EXPERIMENT 5. THE ENTHALPY OF FORMATION OF CAMPHOR BY BOMB CALORIMETRY 1

Transcription

1 EXPERIMENT 5. THE ENTHALPY OF FORMATION OF CAMPHOR BY BOMB CALORIMETRY 1 The enthalpies of combustion and formation for camphor will be determined using a bomb calorimeter and tabulated values for the enthalpy of formation of carbon dioxide and water. Introduction The enthalpy change of a chemical reaction can be calculated if we know the enthalpies of formation of the reactants and products. aa + bb! cc + dd;! r H º= "!! f H º # "!! f H º products reactants Enthalpies of formation correspond to the enthalpies of reaction where one mole of a specified substance is formed from its elements in their reference states. Enthalpies of formation are commonly determined from enthalpies of combustion, and subsequent application of Hess s law. The enthalpy of combustion of a substance can be measured using a bomb calorimeter. A small quantity of the sample is placed inside a sturdy container (called a bomb), oxygen gas is added, and then it is immersed in a large quantity of water inside the calorimeter. The sample is ignited and the released heat causes the temperature of the surrounding water to increase. By measuring the temperature change, we can calculate the enthalpy of combustion. Bomb calorimeters can also be used to determine the caloric content of foods, coal, gasoline, rocket fuels; as well as to conduct energy balance studies in ecology, and for fundamental research into topics such as measuring enthalpies of isomerization and the development of novel fuels and propellants. Camphor is an ancient pharmaceutical, used in some cold remedies. The natural form is (1R)-(+)-camphor, C 10H 16O, shown below. It has found use as a component of synthetic 1 Shoemaker, D. P.; Garland, C. W.; Steinfeld, J. I.; Nibler, J. W. Experiments in Physical Chemistry, 4th ed., McGraw-Hill: New York, NY, 1981; Experiment 7, pp Last updated: 1/8/16

2 plastics, insect repellents, and rust preventatives. Camphor is one of a large class of compounds called isoprenoids. Camphor's fused tricyclic ring system is found in a wide variety of natural products. O Figure 1. Camphor Theory A brief outline of this experiment is as follows: Determine the heat capacity of the calorimeter using benzoic acid. Determine the internal energy of combustion of camphor in the calorimeter. Calculate the enthalpy of combustion of camphor. Calculate the enthalpy of formation of camphor. Bomb calorimeters are constant volume systems, therefore the heat released in a bomb calorimetry experiment is exactly equal to the internal energy change:!u = q V (1) All calorimetry experiments have two basic steps: the first is to determine the heat capacity of the calorimeter and the second is to use this heat capacity to determine the heat of reaction for the compound of interest. In bomb calorimetry, a substance with a known internal energy of combustion is used to determine the heat capacity of the calorimeter. The internal energy of combustion of benzoic acid is accurately known and it is available in very pure form. The heat transferred to the calorimeter from the combustion of benzoic acid is q bz = n bz! c U = m bz! c U s (2) where n bz is the number of moles of benzoic acid, cu is the molar internal energy of combustion of benzoic acid, m bz is the mass of the benzoic acid in grams, and cu s is the internal energy of combustion per gram of benzoic acid. cu s for benzoic acid 1 is kj g -1. In the calorimeter bomb, combustion is initiated by passing a current through a fuse wire. This wire is made from Fe, and the high temperatures convert some of the fuse wire to Fe 2O 3. Therefore, we must add a small contribution to the total heat from the combustion of iron: 2 Last updated: 1/8/16

