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Bomb Calorimetry
Chemicals used:
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Benzoic Acid Description: white solid Risk: eye, skin and respiratory tract irritant, harmful if ingested Precautions: avoid contact with the skin Disposal: solid waste container provided. |
Naphthalene Description: white solid Risk: flammable, harmful by ingestion, inhalation and skin contact. Eye irritant Precautions: avoid contact with skin Disposal: solid waste container provided. |
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General Hazards: ◆ In this experiment, you will work with a high-pressure bomb calorimeter. Always fire the bomb with it firmly held in the screened, clamp holder on the bench. It is important that the bomb is correctly assembled with the heavy locking ring which holds in the bomb head assembly, correctly seated. You must obtain a demonstrator’s signature twice: 1) before you pressurise the bomb, to ensure that the bomb head is correctly seated, and 2) after you load the bomb into the calorimeter to ensure there are no leaks. ◆ When the experimental run is completed and the bomb is removed from the calorimeter, it is still under high internal pressure. The pressure is released through finger grip rotation of the knurled knob on the release valve. Only then can the bomb be disassembled. |
1. Introduction
In this experiment the heat of combustion of naphthalene is determined by rapidly burning a known mass of sample in oxygen, under pressure, in a heavy steel vessel (bomb). From the measured heat of combustion other thermodynamic quantities are calculated. The heat of combustion, as determined with an oxygen bomb calorimeter, may be defined as the heat liberated by complete combustion of a substance, with oxygen, in an enclosed vessel of constant volume. In the reaction the substance and the oxygen are initially at the same temperature, the products of the combustion are cooled to within a few degrees of the initial temperature, and the water vapour formed is condensed to liquid.
The calorimetric procedure involves burning a weighed sample in the oxygen-filled bomb submerged in a measured quantity of water, all held within a thermally insulated chamber. The rise in water temperature resulting from combustion of the sample can be used to calculate the amount of heat liberated.
The bomb is filled with oxygen under high pressure, and the combustion reaction takes place with explosive violence although there is no external evidence of such a reaction. The heat of combustion causes the temperature to rise very rapidly, with a corresponding increase in gas pressure. The increased pressure is reduced quickly as heat is dissipated by the bomb to the surrounding medium, but the cooling rate is so much slower than the heating rate that internal pressures remain high. Since its invention, the oxygen bomb calorimeter has been accepted as the standard for accurate measurement of heats of combustion despite mechanical problems imposed by this condition.
2. Theory
2.1 Calibration
The combined heat capacity of the calorimeter is determined by burning a known weight of benzoic acid (heat of combustion -26.435 kJ g-1) which is the internationally accepted laboratory standard for bomb calorimetry. A sufficient number of calibration runs is carried out so that consistent values for the heat capacity of calorimeter is obtained.
2.2 Calculation of △T for calorimetry experiments
When determining ΔT for an experiment, the effect of background heat exchanges between the bomb and the surroundings must be compensated for by extrapolating from the linear temperature-time change before and after the main temperature rise (e.g. you will need to mathematically determine ΔT and the associated uncertainty).
A numerical calculation can be made by identifying 3 key time points (a, b, and c) and their associated temperatures (Ta – Tc, °C) from your calorimetry plot:
• a = time of firing
• b = time when temperature reaches 63.2% of the total rise (This value is calculated using the rate constant of the exponential curve observed after firing and corresponds to the exponential halfway point of the rise based on integral calculus, which is
beyond the scope of this course).
• c = time at beginning of period when the rate of temperature change is linear
You will also need to calculate the following rates of change of temperature:
• r1 = the rate of change of temperature during the 5 minutes before firing.
• r2 = the rate of change of temperature during the 5 minutes after time c.
To determine these parameters from your data, use the exported .CSV file to plot temperature vs time graph for each experiment (Figure 1). See 6.2 in the laboratory manual for instructions on how to plot your data.
You will need to fit two linear lines to your data to obtain r1 and r2 . See 6.5 in the laboratory manual for instructions on how to split the data into two series and then 6.3 and 6.4 for how to fit linear regression to each set of points and conduct regression analysis to obtain the uncertainty in these extrapolated parameters.
