Bomb Calorimetry
Jonathan Smith
Determination of Resonance Stabilization Energy of Benzene
Adapted from Experimental Physical Chemistry, 2nd edition, Halpern 1997
In this investigation we will measure the heat evolved during the combustion of an organic compound in the presence of excess oxygen under constant volume conditions. The heat evolved, q, thus equals DU rather than DH.. Upon determination of DUcombustion we can then determine a wide range of properties of the compounds examined. DHf is readily determined from DUcombustion and can be used to determine reaction energetics. In examining 1,5,9-trans, trans, cis-1,5,9-cyclododecatriene [2765-29-9] (CDDT) we can determine the resonance energy of benzene from the thermodynamics of the following theoretical reaction.
C6H6(g) (benzene) + C6H12(g) (cyclohexane) ® C12H18(g) (CDDT) + Eresonance
The reason this works is because both the reactants
and products have the same number of each type of chemical bond (C-C, H-C,
C=C). This type of reaction is called an
isodesmic reaction and does not have to correspond to
an actual chemical reaction but is useful in constructing a thermodynamic cycle
which gives us insight into the relative stability of compounds. Isodesmic reactions
produce better theoretical values because of the conservation of each type of
bond. In this case the difference
between reactants and products is the resonance energy of benzene. How do we get these values from bomb calorimetry? We
compute the heats of formation for CDDT from our combustion of this compound
and we use accepted values for benzene and cyclohexane. We need to convert from enthalpy to energy as
well.
Figure 1. CDDT
In preparation for this investigation draw a structure of CDDT and indicate through a thermodynamic cycle how this leads to the resonance energy of benzene. In this thermodynamic cycle, the heat of vaporization of CDDT will be needed. A relationship between log(Pvap) and 1/T has been established1 and from this relationship DHvap can be determined (see for instance: Vapor Pressure of a Pure Liquid).
(1)
The combustion is carried out in a pressure vessel (bomb) immersed in a water bath. The heat evolved is transferred to the bomb and water bath, resulting in a readily measured temperature rise. A second body of water, called the jacket, surrounds the water bath. Its temperature is adjusted to be equal or nearly equal to that of the water bath in order to eliminate or at least minimize heat losses from the bath.
The calorimeter may be operated adiabatically by varying the jacket temperature during the experiment (by adjusting the relative flow rates of hot and cold water) so as to keep pace with the rising temperature of the inner water bath. Alternatively, the calorimeter may be operated isothermally by keeping the jacket temperature constant. You should approximate adiabatic conditions in this experiment.
The heat evolved during the combustion of an unknown, qv, is related to the temperature rise by
qv = DUcombustion = Cs(T2 – T1) (2)
where Cs is the combined heat capacity
in Joules per degree of the calorimeter pressure vessel (bomb), the water in
the heat bath, and the walls of the water bath. We will measure this
temperature rise with a thermometer in tandem with a computer-based
acquisition. The heat evolved is the sum of the internal energy of combustion
of the sample, DUc,
plus the heat of formation of HNO3 from N2, DUf,
{equation 3 below) plus the heat of combustion of the fuse wire. (Note:
Shoemaker and many other texts use the symbol E for
N2(g) + H2O(l) + O2(g) ® 2HNO3(g) (3)
DH = DU + DngRT (4)
Figure 2. Sowmya Gandham (Gustavus '03) at the
Method:
To determine Cs (the heat capacity of the system), a sample of benzoic acid is burned because it has a well characterized heat of combustion and serves as a standard (D Uc = 26.42 kJ g-1.). The length of the fuse wire is measured before and after combustion [DU = 9.6 J (cm wire consumed)]. The bomb is rinsed with water after combustion, and the washings are titrated with 0.1 M NaOH to determine the amount of acid (assumed all HNO3) present. (D Uf(HNO3) is 5.77 J per ml of 0.1 M base used.) If DT is the temperature rise during the combustion of wb grams of benzoic acid, and combustion of d cm of wire, and the acid titration is V ml of 0.1 M alkali, then
(5)
After determining Cs, you will be ready to measure the heat of combustion of several compounds to determine some of their properties based on this standardization. Bomb calorimetry is also used to determine the caloric content of food, so you should choose some type of food to determine how many "calories" it contains, i.e. measure its heat of combustion (1 kcal = 1 food "calorie"). The internal energy of combustion (DUc) for an unknown (on a per gram basis) can be computed using equation 6.