3 q Fe = n Fe! c U = m Fe! c U s (3) where n Fe is the number of moles of iron, cu is the molar internal energy of combustion of iron, m Fe is the mass of the iron that burns 2 in grams, and cu s is the internal energy of combustion per gram for Fe. cu s for Fe is kj g -1. The total heat absorbed by the system is then q system = q bz +q Fe (4) If the change in temperature of the calorimeter is T bz for the combustion of benzoic acid, the heat capacity of the system is c system = q system!t bz (5) The system consists of the bomb, thermometer, stirrer, calorimeter pail, and water in addition to the products, which are CO 2(g) and H 2O(l), and unreacted starting materials. In other words, the system is anything that is heated by the combustion reaction. Having calculated the heat capacity of the system, we can now go on to study the reaction of interest. If the change in temperature for the combustion of n cam moles of camphor is T cam then the heat evolved is: q sys = c sys!t cam (6) Once again we must take into account the internal energy of combustion of the fuse wire and for m Fe grams of fuse wire: Then since the reaction is at constant volume: q cam =!q sys +m Fe " c U s (7)! c U ( camphor)= q cam (8) n cam Once we have calculated cu(camphor), we can forget about the details of how the value was measured, since U is a state function and changes in state functions are independent of the path. In the following calculations we can choose the circumstances that make our calculations easiest. In general, the relationship between internal energy and enthalpy is dh = du +d( pv ) (9) If we neglect the change in volume of any liquid or solid reactants and products:! c H =! c U +!( pv ) gas (10) We can assume that the gases behave according to the ideal gas law and hence calculate "(pv) gas. At constant temperature, only the number of moles can change and 2 Note: not all of the fuse wire burns, so it is important to use the actual burnt mass in this calculation. 3 Last updated: 1/8/16

4 ! c H =! c U +!n gas RT (11) where n gas is the sum of the stoichiometric coefficients of gaseous-products minus that of the gaseous-reactants. In our experiment, take T to be the temperature of the calorimeter at the moment of ignition. For example, in the combustion of methanol: CH 3 OH(l) + 3 / 2 O 2 (g)! CO 2 (g) + 2H 2 O(l) (12) n gas = = 1 2. The enthalpy of formation of camphor can now be calculated using Hess s Law and tabulated values of the enthalpy of formation of CO 2(g) and H 2O(l). 4 Last updated: 1/8/16

5 Calorimetry A diagram of an adiabatic jacket bomb calorimeter is shown in Figure 2. The bomb sits in a pail of water. A stirrer keeps the temperature uniform in the pail, and a 0.01 C resolution digital thermometer measures the water temperature. The pail sits in an insulated plastic jacket. Figure 2. Parr adiabatic jacket bomb calorimeter. In the bomb, the sample is in contact with the fuse wire. An excess of O 2 at 20 atm is added to the bomb through a spring loaded valve. Excess O 2 is vented after ignition through a relief valve. Connections for the fuse wire are through an ignition lead at the top of the bomb and the body of the bomb. The body of the bomb is in contact with the pail, which is in turn in contact with a spring loaded connector in the plastic jacket. A typical plot of the temperature as a function of time in a calorimetry experiment is shown in Figure 3. The temperature before ignition of the bomb is usually not constant. If the calorimeter is warmer than room temperature, the temperature will decrease, and if the calorimeter is colder than room temperature the temperature will increase. Newton's Law 5 Last updated: 1/8/16

6 of Cooling 3 states the rate of change will be proportional to the temperature difference; that is, the drift will be linear. The calorimeter also has a stirrer in the pail surrounding the bomb. The stirrer does work, and in the process heats the water in the pail. The work of stirring will increase the pail temperature. After ignition the temperature increases approximately exponentially. The temperature usually doesn't reach a constant value, because the temperature will slowly drift towards the room temperature after the reaction is complete, again according to Newton's Law of Cooling. If the temperature after the reaction is higher than room temperature the temperature will decrease, and vice versa. Errors in the determination of T caused by temperature drift before and after the reaction can be minimized by an extrapolation procedure. The temperature-time line before ignition is extrapolated forwards and the line after the reaction is complete is extrapolated backwards. T is equal to the difference between these two extrapolated lines. However, at what point should you find the temperature difference? Experiments at the National Institute of Standards and Technology (NIST) show that the most accurate time is when the temperature reaches 60% of its final value (See Figure 3). Let T o be the temperature at the time of ignition, t o; and T max be the maximum temperature. Then select the time, t e, when the temperature is 0.60 # (T max T o). T T max T o! T Calculate!T at the time when the temperature change is 60% of its final value t o t e time Figure 3. A typical thermogram for bomb calorimetry. The extrapolation minimizes errors caused by temperature drift before and after the reaction. Safety There is a hazard of electric shock from the exposed terminals of the ignition circuit. Keep the working space dry. The bomb is expensive and should be handled carefully. Be very careful not to scratch or dent the closure surfaces. Failure of the bomb could result. When the bomb is dismantled, always keep the top in the specially designed stand. 3 Atkins, P. W.; Physical Chemistry, 5th ed., W. H. Freeman: New York, NY, Section Last updated: 1/8/16

