Enthalpy of Combustion via Calorimetry

Transcription

1 Enthalpy of Combustion via Calorimetry Introduction This experiment measures the enthalpy change when a system consisting of a known amount of a substance in the presence of excess oxygen is quantitatively reacted to form simple oxides, i.e., when the substance is burned. For example, benzoic acid, which is the reference material for this experiment, undergoes the exothermic reaction C 6 H 5 COOH(s) + 15/2 O 2 (g) 7 CO 2 (g) + 3 H 2 O(l) For this reaction, the isothermal conversion of one mole of benzoic acid to these products at K liberates 3, ± 0.24 kj of internal energy. Since this energy left the system, (i.e., U(final) < U(initial)), we write the molar internal energy change of combustion, U m,comb, as U m,comb = 3, ± 0.24 kj/mol. The emphasis initially is on the U value, because the reaction is carried out experimentally in a closed, constant-volume apparatus and the experimentally measured quantity is the heat associated with the process. Since U = q V, a measurement of q V gives U directly. If we measure the mass of benzoic acid at the start of the experiment, we can calculate the number of moles reacted (since the reaction goes to completion) and thus calculate the molar quantity U m,comb. These values can be turned into enthalpy changes, yielding standard enthalpies of combustion, (often called heats of combustion in the older literature) by the definition of enthalpy: H = U + PV so that H = U + (PV), and, since V is constant and P changes only from a change in the amount of gas in the calorimeter, we can write H = U + (nrt) = U + RT( n) where n = (# moles of product gases) (# moles of reactant gases). Note that this argument assumes that the reaction is carried out isothermally and that the gases are ideal. In the lab, the experiment is done adiabatically, and the primary data of any one measurement give the temperature rise of the apparatus that accompanies the combustion. Thus, a small correction must be made to bring the system along the hypothetical path from the experimental final temperature back to the experimental initial temperature. Enthalpy of Combustion 1

2 This calculation will involve the heat capacity of the apparatus, a quantity which is measured by burning benzoic acid, which has the internal energy of combustion value quoted above. Once the benzoic acid calibration run is performed satisfactorily, you will then measure H m, comb for one of the following solid hydrocarbons: (1) polyethylene, ( CH 2 ) x (2) phenanthrene (3) anthracene These latter two are C 14 H 10 isomeric aromatic hydrocarbons: Phenanthrene Anthracene Enthalpy of Combustion 2

3 Laboratory Procedure We will be using a Parr calorimeter (shown on this lab s web page). You will make one calibration measurement using benzoic acid (do two if you have extra time at the end of the lab) and duplicate runs on your choice of the other three solids. (You may want to make your choice based on the calculations and problems at the end of this writeup. All three are equally informative experiments, but each reveals a different aspect of the type of molecular information to be gleaned from calorimetric data.) Since data from the calibration measurement are used in subsequent experiments, it is worth performing the calibration measurement as carefully as you can. You will be using compressed O 2 gas at high (30 atm) pressures in this experiment. The calorimeter is called a Bomb calorimeter for just this reason! Follow the detailed steps below carefully. Have an instructor check your gas connections at the appropriate time as indicated below. Directions for use of the Parr Bomb Calorimeter 1. Cut and weigh a piece of Fe ignition wire 9 to 10 cm long, seeing to it that there is at least an 8 cm portion in the middle which is free of kinks. (A piece of wire attached so there is a kink between the leads will burn through at the kink rather than ignite your sample.) The specific internal energy of combustion of Fe is J mg 1. (Note the units; the word specific is often used to mean per unit mass rather than per mole. ) 2. Press a pellet of mass 0.8 ± 0.1 g for benzoic acid (or use a pre-pressed pellet, if available). For the other solids, press a pellet of mass 0.5 ± 0.1 g. 3. Attach the wire to the DC power supply, then slowly raise the voltage while pressing the wire onto the pellet. Keep increasing the voltage until the wire gets hot enough to melt the pellet slightly around the wire, then turn off the supply. The wire should now be stuck to the pellet, insuring good mechanical contact inside the bomb. Weigh the pellet/wire sample. 4. Place the pellet into the pan, attach the ignition wire to the calorimeter lead connections so that there are no kinks or sharp bends, and slide the fasteners over the ignition wire to secure it to the calorimeter leads. Make sure that the wire does not touch the pan, or else it will short circuit and burn through rather than ignite your sample. This is the critical step. Test for electrical continuity between the calorimeter lead connections using a digital ohmmeter, and note the electrical resistance reading. 5. Close the bomb, and screw on the cap until it is hand-tight note that there is Enthalpy of Combustion 3