The parameters can then be read from the fitted line equations obtained from the regression analysis.
Figure 1: Determination of parameters from bomb calorimetry experimental data.
The temperature rise (ΔT) is given by equation (1) with parameters obtained as illustrated in Figure 1.
ΔT = Tc - Ta - r1(b-a) - r2(c-b) (1)
2.3 Calculation of heat capacity of water using benzoic acid calibration.
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Note: • subscript. c, e.g. mc, indicates this value refers to the calibration species e.g. benzoic acid • subscript. s, e.g. ms, indicates the sample e.g. naphthalene. |
The energy (in J) liberated by combustion of mc g benzoic acid, using lc cm offuse wire burned, is:
qc = 26.435× 103 mc + 9.6 lc (2)
The energy liberated is absorbed by the water so that:
qc = ΔTc C (3)
where ∆ Tc is the temperature rise and C is the heat capacity of the water and bomb system.
2.4 Heat of combustion
This determined C can then be used to determine the total energy of combustion of your reaction. For a temperature rise ∆ Ts :
qs = ∆ TsC (4)
The total energy of combustion can then be separated into the naphthalene and wire components:
qs = qs(naphthalene) + qs(wire)
qs = -∆U ms + 9.6 ls (5)
Where ∆ U is the heat of combustion of naphthalene in J g-1, ms is the mass of the naphthalene sample in g, and ls is the length of the fuse wire burned.
The heat of combustion is determined for samples of naphthalene, i.e. the change in internal energy ∆ U for the reaction:
C 10H8(s) + 12O2 (g) → 10CO2 (g) + 4H2O (l) (6)
The enthalpy change for the reaction at constant pressure is:
∆H = ∆U + p∆ V (7)
where ∆V is the volume change for the reaction carried out at constant pressure. At constant temperature, T, and assuming ideal gas behaviour:
p∆ V = ∆nRT (8)
where ∆n is the change in the mole number for the gases only involved in the reaction (i.e. the number of moles of gaseous products minus the number of moles of gaseous reactants). Combination of equations (7) and (8) gives the enthalpy of combustion at temperature T. To a good approximation, the enthalpy of combustion calculated in this way can be equated with the enthalpy of combustion at 298 K, i.e. ΔCH298.
2.5 Heat of formation
The standard enthalpy of formation of naphthalene, i.e. the enthalpy change at 298 K and 101 kPa for the hypothetical reaction (6) is obtained using Hess's Law, from ΔCH298 and the enthalpies of formation of CO2 (g) (-393.5 kJ mol-1) and H2O (l) (-285.8 kJ mol-1).
10C (graphite) + 4H2 (g) → C 10H8(s) (9)
2.6 Delocalisation energy
The delocalisation energy is the difference between the enthalpy of atomisation evaluated from the enthalpy of formation, and the enthalpy of atomisation calculated from bond energies assuming 6 C-C single bonds, 5 C=C double bonds, and 8 C-H bonds. The following data for enthalpies of formation of gaseous atoms, and bond energies are required:
1/2 H2(g) → H (g) ΔfH = 218.0 kJ mol-1
C (graphite) → C (g) ΔfH = 715.0 kJ mol-1
bond ΔH / kJ mol-1
C-H 416.3
C-C 347
C=C 611
The enthalpy of sublimation of naphthalene at 298 K is 72.5 kJ mol-1. The delocalisation energy calculation is illustrated below for the case of benzene.
Assuming a structure of 6 C-H bonds, 3 C-C bonds and 3 C=C bonds, the estimated enthalpy of atomisation i.e. the enthalpy change for the reaction is 6×416.3 + 3 ×347 + 3×611 = 5372 kJ mol-1.