7 Procedure Ask Dr. Napper to demonstrate how to use the bomb calorimeter and pellet press before attempting this lab. The bomb should be clean and dry. Wipe out the pail, also. A diagram of the assembled bomb is in Figure 4. Weigh out a pellet of benzoic acid. Cut the fuse wire to a length of 10 cm. Weigh the fuse wire on an analytical balance. Attach the fuse wire to the bomb electrodes. The sample pellet should sit in the metal sample cup, and the fuse wire should touch the surface of the pellet. Be careful to ensure that the fuse wire touches only the terminals and not any other metal parts. Place a drop of water on top of the pellet to ensure good contact with the fuse wire. Using a graduated cylinder, add 1-mL of de-ionized water to the bottom of the bomb. (Not on top of the pellet!) This will saturate the inside of the bomb with water vapor, and will result in formation of liquid rather than gaseous H 2O after combustion. Replace the bomb top and securely tighten the screw cap. Attach the bomb to the filling apparatus. Slowly admit O 2 until the pressure reads 20 atm. Slowly release the pressure using the relief valve on the bomb to flush out most of the atmospheric nitrogen originally present in the bomb. Refill the bomb to 20 atm. Remove the bomb from the filling apparatus. Figure 4. Cross section of the Parr oxygen bomb. 7 Last updated: 1/8/16

8 Dry the bomb. Place the pail in the jacket. Place the bomb in the pail. Attach the ignition lead to the connector on the top of the bomb; notice that the lead slips into vertical slots on the bomb connector. Fill a 2000-mL volumetric flask with water. Make sure your water is at the same temperature every time (within 1 ºC). Use the digital thermometer to measure this. Be sure not to immerse the thermistor probe past the tapered transition point! You can add small amounts of hot/cold water to raise/lower the temperature. Note: tap water is OK for this experiment. Slowly pour this water into the metal pail, being careful to avoid splashing. On your first run, check for leaks by immersing the bomb in water. An occasional bubble one every 10 sec is inconsequential. Put the calorimeter lid in place and turn on the stirrer. Connect one of the lead wires from the calorimeter into the 10-cm binding post on the ignition unit, then connect the second wire to the middle terminal. Plug in the ignition unit to the wall. 8 Last updated: 1/8/16

9 Begin time-temperature readings, reading the calorimeter thermometer every 30 seconds and record both the time and temperature. Do not interrupt the time-temperature readings until the run is finished. The temperature should change at a very slow rate (of the order of 0.01 C min -1 ). After this steady rate has persisted for four minutes, the bomb may be ignited. To ignite the bomb, briefly press the button on the top of the ignition box. You should be as far away from the calorimeter as possible when you fire the bomb. Do not approach the bomb for at least seconds after firing. The temperature should begin to rise after a delay of seconds. After a few minutes the temperature should again show a slow, steady rate of change. Do not discontinue readings; continue your readings for 7 8 minutes. The purpose for the continued readings is to ensure that you have sufficient readings for the extrapolation. Disconnect the firing unit from the wall, then from the calorimeter. Remove the bomb from the pail and slowly relieve the bomb pressure. Remember that we added an excess of O 2 so that the pressure will still be quite high after ignition. Remove and weigh any unburned fuse wire, ignoring any globules of metal oxide. Subtract the weight of unburned wire from the initial fuse wire weight to obtain the net weight of fuse wire burned. Wipe dry all bomb parts and the calorimeter pail. In total, you should plan on doing two runs using benzoic acid and two runs with camphor. You should use 0.5 ± 0.1 g of camphor for each pellet you press. You should start by weighing out a little more than 0.5 g of camphor (which can be ground up with a mortar and pestle if desired), followed by pressing the pellet. After pressing the pellet be sure to weigh it on an analytical balance and use as soon as possible. Consider keeping the pellets of camphor in an air-tight vial if you do not use them straight away. Camphor sublimes easily and the pellets can lose a significant amount of mass if you do not use them immediately! Clean-up When you are finished, be sure to dry out the bomb and pail and return all the equipment to its original location. Make sure that you close the valve on the top of the oxygen tank before you leave. 9 Last updated: 1/8/16

