1 Combustion Calorimetry 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 CHΔ or fHΔ 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 CUΔ for benzoic acid without outside help. I. lntroduction In this experiment ΔCHO will be determined and use to calculate ΔfHO for your chosen compound. ΔCH 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. vHC hydrocarbon(solid or liquid) + vO2 O2(g) -----> vCO2 CO2(g) + vH2O H2O(l) + heat where vX = 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 an2 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 Co., 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 qv which is equal to ΔCU. Figure 2 - Parr oxygen bomb3 II. Equations The fundamental adiabatic, bomb calorimeter equation is given by equation 1a. Recall that in a thermodynamic change at constant volume, no work is done. Total SystemCVUC TΔ=− Δ (1a) Where: Total SystemVC= total constant volume heat capacity of the system Total System Water Calorimeter AnalyteVVVVCCCC=+ + (1b) Total SystemVCwill be a constant as long as the following conditions are met. Using the same bucket and bomb for all work will keep CalorimeterVC constant. AnalyteVCwill be hard to keep constant but its contribution is so small that any error caused by varying sample size will be negligible. WaterVCaccounts for about 75% of the total heat capacity of the system. This value is very sensitive 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: C Total C Benzoic Acid C Fuse WireUU UΔ=Δ +Δ (2) For unknown solid samples: C Total C X C Fuse WireUUUΔ=Δ +Δ (3) For unknown liquid samples: C Total C X C Fuse Wire C Sample PouchUUU UΔ=Δ +Δ +Δ (4) Total SystemVCis determined by burning samples of benzoic acid (the combustion calorimeter standard with an accurately known value for ΔCU) and then solving equation 1a for Total SystemVC. Once Total SystemVChas been determined, ΔCUX (kJ/mol) for the chosen compound can be determined from equations 1 and 3 or 1 and 4. ΔCHX (kJ/mol) can be calculated using equation 5. ()()CX CX gasH kJmol U kJmol n RTΔ=Δ+Δ (5) Care must be taken while doing the above calculations. The units on ΔCUBenzoic acid, ΔCUfuse wire and ΔCU sample pouch will all be J/g. Total SystemVCwill have the units of J/°C. The units on ΔCHX must be J/mol or kJ/mol.4 III. Correction of ΔCHX to standard state conditions Remember standard state conditions for chemical reactions requires all reactants and products to be at 298.15 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. PTHHdH dT dPTP∂∂⎛⎞ ⎛⎞=+⎜⎟ ⎜⎟∂∂⎝⎠ ⎝⎠ (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 ΔHpressure, will help us make this correction. From lecture remember (∂H/∂P)T = -µ CP so; oo22 2 2 2 2exp exppppressure OO P,O COCO P,COppH C dP C dPΔ=υμ −υμ∫∫ where: υ = stoichiometric coefficient of oxygen or carbon dioxide, μ = Joule-Thompson coefficient, and CP = molar heat capacity Thermodynamic Data for CO2 and O2 Gas CP/(J mol K-1 ) μ /K atm-1 O2 29.72 0.31 CO2 37.14 1.10 Finally, ΔCHXO = ΔCHX + ΔHpressure. III. Procedure 1. 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°C. This temperature is not critical. Anywhere between 24 and 26°C 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 If the jacket temperature is below about 20 °C, the automatic controller will not work properly and hot water will have to be added manually using a toggle switch