C6H6(g) → 6C (g) + 6H (g) (10)
The “experimental” value is obtained from
C6H6(l) → 6C (graphite) + 3H2 (g) ∆H = -∆fH = -49 kJ mol-1
C6H6(g) → C6H6(l) ∆H = -∆vapH = -33.8 kJ mol-1
6C (graphite) → 6C (g) ∆H = 4290 kJ mol-1
3H2 (g) → 6H (g) ∆H = 1308 kJ mol-1
C6H6(g) → 6C (g) + 6H (g) ∆H = 5515 kJ mol-1
The delocalisation energy (ΔdelocE) is therefore 5515 - 5372 = 143 kJ mol-1, i.e. the benzene molecule is 143 kJ mol-1 more stable than it would be if it had the Kekulé alternating single and double bond structure.
3. Experimental Procedure
The following procedure is used for both calibration (using benzoic acid) and test (using naphthalene) runs. At least two calibration runs and at least two test runs should be completed during this experiment, however, more maybe completed if time permits.
3.1 Sample preparation
Place approximately, but no more than 0.9 g of benzoic acid (or 0.6 g of naphthalene) in the pellet press. This sample size is sufficient to produce a temperature rise of a few K. Push the lever down firmly to form. a dense pellet. To remove the pellet from the press, lift the lever and turn the bottom plate over, revealing a cavity. Press the lever down again and the pellet should be expelled into the cavity. Precisely weigh and record the mass of the pellet.
You can change the base level of the press by screwing/unscrewing it by hand, which will be necessary to do throughout the experiment to make sure the pellet die is high/low enough to fully press your pellet (if you cannot get your pellet to stay solid after pressing, make sure the base is high enough).
3.2 Bomb loading
With the bomb head in its holder, place the precisely weighed pellet in the metal combustion cup. Cut a 10-12 cm length of fuse wire (record the length used) and thread it through the eyelets on the electrodes that are revealed when the metal hoods are raised. Secure the wire on the electrical contacts by pulling down the metal hood over the connection. Form. a loop in the middle of the taught wire which presses against the top of the sample pellet, refer figures 1 and 2.
Figure 2: Attachment of the fuse wire. The caps on the electrodes ensure good electrical contact.
Figure 3: Schematic of the bomb head. Note positioning of the fuse wire, touching the sample but not the cup.
The main reason your bomb may fail to fire later is because your wire was not in sufficient contact with the pellet during this step, or the pellet shifted during the handling of the bomb and is no longer in contact with the wire. So, make sure to take extra care during this step and later when moving the bomb.
Add approximately 1 mL of deionised water into the bottom of the bomb to absorb any acid formed (have a think about what this acid might be). Ensure that the gas relief valve is open and fit the bomb head into the cylinder, taking care not to disturb the sample. Close the bomb by screwing the retaining ring down and tightening by hand as tight as possible.
At this point you must have your demonstrator or Dr Ware sign off the safety sheet before you can proceed. This is to ensure the retaining ring is as tight as possible. Ensure the safety sheet is fixed into your lab notebook.
After you have been signed off, close the gas relief valve (finger tight only – do not overtighten, as this will ruin the seal over time) and then transfer, using the bomb lifting tongs, to the bench with the oxygen cylinder (try keep the bomb as level as possible to ensure the wire and pellet stay connected). Attach the slip connector to the oxygen filling tube on the top of the bomb. Pressurise the bomb to 30 atmospheres (note that if your gas relief value is not fully closed, you can hear and feel a large release of gas from the valve, and this will result in a failed pressurisation which you will then have to undergo again). After pressurisation, remove the filling tube and return the pressurised bomb to the bench with your calorimeter. Do not open the gas relief valve after you have successfully pressurised your bomb.
3.3 Calorimeter preparation
Measure 2000 mL of deionised water and pour it into the stainless-steel bucket inside the calorimeter. Note that it is assumed in the calculations below that a constant volume of water was used in the calorimeter bucket, for all calibration and test runs. Care must be taken to ensure that minimal water is lost when transferring the bomb into and out of the calorimeter (consider methods to reduce water loss when removing or placing the bomb).
Using the bomb lifting tongs, place the bomb in the water over the circular protuberance in the bottom of the bucket. The bomb should be completely immersed: if it is not, there is insufficient water in the bucket. Check that there are no bubbles coming from the bomb pressure release valve as this would indicate an oxygen leak (note that bubbles may emerge from holes in the retaining ring due to trapped air, but this is not an issue).