10 Calculations For each run: use a spreadsheet (such as Microsoft Excel) to plot the temperature versus time curves. You may wish to use three columns for the temperature data: the first for the linear portion before ignition, the second for the rapidly changing temperature after ignition, and the third column for the linear portion after ignition. Fit the linear portions of the curve (before and after ignition) using a linear trend line. Choose the time, t e, that corresponds to ~60% of the total temperature change. One way you can estimate this time is by plotting the temperature vs. time graph, and using a ruler and a pencil to estimate 4 the time at which ~60% of the total temperature change has occurred. See figure 3 for details. Use t e to calculate the temperature change: if the pre-ignition slope and intercept are m b and b b and the slope and intercept after ignition are m a and b a then!t = ( m a t e +b a )!( m b t e +b b ) (13) Determine the heat capacity of the system using your benzoic acid runs, eqs. (2) (5), and your benzoic acid and fuse wire masses. Use the average of the three heat capacities for the 4 It is not necessary to be too precise when determining this time. As long as you are within seconds, you should be fine! 10 Last updated: 1/8/16

11 analysis of camphor. Determine the molar internal energy of combustion of camphor using your camphor runs, eqs. (6) (8), and the masses of camphor and fuse wire. Write the balanced chemical reaction for the combustion of one mole of camphor. Calculate the molar enthalpy of combustion using eq. (11) and your balanced reaction. Finally, use Hess s Law and tabulated values of the enthalpy of formation of CO 2(g) and H 2O(l) to calculate the enthalpy of formation of camphor using your balanced reaction. When you report " cu, " ch, and " fh, be sure to report the average value. You may use the standard deviation as an estimate of the error. There is no need to carry out a full propagation of errors for this lab report. Be sure to compare your enthalpy of formation to the literature value. Useful Information Dr. Gary Bertrand at the University of Missouri-Rolla has a great webpage concerning bomb calorimetry: practical operation and theory. There are some nice animations of the calorimetry process worth looking at. Warning!!! The following appeared in the October 1985 edition of the Journal of Chemical Education. 11 Last updated: 1/8/16

12 Questions Q1. From the following data (determined using bomb calorimetry), determine " fhº for diborane, B 2H 6(g), at 298 K: (i) B 2H 6(g) + 3 O 2(g)! B 2O 3(s) + 3 H 2O(g); " rhº = kj mol 1 (ii) 2 B(s) O 2(g)! B 2O 3(s); " rhº = 1264 kj mol 1 (iii) H 2(g) O 2(g)! H 2O(g); " rhº = kj mol 1 Q2. Calculate the percent difference between " ru and " rh for the combustion of camphor. How much error would it introduce to your final value of " fh(camphor) if you used " ru in place of " rh? Q3. When calculating "T for the reaction, does it matter if the temperature is expressed in degrees Celsius or Kelvin? Explain. Q4. How different was the calorimeter constant (c sys) from that of the heat capacity of the jacketing water? Explain the difference. Q5. Using bond-enthalpies, estimate the enthalpy of combustion of camphor. How does this compare to the literature value? 12 Last updated: 1/8/16

13 Pre-Lab Questions 1. If we assume that the bomb calorimeter has a heat capacity identical to that of 2000 ml of water at 22 ºC, calculate the change in temperature of the calorimeter after 0.5 g of benzoic acid combusts. Assume that the specific heat of combustion of benzoic acid is kj/g when it is burnt in an isochoric calorimeter. 2. If 0.5 g of benzoic acid, C 6H 5CO 2H, is burnt inside a calorimeter with a volume of 250 ml, what is the minimum pressure of O 2(g) (in atm) required for complete combustion at 22 ºC? State what assumptions you are making. 3. Visit the following website: and go through the discussion, bomb, pressure, and apparatus links. After you have viewed these pages and animations, choose the simulated experiment link. Click on choose a sample and pick either the first, second, or third sample (depending on whether you are the first, second, or third member of the group alphabetically by last name). Calculate " ch and " cu based upon the data you are given. Be sure to print out the raw data sheet (click on view data) and attach to your prelab. 13 Last updated: 1/8/16

