22 November 2005
††††††††††† The purpose of this experiment is to determine the heat of combustion for naphthalene and to learn basic bomb procedures. This determination will be accomplished by using an adiabatic bomb calorimeter. First benzoic acid will be combusted in the bomb to yield the specific heat of the bomb and to provide a sample run. Then the specific heat of the bomb will be used to determine the heat of combustion of naphthalene. Once the heat of combustion is found the heat of formation for naphthalene can be determined. Data will be taken and plotted to determine ∆T for the substances, with this value the specific heats may be determined.
††††††††††† A bomb calorimeter functions as a sealed system under pressure. The bomb consists of a sealed cylinder, a sample cup, and a surrounding compartment filled with water. This water will act as an insulator and absorb the heat given off by the combustion of the sample. As the water temperature rises the temperature is read from a thermometer sticking out of the top of the insulating compartment. With these temperature readings one can create a graph of temperature versus time and determine ∆T. Once this value is found it can be used in various equations to determine specific heats.
In order to create combustion in the bomb it is filled with oxygen gas. The oxygen will be ignited by two electrodes placed into the top of the bomb. Once an electrical current passes through the leads it ignites the oxygen and combustion occurs. When the combustion occurs the sample in the cup is combusted and a temperature change is observed in the water of the insulating compartment. All of this process is carried out under adiabatic conditions due to the insulating properties of the air layer above the water and the water itself. If the conditions were not adiabatic the experiment would fail, because there would be heat loss into the atmosphere and an accurate reading would be impossible to achieve.
To determine the heat of combustion for naphthalene the specific heat of the bomb calorimeter must be determined. To determine this three runs of benzoic acid will be combusted. The benzoic acid is a good base to use, because its thermodynamic values have been determined accurately and can be found in various data tables. Once the three runs have been completed and all data is taken a graph of time versus temperature can be plotted. From this graph ∆T can determined and using this value the specific heat of the bomb can be determined.
In order to find the specific heat of the bomb the heat of benzoic acid must be found first. To do this equation 1 must be used.
Qbz = mbz∆cUs†† (1)
Qbz is equal to the heat energy of benzoic acid. m is equal to the mass of acid in grams and ∆U is equal to molar internal energy which is 26.43kJ/g. Now that the heat of benzoic acid has been found the heat of the iron wire must be taken into account, because as the iron wire is combusted some Fe2O3 is formed and this adds to the heat of the system. In order to find this value equation 2 will be used.
QFe = mFe∆cUs††††††††††††† (2)
Q is equal to the heat of iron. m is equal to the mass of iron wire used during combustion. ∆U is equal to the internal energy of iron which is 5.858kJ/g. Now the total heat absorbed by the system may be calculated. To this equation 3 will be used.
Qsys =† Qbz + QFe††††††††† (3)
Now that the heat of the system has been found the heat capacity of the bomb can be determined by using equation 4.
††††††††††† Csys = Qsys/∆Tbz††††††††††† (4)
††††††††††† Once the heat capacity of the system has been determined the heat of combustion of naphthalene can be calculated. First the heat energy of system needs to be found. To do this equation 5 will be used.
Qsys = Csys∆Tnap††††††††††††††† (5)
Once again ∆T will be determined by the graphs created using the data colleted.
Now that the heat of the system has been found the heat of naphthalene can be calculated. Equation 6 will be used to do so.
Qnap = Qsys -- mFe∆cUs††††††††††††† (6)
Now the heat of combustion can be determined using equation 7.
∆cH = ∆cU + ∆ngRT††††† (7)
∆cU is equal to the heat of naphthalene, R is equal to the gas constant 8.314J/molK, T is equal to temperature in K, and ∆ng is equal to 16.5 according to the formula for the combustion of naphthalene which is given below.
C14H10 + 33/2O2ŗ 14CO2 + 5H2O
Now the heat of formation can be determined using Hessís law.
††††††††††† Weigh three one gram portions of benzoic acid and set aside. Weigh out three iron wires and set aside. Now take the fuel cup for the bomb and place the sample in the cup with the wire wrapped around it to provide proper combustion. Then wrap the wire around the leads coming out of the top of the bomb. Next place a little amount of water in the bottom of the bomb just to cover the top. Then place the top down and screw on the top making sure everything is tighten down. Now the bomb must be filled with oxygen from the tank. Fill once through the fill valve and release the air and fill once again, now there is only pure oxygen present. Now place the bomb in the pail which has 2 liters of water in it. Next set the leads and put the top of the insulating container on. Next switch on the agitator and let the system run as is for two minutes will recording the temperature. After two minutes ignite the system and record for data for four more minutes. After combustion weigh the remainder of wire and plot the graphs to perform the calculations discussed in the introduction.
††††††††††† Repeat these steps for two naphthalene runs.
Trial 1 Benzoic Acid:
Trial 2 Benzoic Acid:
Trial 3 Benzoic Acid:
Average heat capacity: 10.82 kJ/K
Trial 1 Naphthalene:
††††††††††††††††††††††† ∆cH= 77.07kJ
Trial 2 Naphthalene:
††††††††††††††††††††††† ∆cH= 80.76kJ
††††††††††† The bomb calorimeter is a very useful tool to have in the lab. It allows us to discover the thermodynamic properties of a substance that is unknown. It accomplishes this by running a known substance in the bomb such as benzoic acid and then comparing the values of the unknown substance such as naphthalene.
††††††††††† The behavior of the graphs is what was to be expected. They begin with a stable temperature before ignition and a rapid increase after ignition is initiated. Eventually the graph settles back to an even temperature that is higher than the starting temperature. This behavior occurs because of the nature of a combustion reaction. As the material is being combusted bonds are breaking and this requires energy to be released in the form of thermal energy. This energy is then transferred to the water and the temperature is registered and recorded. However this combustion is quick and that accounts for rapid increase in the slope of the graph. Once the sample is completely combusted the temperature of the water begins to stabilize and there is a new gentle slope that is higher than that of the starting temperature.
††††††††††† Also naphthalene displayed higher values of energy than benzoic acid due to its structure. Naphthalene is a complex organic ring structure with carbon carbon bonds that more difficult to break than the carbon oxygen bonds of the acid. This leads to greater energy needed to break the bonds and a greater energy released as heat. Therefore, the naphthalene creates more heat than benzoic acid during combustion, as is evident by the graphs and the calculations. Also, during the experiment the water after naphthalene combustion was actually slightly warm; however during the benzoic acid combustion reactions the water temperature felt nearly the same.
††††††††††† The experiment is a necessary one, because it teaches valuable calorimeter techniques which will be essential in the field of chemistry and even biology to some extent. The experiment also yielded results that were to be expected when it came to the graphs and calculations.
††††††††††† The Enthalpy of Formation of Camphor by Bomb Calorimetry. D.P Shoemaker, C.W. Garland, J.I. Steinfeld, and J.W. Nibler, Experiments in Physical Chemistry, 4th ed., McGraw-Hill, New York, NY, 1981. Experiment 7-pp125-138.