Combustion Calorimetry

Transcription

1 ombustion alorimetry 1 Please Note: Each lab group will be required to pick the compound they use in this experiment. The compound must contain only carbon and hydrogen or carbon, hydrogen and oxygen. The Δ H or Δ H f for this compound must have been determined in the last 20 years or longer than 70 years ago. Also the compound must be relatively inexpensive, be not acutely toxic and be combustible (not overly volatile or deliquescent). Please provide a vendor as well as a price for your sample. Your information is due one week before the lab begins. Each person in each group must a separate primary reference for the without outside help. I. lntroduction Δ U for benzoic acid In this experiment Δ H O will be determined and use to calculate Δ f H O for your chosen compound. Δ H will be obtained using an adiabatic, constant volume calorimeter (bomb calorimeter) (Figure 1). Figure 1- Schematic of adiabatic bomb calorimeter The following reaction represents the typical combustion reaction in an oxygen bomb calorimeter. v H hydrocarbon(solid or liquid) + v O2 O 2 (g) -----> v O2 O 2 (g) + v H2 O H 2 O(l) + heat where v X = the stoichiometeric coefficient for compound x The heat released during combustion (q) is detected by the temperature increase recorded in the calorimeter bucket. The system is composed of the sample, the oxygen gas, the bomb, the bucket and water, stirrer and thermometer. This system is separated from the surroundings by an

2 2 adiabatic boundary. The adiabatic boundary is achieved by keeping the adiabatic jacket temperature the same as the system temperature. For the calorimeter used in the experiment, this equilibrium is accomplished by the addition of hot or cold water to the adiabatic jacket. A Parr Model 1241 (Parr Instrument o., Moline, IL) automatic adiabatic calorimeter will be used in this experiment. The temperature adjustment keeping the jacket temperature the same as the system temperature is done automatically. Figure 2 is a more detailed picture of the oxygen bomb you will be using. The oxygen filling valve is not shown but is directly behind the venting valve. Our bombs use two ignition leads and solid samples are placed directly in the sample cup and not suspended above the cup as shown in this drawing. Best results are obtained using ~ 1.0 g samples and 30.0 atm of oxygen gas. Obviously the system must be sealed, consequently the bomb calorimeter reaction occurs under constant volume rather than constant pressure conditions and the measured heat transfer is q v which is equal to Δ U. Figure 2 - Parr oxygen bomb

3 II. Equations 3 The fundamental adiabatic, bomb calorimeter equation is given by equation 1a. Recall that in a thermodynamic change at constant volume, no work is done. Δ U = Δ T (1a) Where: = total constant volume heat capacity of the system = + + (1b) Water alorimeter Analyte will be a constant as long as the following conditions are met. Using the same bucket and bomb for all work will keep constant. will be hard to keep constant alorimeter Analyte but its contribution is so small that any error caused by varying sample size will be negligible. accounts for about 75% of the total heat capacity of the system. This value is very sensitive Water to the mass of water used. It is very important that the mass of water used in all work be kept constant. For benzoic acid samples: Δ UTotal =Δ UBenzoic Acid +Δ UFuse Wire (2) For unknown solid samples: Δ UTotal =Δ UX +Δ UFuse Wire (3) For unknown liquid samples: Δ UTotal =Δ UX +Δ UFuse Wire +Δ USample Pouch (4) is determined by burning samples of benzoic acid (the combustion calorimeter standard with an accurately known value for Δ U) and then solving equation 1a for. Once has been determined, Δ U X (kj/mol) for the chosen compound can be determined from equations 1 and 3 or 1 and 4. Δ H X (kj/mol) can be calculated using equation 5. ( ) ( ) Δ H kjmol =Δ U kjmol+δ n RT (5) X X gas are must be taken while doing the above calculations. The units on Δ U Benzoic acid, Δ U fuse wire and Δ U sample pouch will all be J/g. will have the units of J/. The units on Δ H X must be J/mol or kj/mol.