14 VISUAL GUIDE TO USING THE BOMB CALORIMETER 1. This is the bomb: 3. This special ring stand is to hold the head of the bomb: 2. The jacket of the calorimeter looks like this: 4. Place the bomb head in its ring stand. The ignition posts and sample support loop extend beneath the bomb head. BE SURE TO READ THE PROCEDURE FOR THE EXPERIMENT IN ADDITION TO THE PICTURE GUIDE! (There is additional information not mentioned in the picture guide ) 14 Last updated: 1/8/16

15 5. This is the special iron ignition wire: 7. Fasten one end of the weighed ignition wire to one binding post beneath the bomb head, and the other end to the other ignition post. 6. Cut off a piece of wire about 10 centimeters long. Weigh the wire precisely (on the analytical balance). 8. Bend the wire down in the middle so that it will dip into the sample. 15 Last updated: 1/8/16

16 9. The sample is weighed into a small open metal dish called a combustion capsule. The metal capsule needs to be cleaned occasionally with emery cloth. 10. Solid samples will need to be pressed into a pellet. The standard benzoic acid samples are already pressed for you. They just need to be weighed before use. For the camphor samples, weigh out approximately 0.5 g and press it into a pellet. Weigh the pellet on an analytical balance and record its mass. Camphor has a large vapor pressure, so be sure to store the pellet in a small closed container (such as a vial) until you are ready to use it. 11. Your sample should be about 0.5 grams (camphor) or 1.0 grams (benzoic acid). Too big a sample would give too big a temperature change to be measured with this apparatus, and might be dangerous. Too small a sample would give too small a temperature change for good precision. 12. Place the combustion capsule, containing the sample, in the loop beneath the bomb head. The ignition wire should make good contact with the solid pellet. (If not, bend the wire carefully.) Add a single drop of distilled water to the top of the pellet where the wire touches. This improves the chance of a successful ignition! 16 Last updated: 1/8/16

17 13. Take extreme care throughout this experiment not to disturb the sample out of the combustion capsule. Lift the bomb head and screw it fingertight into the bomb. If the valve on the top is left open while your do this (and closed afterwards) it will make this step much easier! A drop or two of water smeared around the O- Ring makes it easier to remove the head after ignition. 14. Place the bomb in the bench clamp, and tighten the clamp. 15. Attach the filling connection from the compressed oxygen cylinder to the bomb head. Slowly pressurize the bomb to 20 atmospheres. The gaskets are self-seating under pressure, so the system may leak until you get a little pressure built up. 16. Now release the pressure slowly through the outlet valve. Repeat pressurizing the bomb and releasing the pressure. This is to purge the bomb of nearly all of the nitrogen in the air it originally contained. 17 Last updated: 1/8/16

18 17. Finally, pressurize the bomb to 20 atmospheres. Keep the outlet valve closed this time. The inlet valve is a check valve and will not let oxygen out of the bomb. There is a special valve on the filling connection, beneath the pressure gauge. Use this to let the pressure out of the filling connection, so that the filling connection can now be disconnected. 18. Use a 2-liter volumetric flask to measure exactly 2000-mL of water. Measure the temperature using the digital thermometer. For all other runs, be sure that the water temperature is within 1 ºC of your first benzoic acid run. Add small amounts of hot/cold water to raise/lower the temperature. Use the digital thermometer to measure the water temperature. Be careful not to immerse the thermistor below its tapered end, approximately 4.5 inches from the tip. 19. The bucket is a polished pail. Pour the 2,000 ml of water into the clean, dry bucket. Make sure the pail is dry before adding the 2.0 L of water from the volumetric flask. 20. The three bumps in the bottom of the bucket rest on three knobs in the bottom of the jacket. The bucket handle is left on during the run. 18 Last updated: 1/8/16

