, there were two pieces of 7 cm fuse that were prepared, one for each of the two trials. The bomb head was then placed at the stand and the fuse was attached between the two electrodes. D. Securing of the sample in the bomb head The pelletized Benzoic acid sample was carefully placed in the steel capsule and the fuse was bent using a forceps to touch the top surface of the sample. The connection between the fuse and the top surface of the sample was carefully checked because errors in the experiment could arise once this part of the procedure is not correctly performed.

E. Closing the Bomb calorimeter The bomb head from the stand was removed and was carefully placed in the bomb cylinder. The screw cap was then secured and the oxygen and the gas release valves were then closed. The bomb calorimeter used in the experiment is shown in Figure 3 of the Appendix. F. Filling the Bomb calorimeter with oxygen With the assistance from the laboratory technician, the hose from the oxygen tank was secured to the bomb and the oxygen tank valve at the oxygen tank was opened. The oxygen control valve was then slowly opened and the increase in the bomb pressure approximately not exceeding to 590 psig was observed. After obtaining the desired pressure, the control valve was carefully closed.

G. Pre-firing and firing the Bomb calorimeter

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The two ignition wires were attached to the terminal sockets of the bomb head. The oxygen-filled bomb calorimeter was then placed in the bucket of water and the screws at the calorimeter cover were secured to close the calorimeter. The bomb calorimeter was examined if it has any sign of weakness or deterioration to prevent errors in the results of the experiment. The thermometer was lowered, the stirrer was turned on and temperature was recorded for every 30-second interval for 6 minutes. For the firing proper, the firing button was pressed for about 1 to 2 seconds on the ignition unit. Temperatures were also recorded for every 30-second interval for additional 12 minutes. Hence, the total time for the recording of temperatures was 18 minutes. H. Recovery of the combustion products The stirrer was turned off and the calorimeter was opened to take the bomb out. The bomb was wiped and cleaned by a towel in order to prepare it for the next trial. Next, the bomb was brought to an open area outside the laboratory. The gas release valves were slowly opened before the screw cap to remove the residual gas pressure. It was brought back inside the laboratory and necessary observations were made. The cap was unscrewed and the bomb head was lifted to check for incomplete combustion inside. The mass of the capsule and the fuse were measured after combustion using the analytical balance. Lastly, all parts of the calorimeter were washed with distilled water and were dried using a clean towel. Procedures from A-H were then repeated until a minimum of two trials were attained. Unfortunately in the experiment, each group was just limited to having two trials each because of time constraints and availability of the bomb calorimeter. All necessary data gathered from the experiment were systematically tabulated for the interpretation of results.

IV. Discussion of Results

The goal of the experiment is to calibrate the bomb calorimeter by measuring its heat capacity. Sample Benzoic acid was used as a standardizing material whose heat of combustion is known. Temperatures at different time intervals were recorded in a total duration of 18 minutes. Data taken from the different temperatures with respect to time within the pre-firing and post-firing period are plotted in the Temperature rise curve as shown in Figure 2 below. Parts of the graph are labelled correspondingly based on the different periods observed in the experiment. The pre-period describes the initial period at time = 0 to the first 6 minutes. This portion represents the data obtained before firing the bomb calorimeter. Based from the graph, trial 1 has higher temperature values compared to trial 2. This was probably because the amounts of water used in each trial were not the same thus, different temperature rise were achieved. On the other hand, the portion of the graph labelled as rise period describes the post-firing data obtained after firing the bomb calorimeter. It was observed that a gradual increase in temperature accompanied the firing of the bomb. Based from the graph, an almost steep line was plotted which represents large temperature changes in the sample as compared to the pre-period. Lastly, post-period portion of the graph describes the temperatures obtained after the subsequent temperature rise. These were the data obtained until the rest of the 18 minutes of the experiment. Temperature changes of the water were ed in its heat capacity which describes the amount of heat required to raise its temperature by 1 degrees Celsius. Hence, greater change in temperature of water was observed in Trial 1 with the higher amount of water.

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28.5

TTemperature (°C)

28 27.5

27 26.5 26 25.5

25 24.5

Pre-period

0

Rise period

5

10

Post-period

15

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Time (minutes) Trial 1 Trial 2

Figure 3. Temperature rise curve for Benzoic acid

To promote complete combustion of the sample, more than 97% excess air was introduced. Benzoic acid reacts with the oxygen to produce combustion products such as soot and moisture. This is represented by the combustion reaction: C6H5CO2H + 7.5O2 = 7CO2 + 3H2O This reaction was validated by observing the contents of the bomb head after the experiment. Along the sides of the bomb head were minimal amounts of black substance which is probably the soot and ample amounts of water. Data obtained after the experiment are summarized in Table 1 below. To the corrections in the calculated delta T, RC was calculated and subtracted to the obtained the delta T to have the corrected delta T. Larger RC was obtained in trial 2 probably because of larger temperature intervals since less heat was lost because most of the heat was absorbed by the greater amount of water.