4 III. orrection of Δ H X to standard state conditions 4 Remember standard state conditions for chemical reactions requires all reactants and products to be at K and all gases to have a partial pressure of 1.00 Bar (0.987 atm). The total differential of enthalpy (equation 6) will give us the correcting equations we need. The first term in equation 6 is the temperature correction term. H H dh = dt + dp T P P T (6) Since our procedure reasonably approximates the standard state temperature conditions, no temperature correction will be needed. However the partial pressures of oxygen gas and carbon dioxide gas will be significantly different from standard state conditions, this correction will need to be made. The second term in equation 6, which we will call ΔH pressure, will help us make this correction. From lecture remember ( H/ P) T = -µ P so; o p Δ H = υ μ dp υ μ dp pressure O2 O2 P,O2 O2 O2 P,O2 exp exp p p o p where: υ = stoichiometric coefficient of oxygen or carbon dioxide, μ = Joule-Thompson coefficient, and P = molar heat capacity Thermodynamic Data for O 2 and O 2 III. Procedure Finally, Δ H X O = Δ H X + ΔH pressure. Gas P /(J mol K -1 ) μ /K atm -1 O O Turn-on the water to the water heater and to the water cooler (same valve). 2. Turn-on the power to the water heater and water cooler. 3. After the water heater has come to temperature turn on the calorimeter power and place the run/purge switch into the purge position. 4. The calorimeter should automatically adjust the adiabatic jacket to about 25. This temperature is not critical. Anywhere between 24 and 26 will be OK. If adjustment is needed, the purge temperature can be changed using the jacket adjust dial. Be careful, a small change in this setting makes a fairly large change in the resulting purge temperature.

5 5 If the jacket temperature is below about 20, the automatic controller will not work properly and hot water will have to be added manually using a toggle switch located on the top control panel. This condition is usually indicated by a constant addition of cold water to the jacket. To correct this problem hot water has to be added manually until the controller seems to be OK. orrect functioning is usually indicated by a cycling of hot and cold water being added to the jacket. 5. If this is the first run of the day, you will need to activate the calorimeter data acquisition system by; (a) loading Labiew / BOMBRUN) (b) mounting two calibrated thermistors in the calorimeter, (c) connecting the thermistors to the computer interface control box and (d) enter the A, B and determined for the 20 to 30 calibration of each thermistor thermometer. When you run BOMBRUN, the program will take you through the steps required to correctly incorporate the thermistors into the data acquisition program. Once you have verified that you are reading correct temperatures, save the correctly configured program to your flash drive (using a name other than BOMBRUN) and go to step Disassemble the oxygen bomb and prepare the sample to be burned. For most materials one gram of solid sample or one milliliter of liquid sample is usually sufficient. One milliliter of water must also be added to the bomb before the bomb is sealed. ontemplate why for the lab interview! 7. Specific instructions concerning fuse wire, sample size, sample preparation and handling, bomb assembly and oxygen pressurization will be given in lab. 8. Remove the calorimeter bucket. Empty and dry the bucket if it contains water from a previous run. 9. Place the assembled and pressurized bomb into dried bucket. 10. Using the thermostated filling burette, carefully add an accurately known mass of about 2000 g of 25 water to the bucket. Remember all subsequent runs must use this same mass of water. 11. Place the filled bucket with bomb assembly into the calorimeter. 12. onnect the ignition wires, close the calorimeter lid, and carefully lower the calorimeter lid. 13. Move the run/purge switch to run and begin data acquisition. This step is critical; no useful data will be collected if this step is omitted. 14. Start BombRun. 15. Using the balance control dial, adjust the adiabatic jacket temperature to match the bucket temperature.

6 6 16. Once the jacket and bucket temperatures are the same, take about 4 additional minutes of data to establish a good starting temperature. Record this temperature in your lab notebook. Ignite the sample. 17. Ignition of the sample is accomplished by depressing the ignition switch. Successful ignition will be indicated by a rapid increase in the bucket temperature. (A noticeable change of temperature occurs within 30 seconds after ignition.) The automatic controller will keep the bucket and jacket temperatures nearly equal. 18. ontinue collecting data for about 12 more minutes so that the temperature of the system and the jacket have stopped increasing and are clearly constant. Record the final temperature in your notebook. Stop BombRun. Once BombRun has stopped taking data, the run is now completed, change the run/purge switch to purge. Purging will drop the jacket temperature to about 25 in preparation for the next run. 19. While the jacket temperature is being reduced, you can remove the bomb and bucket from the calorimeter. Remove the bomb from the bucket. 20. Pour the bucket water back into the holding tank of the filling burette. 21. Release the excess oxygen gas and exhaust from the bomb and then disassemble the bomb. 22. Set aside the left over fuse wire for weighing. Weigh only unburned wire, fused globs of the fuse wire are considered to have burned. 23. heck the inside of the bomb for complete combustion. If the inside of the bomb is sooty, indicating incomplete combustion, the run probably will not be usable. 24. Thoroughly dry the bomb and bucket and prepare the bomb for the next run. 25. alculate sys or Δ U X and Δ H X for the run. 26. Ordinarily three calibration runs will be done with benzoic acid and three runs with the chosen compound. It may take more than three runs with the chose compound because not everything burns as easily or as completely as benzoic acid. I. omputer Programs 1. Using a specifically constructed Excel program, calculate. This program must be ready before your first week with this experiment. 2. Using a specifically constructed Excel program, calculate Δ H O (kj/mol) for the chosen compound. This program must be ready before you begin the second week of this experiment.