At this point you must have your demonstrator or Dr Ware sign off the safety sheet before you can proceed. This is to ensure there are there are no oxygen leaks from the bomb before the reaction.
If there are no leaks and you have been signed off, connect the two electrical leads to the electrode terminals on the bomb head (you may use a multimeter to test the electrical contacts of your bomb electrode terminals before connecting the leads if you are worried that your wire is not secured fully). Fit the calorimeter lid with the diode thermometer probe and stirrer, making sure the arrows on the calorimeter lid and stirrer motor line up. Connect a rubber band to the stirrer motor and turn it on by turning the dial in the direction of the arrow. Check that the stirrer rotates freely. Place the Perspex enclosure over the calorimeter with wires passing through the cut-outs on the back bottom sides of the shield.
The accuracy of the experiment depends heavily on accurate determination of ∆ T, which is measured with a diode thermometer in conjunction with a data logger and PC.
3.4 Temperature change measurements
Start the recording and run the stirrer to establish thermal equilibrium (up to about 3 minutes should be sufficient). Refer to the instructions provided for use of the computer acquisition software for recording temperature rises, or consult a demonstrator or the Laboratory Technician. Once readings have been taken for at least 3 minutes, ignite the bomb by holding down the ignition button for 5-10 sec (holding down this button completes a circuit which heats up the fuse wire. If the button is not held down for long enough, the fuse wire will not heat up enough to ignite the pellet.) The temperature will quicky rise, and then level out and become linear again. Continue to take readings for at least 5 minutes after the temperature has become linear. You will be using the linear slopes measured from before and after firing to analyse your data, so it is important to record these linear sections for reasonable amounts of time.
If your temperature readings have still not changed a minute after you have stopped holding down the ignition button, attempt to use the ignition button again. If your bomb has still failed to fire after your second ignition attempt, ask your demonstrator for assistance. You may need to dismantle your pressurised bomb and attempt the whole set up again.
Before collecting data for your next measurement, make sure to export and reset the already collected data.
3.5 Opening the bomb
Carefully remove the Perspex shield. Turn off the motor, remove the rubber band and carefully remove the calorimeter lid and place it back on its stand. Disconnect the electrical leads from the bomb and lift the bomb out of the bucket with the bomb lifting tongs. Pour any water from the top of the bomb back into the bucket (Note: Before firing, it was important to keep the bomb as level as possible to ensure the wire and pellet stayed connected. However now, it is important to return as much water as possible to the bucket, which you can do by fully lifting the bomb out of the bucket and then tipping it on its side while holding it with the tongs.) After you have recovered as much water as you can, return the bomb to the bench and proceed to dry the outside with a towel.
Release the pressure in the bomb by slowly opening the gas relief valve (venting should take longer than one minute). Unscrew the bomb head retaining ring after the pressure has been released.
Prepare to remove the bomb head by holding the bomb body flat and steady with one hand. With your other hand, pinch the electrode terminals between your thumb and index finger. To remove the head, gently rock it side to side using the hand that is holding the terminals, while keeping the body steady with your other hand. When the head is removed, return it to its stand. Inspect the bomb for soot or other evidence of incomplete combustion: if such evidence is found, the results are unreliable, and the experiment should be repeated. Record the length of any residual fuse wire. Subtract this length from the initial length to give the length of fuse burned.
Clean out the body of the bomb with deionised water and dry. The surfaces of the bomb affected must be kept scrupulously clean and every precaution taken to avoid blemishing and damaging them (as this could result in the bomb becoming unusable). Always place the bomb head on its stand and the bomb body on a towel when not in use.
Results
Export your temperature vs. time results as .CSV files. Email these files to yourself and your partner to be used in your plot to find ∆T.
4. Report
A Laboratory Report template for this experiment (Experiment 1) is available as a Word file through Canvas. Please complete all sections of the template then convert your resulting Laboratory Report into a PDF file before submission through Canvas. Much of the report involves rearranging equations, so double check your working and make sure to include appropriate units for your answers and during your calculations.
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