4 no need to go beyond hand-tightening the screw cap. Re-test for electrical continuity between the calorimeter lead connections. (If this electrical resistance reading is very different from that obtained in step 4, open the bomb, and recheck the position of the pellet and the ignition wire.) 6. Hand-tighten the knurled venting nut. Attach the bomb to the oxygen filling line. Consult an instructor at this point. 7. Pressurize the bomb to 400 psi (about 30 atm). 8. Unscrew the knurled venting nut carefully to release the pressure and thus flush most of the air out of the bomb. When the bomb pressure has fallen back to the ambient pressure, hand-tighten the knurled venting nut, and repeat this procedure (steps 7 and 8) to flush the bomb a second time. 9. Hand-tighten the knurled venting nut, and pressurize the bomb again to 400 psi. Open the venting valve to depressurize the filling line, and detach the bomb from the oxygen filling line. Re-test for electrical continuity between the calorimeter lead connections. If this electrical resistance reading is very different from those obtained in steps 4 and 5, unscrew the knurled venting nut carefully to release the pressure, open the bomb and recheck the position of the pellet and the ignition wire. Make any required adjustments, and repeat steps 6 through Seat the water pail in the calorimeter jacket and the bomb in the pail. Attach the ignition leads to the bomb and again test for electrical continuity across the ignition contacts on the side of the jacket. If this resistance reading differs appreciably from those measured in previous steps, consult an instructor. Fill the pail with 2000 ml of water at 25 C, using a volumetric flask. Mix hot and cold water in the flask to reach the desired 25 C. 11. Put the calorimeter lid in place and insert the precision thermometer so that its bulb is near the mid-point of the water depth. Be careful! This thermometer has a thin glass bulb at its end containing mercury. Go slowly and carefully to ensure that you do not break the thermometer. Attach the thermometer viewer to the thermometer, and practice taking temperature measurements. 12. Turn the stirrer shaft by hand to ensure that the stirrer blades are not obstructed. Attach the stirrer motor to the stirrer blade with the motor belt, plug in the motor, and start the stirrer. 13. Attach the ignition wires to the leads marked 10 cm on the ignition box. 14. Begin measuring temperature as a function of time, making a reading every Enthalpy of Combustion 4

5 30 s. Read the temperature to the nearest C. You will continue these readings until the entire run is over. 15. After you have observed a steady but small rate of temperature change on the order of 0.01 C/min for at least 5 minutes, push the black button on the ignition box. The red light on the box should light briefly, and then fade, indicating an ignition current flowing through the Fe wire, followed by an open circuit due to the wire's consumption. A notable temperature rise should begin within 20 to 30 s. Follow the temperature for an additional 10 to 15 min, until the rate of change is again about as slow as before ignition. 16. Unplug the stirrer, disassemble the apparatus, release the bomb pressure carefully, and open the bomb. If the inside of the bomb is coated with soot, then there was insufficient O 2 present to give complete combustion, and the run will have to be repeated. If not, weigh any unburnt wire. Clean and dry all bomb parts before you begin a new run and after your last run. Questions and Calculations Your primary data for each run are the various (time, temperature) measurements you recorded. From these data, we need an accurate a value for the overall temperature change, T, that was caused by the burning reaction. You should plot these data for each of your runs, and when you do, you should find plots that look somewhat like the lefthand schematic plot below. Our T = T 1 T 0, and thus we need to know these initial and final temperatures as accurately as we can. To that end, make a second plot of your data in the vicinity of the ignition time in which you expand the temperature scale, as shown on the right-hand plot below. Extrapolate the temperatures on either side of the ignition time in order to obtain the best T 1 and T 0 values that you can. 28 Raw Data Plot 27.4 Raw Data on Expanded and Interrupted Temperature Plot 27 T T 1 T/ C T/ C T 0 25 T time time Enthalpy of Combustion 5