19 21. There is a bomb lifting handle which fits in the holes in the sides of the bomb head screw cap. This handle will be used to lower the bomb into the water in the bucket. 23. The bomb is then lowered so that the knobs on the bottom of the bomb fit around the indentation on the bottom of the bucket. Then remove the bomb lifter handle. Be sure that all water droplets that stick to the handle are shaken back off into the bucket. 22. The electric leads from the ignition unit are inserted into thrust terminals in the bomb head, while you are holding the bomb above the bucket. 24. Here is the calorimeter cover, bearing the stirrer. There is a very large ring stand ring to support it. Note: we will not be using the mercury thermometer. 19 Last updated: 1/8/16

20 25. Fit the cover and stirrer in place, lining up the placement pin in the rear. 27. This is the ignition unit. Make sure it is plugged in. Attach one of the firing leads to the center post and the other to the 10-cm post. 26. Start the stirrer. Everything is now assembled, and you are ready to go! The thermistor probe can be inserted through the inlet that the mercury thermometer used to go through. Be careful not to immerse the thermistor below its tapered end, approximately 4.5 inches from the tip. 28. Now that you have everything all set, you are ready to run. One partner calls time every 30 seconds and records the data. The other partner reads the thermometer and calls out the temperature. Do not disturb the thermistor cable during the run or it may cause slight changes in the temperature readings! 20 Last updated: 1/8/16

21 29. After the initial water temperature has been read over a time interval of at least 4 minutes, on the next time mark, push down on the ignition button for no more than three seconds. (The red light should flash on then off to indicate firing.) 31. Once data collection is complete, open the jacket cover. Disconnect the ignition leads. Remove the bomb and place the bomb on a towel on the bench. You should be as far away as possible from the bomb when it fires. After 15 s has passed, you can return to your usual position. 30. About 8,000 calories will now be released very rapidly inside the bomb. You record the temperature of the water in the bucket every thirty seconds thereafter. After a few minutes the temperature will peak. Once the temperature starts dropping, continue readings for 7 8 more minutes. 32. Lift out the bucket. Pour the water in the sink. Dry the bucket and keep it clean. 21 Last updated: 1/8/16

22 33. Wipe the outside of the bomb dry, and release the gas pressure through the outlet valve. 35. Recover all fragments of unburned pieces of ignition wire. Use tweezers if necessary, and remove any (round) nodules of iron oxide. Weigh the pieces of wire (but not the iron oxide!), and subtract this from the initial mass of wire to get the milligrams of wire consumed. 34. Unscrew the bomb head, lift it out, and put it on its special ring stand. Look for signs of incomplete combustion: soot or residue of sample. 36. The End. 22 Last updated: 1/8/16

23 USING THE PARR PELLET PRESS Pellet making with a Parr pellet press 1. Fill the die. Set the die and its holder on the base of the press with the beveled edge of the die resting on the flat surface in the reversible holder. Pour the charge into the die cavity and tamp with a stirring rod, if necessary Compress the charge. Transfer the die and its holder to the press and push the lever down to compress the charge. To obtain maximum compression, the lever should require a firm push as it moves through its full stroke. If a full stroke is not obtained, turn the anvil to lower the die until the full mechanical advantage of the press can be utilized. Conversely, if the lever moves through its full stroke without encountering sufficient resistance, raise the die until firm compression can be applied. 3. Reverse the die holder. Raise the lever and slide the die and its holder out of the press. Reverse the holder to bring the deep cavity under the die and return the parts to their original position. The clearance under the punch will be limited when making thin pellets. In such cases it will be more convenient to grasp the die with one hand and slide it upward on the punch, holding it in that position while reversing the die holder with the other hand Last updated: 1/8/16

24 4. Eject the pellet. Bring the lever down gently to eject the pellet into the cavity in the holder. If a thick pellet is not ejected by this stroke, turn the anvil to raise the die. The pellet will then drop out freely. Remove the pellet with tweezers or forceps; reverse the holder and repeat the cycle if additional pellets are required Last updated: 1/8/16