7 . Test Data for omputer Programs 7 Program 1- alculation of mass benzoic acid g Δ U fuse wire J/g mass of wire burned g initial temp final temp Expected result: = J = J x deg -1 (Your value may differ slightly, about ± 10 J depending on the value of Δ U of benzoic acid you use.) Use this value of for the following calculations. Program 2 - alculation of Δ U and Δ H O for 6 H 6 O 2 (s). Use this data if the unknown is a solid J deg -1 Δ U fuse wire J/g pressure of O atm mass of sample g mass of wire burned g initial temp final temp volume of bomb 336 ml Expected results: Δ U = kj/mol Δ H = kj/mol Δ H O = kj/mol

8 I. Grading 8 Please note any data taken before you arrive will not be available for your use and will need to be repeated by you at a later date. You must clearly demonstrate that you are prepared to do the required lab work. Lab Report (125 points) Your report on this work must include at least the following information. 1. Give the balanced combustion reaction of the chosen compound. 2. Report value(s) of Δ U (benzoic acid) used in System calculation. Give correct reference(s). 3. alculate sys for your calorimeter system and report the average at 95% confidence limits. (A number of points will be given based on your standard deviation.) 4. alculate Δ H O (kj/mol) for your chosen compound and report the average at 95% confidence limits. (A number of points will be given based on your standard deviation.) 5. Data tables as appendices. One table for standardization runs and one table for your unknown compound runs. Tables must include proper column heading and appropriate table titles. 6. From tabulated values of Δ f H for O 2 (g) and H 2 O(l), calculate Δ f H O (kj/mol) for your chosen compound. You can use Δ f H O values from your textbook. Show calculation. 7. ompare either Δ H O or Δ f H O for your compound with primary literature values. Give the correct literature reference for your source. Attach a copy of the article. 8. alculate the most probable errors in total system and Δ H. learly indicate the estimated errors you used in these calculations. 9. Discussion of why any run was discarded. 10. Intelligent discussion comparing experimental results to literature value, including the significance of errors.

9 II. Interview Questions 9 1. Derive the adiabatic combustion calorimeter equation: Δ U = Δ T. Total system Total 2. How is Total system determined? 3. What is the combustion equation for benzoic acid? 4. What is Δ U for benzoic acid? 5. Structure of the unknown compound. Any special handling or safety concerns? 6. What is the combustion equation for the unknown compound? 7. What is the value of Δn in equation 5 for the unknown compound? 8. Using a bomb volume of 336 ml and assuming O 2 (g) is ideal, calculate the partial pressure of O 2 in the bomb after the complete combustion of 1.00 g of your unknown substance. You can assume the final temperature of the reaction is Review the basic procedure for this experiment for the combustion of benzoic acid. 10. Why is 1 ml of H 2 O added to reaction chamber? 11. What are the major sources of error in the determination of 12. Others??? Total system? III. References 1. Sime, R. J. Physical hemistry: Methods, Techniques and Experiments; Saunders: San Francisco, 1990; p420. Please note: Sime's discussion is fine but her main working equation (eq 3-6) is not entirely correct. Also, since we can flush our systems, we do not have to correct for the formation of nitrogen oxides. 2. Shoemaker, D. P., Garland,. W., Nibler, J. W. Experiments in Physical hemistry; M Graw- Hill: New York, 1996; p152. Author: Roger Hoburg Editing: Ed Tisko