(6)
Where ws is the weight of the sample all other definitions from equation 5 hold.
The internal energy of combustion computed from equation 6 is without sign, but of course the heat of combustion must be negative since all combustions are exothermic. Be wary of significant figure conventions and convert values from kJ/gram to kJ/mole in consulting the literature and in comparing texts. You should report your internal energy of combustion as a negative quantity. We will determine the heat of combustion for cis-1, 5, 9-cyclododecatriene and a piece of candy of your choice. The heat of combustion determination for cis-1, 5, 9-cyclododecatriene will allow us to investigate the resonance energy of benzene based on the consideration of a thermodynamic cycle described in an accompanying handout.(Anonymous1997) We will determine the caloric content of the candy we examine, so you should choose some type of food to determine how many "calories" it contains, i.e. measure its heat of combustion (1 kcal = 1 food "calorie"). Those with a small water content and containing only carbohydrates work best. Some that have been run successfully in the past include chocolate bars, M&M’s, Reese’s Pieces, and Lifesavers. The link between combustion and our own use of the energy is that the respiration process involves the same net chemical reaction equation.
Procedure:
Follow the steps given below exactly in the correct sequence. As always, wear your safety goggles.
1. Weigh out a sample of benzoic acid or other substance. The sample size must not exceed 1.1 g. Larger samples would evolve more than the calorimeter maximum of 10,000 cal and might cause an explosion.
2. Form the sample into a pellet using the press. It is probably not necessary to form your food sample into a pellet if it is already in compact form or a liquid.
3. Attach a measured length of fuse wire (about 10 cm) between the electrodes. See p. 28 of the Parr Instruction Manual. Let the wire touch the pellet.
4. Place 1 ml of water in the bomb.
5. Carefully lower the head onto the bomb. Turn the screw cap down hand tight; never use a wrench.
6. If this is your first sample, ask your lab instructor for help with the oxygen filling described in the next few steps.
7. Remove the inlet valve thumb nut from the bomb. Attach the filling connection from the oxygen tank. Draw tight by hand (no wrench).
9. Close the control valve (black knob) on the tank regulator.
10. Crack open (1/4 turn) the main valve on the oxygen tank.
11. Slowly open tile control valve. Let the bomb pressure increase slowly to 25 atm. Close the control valve. NEVER CHARGE TO MORE THAN 30 ATM.
12. Relieve the pressure in the connecting tube by flipping the relief valve (underneath the control valve).
13. Remove connecting tube from bomb. (The bomb has a one-way filling valve which automatically closes.)
14. Fill a 2-liter volumetric flask with distilled water. This should be 1-2°C below room temperature. Pour it into the oval bucket.
15. Submerge the bomb in the bucket, attaching the electric cable.
16. Close the calorimeter cover. Carefully lower the thermometers. Make sure both stirrer shafts are properly engaged.
17. Turn on the faucet and the heater. Adjust the temperature of the outer jacket to be equal or slightly lower than the bucket temperature by adjusting the flow of hot and cold water. Turn on the stirrer motor. (You may need to manually start the shaft wheel turning). After a few minutes, readjust the jacket temperature.
18. Take several readings at 30 s intervals until the inner bath temperature is nearly constant. Begin recording with the Labview computer program and the thermistor.
19. Fire the fuse wire by pushing the button, and hold on until the light goes out. Never hold it down for more than a few seconds.
20. Continue to read the inner bath temperature at 30 s intervals after combustion. Examine the plot of temperature vs. time. After firing the bomb the temperature will start to climb and then you should record a fair amount of data to verify that the temperature has leveled out.
21. After completion, raise the thermometers before swinging open tile cover.
22. Take out the bomb. Slowly relieve the pressure by turning tile needle valve with the key.
23. Unscrew the cap. Measure the length of wire left. Rinse the bomb, and titrate the washings with standardized NaOH.
You should do at least two runs for each sample (a total of six runs). Do a third run for as many of the samples as you have time.
1.Rauh, H. J.; Geyer, W.; Schmidt, H.; Geisler, G. Z. Phys. Chemie (Liepzip) 1973, 253, 43.
2.Halpern, A. M. Experimental Physical
Chemistry, second ed.; Prentice Hall:
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