6 Next, calculate the system heat capacity, C, as determined by your benzoic acid calibration run(s), and calculate the absolute uncertainty associated with this quantity. (See the discussion on Propagation of Errors in the handout distributed in class.) This heat capacity is simply the total internal energy change associated with burning your sample of benzoic acid (of known mass) and the segment of Fe wire that burned (also of known mass) divided by the observed temperature change. For the other substance, find the molar internal energy change of combustion, U m,comb, using C, the mass of the burned substance (what about the Fe wire that burned, too? how should you account for that?), and the observed temperature change. Write a balanced combustion reaction for the oxidation of your substance to CO 2 and H 2 O. From the stoichiometry of this reaction and the amount of sample you used, find (PV) and thus calculate H m,comb and its associated uncertainty. Compare your value of H m,comb with the literature value. Continue with the calculations outlined below for the appropriate substance. Polyethylene A table of standard enthalpy of formation data is at the end of this handout. Use these H f,m data and your experimental results to calculate: and (1) the standard enthalpy of formation of solid polyethylene, per mole of CH 2 units, (2) the reaction enthalpy change for the polymerization reaction C 2 H 4 (g) (2/x) ( CH 2 ) x (s). Next, find the C C bond energy in the polymer chain. (Recall that a bond energy is really an enthalpy.) In the present case, H for the reaction ( CH 2 ) x (g) x C(g) + 2x H(g) is the sum of (x 1) times the molar C C bond energy plus (2x) times the molar energy of a C H bond. Use 413 kj/mol for the C H bond energy. Note that the reaction above is written for gaseous polyethylene. To take the solid-to-gas transition into account (i.e., the process of sublimation), use the value of 9 kj/mol per CH 2 group for the enthalpy of sublimation of polyethylene. The standard molar enthalpies of formation of C(g) and H(g) are tabulated below. How does your answer compare to (i) the accepted average C C single bond energy of 348 kj/mol, and (ii) to the ethane C C bond energy of 368 kj/mol? Enthalpy of Combustion 6

7 Phenanthrene and Anthracene 1. Using the value of H m,comb you determined experimentally, and the standard molar enthalpy of formation ( H f,m ) data tabulated below, calculate the standard molar enthalpy of formation ( H f,m ) of phenanthrene or anthracene (C 14 H 10 (s) hereinafter) and its associated uncertainty. Compare your value of H f,m with the literature value: 121. ± 10. kj/mol for anthracene and ± 2.2 kj/mol for phenanthrene. 2. Calculate the standard molar enthalpy change to break C 14 H 10 (g) apart into its atomic constituents, i.e., H m,atom, the molar enthalpy of atomization of (gas phase) C 14 H 10 : C 14 H 10 (g) 14 C(g) + 10 H(g). You will need to use your value of H f,m for C 14 H 10 (s), the tabulated values for the enthalpies of formation of C(g) and H(g), and the molar enthalpy of sublimation of C 14 H 10 (s): which is 91 kj/mol for phenanthrene and 102 kj/mol for anthracene. 3. Compare the value of H m,atom calculated in 2 with a sum of the appropriate number of C C, C=C, and C H bond energies, based on the Kekulé structures given earlier in the writeup, using 348 kj/mol for C C, 614 kj/mol for C=C, and 413 kj/mol for C H. This sum should be a smaller number than your calculated H m,atom ; the difference is the stabilization energy associated with delocalization of the electron system in these aromatic hydrocarbons. The literature values for these energies are 351 kj/mol for anthracene and 385 kj/mol for phenanthrene. How does your value compare? The resonance energy for benzene is about 151 kj/mol. What does this tell you about the additional stability induced by the center ring system in your substance? Acknowledgements: The figure of data plots is based on a similar figure in Experiments in Physical Chemistry, C. W. Garland, J. W. Nibler, and D. P. Shoemaker, 8 th Edition, McGraw Hill, New York, 2009, page 148. Enthalpies of formation of anthracene and phenanthrene taken from the NIST Chemistry WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, accessed December, Enthalpy of Combustion 7

8 Standard Molar Enthalpies of Formation/kJ mol 1 H(g) C(g) O(g) H 2 O(g) CO 2 (g) C 2 H 4 (g) O 2 (g) 0 H 2 O(l) Enthalpy of Combustion 8