Table 1. Summary of the calculated data Parameters mass of combusted BA (g) mass of unburned BA (g) mass of combusted fuse (g) RC (K) ΔT (K) Cv (cal/K) Cv, average (cal/K) %excess air

Trial 1 0.9924 0.0002 0.00250 0.0267 0.8733 4309.01

Trial 2 0.9700 0.0040 0.0031 0.0342 1.0258 3772.56

3586.24 cal/K 97.044

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250.72

Consequently, the change in temperature of the water in the second trial was lower compared to trial 1 as exhibited by the value of its calculated heat capacity. The heat capacity of the calorimeter was determined using the mass and internal energy of both the benzoic acid and the platinum fuse by using equation (8):

(8) Results showed that the heat capacity (CV) of trial 1 was greater than trial 2. This obtained value illustrates the effect of the amount of water used in the bomb calorimeter. For trial 1, a value of 3854.55 calorie per Kelvin indicates that 3854.55 calories of heat energy is needed to raise the temperature of the bomb calorimeter by 1 Kelvin. Compared with trial 2 of 3317.94 cal/K, the lesser water used yielded the lower heat capacity. The average heat capacity 3586.24 cal/K cal/K. The bomb calorimeter is now calibrated and can now be used to determine the heat of combustion of other samples.

V. Conclusion Bomb calorimeter calibration was accomplished by burning completely a sample of known heat of combustion which is benzoic acid and in a pure state with known heat of combustion. Other factors that could affect the determination of the heat capacity such as heat leaking out of the calorimeter, the work done by the stirrer, and the heat generated by the fuse should be carefully considered. The heat evolved from burning the benzoic acid and the platinum fuse wire was divided by the corrected temperature change in order to calculate the heat capacity of the bomb calorimeter. Errors could arise if values in temperature changes, precision of equipments used and temperature of water that were calculated.

VI. Recommendation More accurate results could have been obtained if more advanced equipment were used and human errors were reduced.

VII. References

Eloranta, J. (2009). Experiment 1 : Adiabatic Bomb Calorimeter, 0(5), 1–7. Francis, W. M. (2016). Fuels and Fuel Technology: A Summarized Manual in Two Volumes. Elsevier. Helmenstine, A. M. (2015, November 17). Combustion Reactions. Retrieved from About Education: http://chemistry.about.com/od/chemicalreactions/a/Combustion-Reactions.htm Jones, A. Z. (2015, October 31). Introduction to Heat Transfer - Does heat transfer? Retrieved from about education: http://physics.about.com/od/thermodynamics/f/heattransfer.htm

Parr Instrument Company. (2007). Introduction to Bomb Calorimetry. Parr Instrument Company, (483), 1–11. Retrieved from http://www.scimed.co.uk/wp-content/s/2013/03/Introduction-tobomb-calorimetry.pdf Perry, R. H. (2008). Perry's Chemical Engineering Handbook 8th Edition. The McGraw-Hill Companies, Inc.

. H. (2008). Perry's Chemical Engineering Handbook 8th Edition. The McGraw-Hill Companies, Inc.

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VIII. Appendix Raw data and Sample Calculations Table 1. Raw Data for Masses and Volume Parameters Mass of Benzoic Acid (g) Mass of Fuse (g) Mass of the Capsule (g) Mass of the Capsule and Pelletized BA (g) Mass of the Capsule and Soot (g) Mass of Unburned Fuse Pressure (psia) Volume of the Bomb (L) *Mass of Burned Fuse(g) **Mass of Combusted Benzoic Acid (g)

Trial 1 1.0092 0.0122 12.4420 13.4512 12.4588 0.0097 122.787 0.350 0.0025 0.9924

Trial 2 0.9772 0.0112 12.8032 13.7804 12.8104 0.0081 114.263 0.350 0.0031 0.9700

Table 3. Raw Data for Temperatures with respect to time Trial 1

Trial 2

Time (min)

Temperature (°C)

0.0 (initial) 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

26.000 26.100 26.140 26.160 26.170 26.180 26.200 26.200 26.220 26.220 26.225 26.240 26.260

6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0

26.500 26.800 27.400 27.600 27.700 27.780 27.820 27.900

Time (min)

Temperature (°C)

0.0 (initial) 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0

25.140 25.360 25.400 25.440 25.440 25.480 25.480 25.500 25.500 25.520 25.540 25.540 25.550

6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0

25.780 26.280 26.700 26.980 27.140 27.260 27.320 27.400

Pre-firing

Post-firing

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10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 16.0 16.5 17.0 17.5 18.0

27.900 27.920 27.920 27.960 27.965 27.980 27.980 27.980 27.990 27.990 27.990 27.990 27.990 27.990 27.990 27.995

10.5 11.0 11.5 12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 16.0 16.5 17.0 17.5 18.0

Sample Calculations: Data from Trial 1 a. Mass of Combusted BA = (Mass of Capsule + BA) – (Mass of Capsule + Soot) = 13.4512g – 12.4588g = 0.9924 b. Mass of Combusted Fuse = Mass of Fuse - Mass of Unburned Fuse = 0.0122g - 0.0097g = 0.0025 g Calculating the radiative correction using equation (7): 5(T18− T12 )+(T6 −T0 ) 6 5(27.995−27.960)+(26.260−26.0) 6

c. Radiative Correction (RC) = = RC

= 0.0725 K

d. ΔT = T12 – T6 – RC ΔT = 27.960 – 26.260 – 0.0725 = 1.6275 K

e. Heat Capacity of Calorimeter

ΔU (benzoic acid) = -6318 cal/g ΔU (platinum fuse) = - 2.3 cal/cm (Parr Instrument Company, 2009) = - 2.3 cal/cm (7 cm) = - 16.1 cal (1/0.0122g) = - 1319.6721 cal/g ΔT = 1.6275 K Mass of burned BA = 0.9924 g Mass of burned Pt = 0.0025 g

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27.420 27.460 27.480 27.500 27.500 27.515 27.520 27.520 27.520 27.520 27.525 27.525 27.530 27.530 27.540 27.540

(ΔUsample)(mass sample) + (ΔUburned fuse)(mass burned fuse) = −𝐶𝑣 ΔT

CV = − =−

ΔUBA mBA + ΔUPt mPt ΔT

(−6318cal/g)(0.9924g) + (−1319.6721cal/g)(0.0025g) 1.6275 K

CV = 3854.55 cal/K

Taking the average of the heat capacities of the two trials: Cv,average = =

Cv,1+Cv,2 2

3854.55 + 3317.94 2 = 𝟑𝟓𝟖𝟔. 𝟐𝟒 𝐜𝐚𝐥/𝐊

Percent excess air Data from Trial 1 Volume of Bomb = 350 ml = 0.350 L Pressure in Bomb = 108.097 psig + 14.7 psi Pressure in Bomb = 122.797 psia Gas constant, R = 0.08206 L-atm/mol-K (Perry and Green, 2008) Temperature = 298.15 K

f.

Oxygen supplied (mol) in the bomb at 298.15 K: = PV/RT 1atm (122.797 psia) ( (0.350L) 14.7psi) L − atm (0.08206 ) (298.15 K) mol − K = 𝟎. 𝟏𝟐𝟎 𝐦𝐨𝐥 𝐎𝟐 Combustion reaction of Benzoic acid: C6H5CO2H + 7.5O2 = 7CO2 + 3H2O Molecular weight of Benzoic acid = 122.12 g/mol (Perry and Green, 2008) Theo O2 for BA = (mass of BA/122.12g-mol-1)(7.5) 0.9924

= (122.12) (7.5)

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(Polik, 2000)

Theo O2 for BA = 0.0609 mol Combustion reaction of Platinum fuse: Pt (s) + O2 = PtO2 (g) Molecular weight of Pt = 195.08 g/mol (Perry and Green, 2008) Theo O2 for Platinum fuse = (mass Pt/ 195.08)(1) = (0.0025 g/195.08) (1) = 1.2815 x 10-5 mol Total Theo O2 =0.0609 mol + 1.2815 x 10-5 mol = 0.0609 mol

Calculating the percent excess air: g. Percent Excess air =

O2 supplied − theo O2 x 100 theo O2 0.120 − 0.0609 x100 0.0609 = 𝟗𝟕. 𝟎𝟒𝟒 % 𝐞𝐱𝐜𝐞𝐬𝐬 𝐚𝐢𝐫

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Photos/Pictures

Figure 4. Sample Pelletizer

Figure 5. Bomb Head

Figure 6. Bomb Calorimeter

Name: Bryle Kristiann C. Camarote Groupmates: Dion Paul Caspe Smith Nuevaespana Emmanuel Plaza Date of Experiment: September 15, 2016 Date Submitted: September 26, 2016

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