Exp1 Combustion bomb calorimeter PDF

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Experiment 1

Avzianova/Bozzelli

Bomb Calorimetry and Heat (Enthalpy) of Combustion Watch this video! https://www.youtube.com/watch?v=bm3Tn6DXJrI Safety: - Always keep the pressurized reactor (cylindrical, heavy wall vessel) away from you – do not put any part of your body over the top of the bomb when it is pressurized. - Always be sure to close (screw down) the cap before filling - firm but not too tight. - You are working with chemicals: do not spill powder or leave around the balance or your bench area. For these chemicals can clean up with wet paper towels

Introduction Calorimetry is an important field of physical chemistry which deals accurately measuring heats of reaction and finds application in fields ranging from nutritional analysis to explosive yield tests. In this experiment you will use a Parr bomb calorimeter to accurately determine the molar enthalpy change, ΔHcomb, for the combustion of solid and liquid samples.

Theory Heat released in a chemical reaction can be determined experimentally by using a calorimeter. The reaction must proceed without any side reactions and the heat exchange with the surrounding must be negligible. The heat of combustion can be most conveniently measured using an adiabatic bomb calorimeter. In such calorimeter, the combustion reaction occurs in a closed container under constant volume (“bomb”). The bomb is immersed in a weighted quantity of water and surrounded by an adiabatic shield that serves as a heat insulator. The sealed bomb acts as a closed system, and the energy from the adiabatic combustion of a known mass of sample will heat the bomb calorimeter and the measurable amount of water. We assume that no heat is exchanged through the walls of the insulated container, the heat that is exchanged with the water causes a temperature change of ΔTwater in the water. Most elements and compounds react with oxygen, and many of these reactions are highly exothermic, making the measurement of their heats relatively easy. Through the use of a calibration sample of known Hcomb, the heat capacity of the calorimeter system can be determined, allowing the calculation of the heat of combustion of a sample of known mass by the net temperature change. Experimental data is used to determine enthalpy of combustion and enthalpies of formation of the assigned compounds. The most important of these are the standard enthalpies of combustion. The standard enthalpy of combustion for a substance is defined as the enthalpy change, Hocomb, which accompanies a process in which the substance reacts with oxygen gas to form specified combustion products [such as CO2(g), H2O(l), NO2(g), SO2(g)]. Combustion is always exothermic, the enthalpy

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change for the reaction is negative, Hocomb is negative. Heat of Combustion of a substance is the energy released as heat when 1 mole of the substance undergoes complete combustion with oxygen at constant pressure. By definition, the heat of combustion is minus the enthalpy change for the combustion reaction: Hocomb. By definition, the heat of combustion is a positive value. Because most substances cannot be prepared directly from their elements, enthalpies of formation of compounds are seldom determined by direct measurement. Instead, Hess’ law is employed to calculate enthalpies of formation from more accessible data:  Heat of combustion = ∆Hrxn = ∑Hf Products – ∑Hf Reactants

(Eq. 1)

All reactants and products are in their respective standard states at the given temperature T. As an example the standard enthalpy for combustion of benzene at 298.15°K ( Hocomb) can be calculated from standard enthalpies of formation of reactants and products in the reaction:

fHo (H2O(l)) = –285.8 kJ/mol, fHo (C6H6(l)) = +40.9 kJ/mol,  fHo (CO2(g)) = –393.5 kJ/mol.

This is the energy released when two moles of benzene combusts, we want the energy per one mole:

Heat of combustion = Hrxn = (3259) = 3259 kJ/mol In this lab, you will calculate  fHo of the hydrocarbons samples (make sure your equation(s) are balanced). Assume that: fHo values in kcal mol-1: O2(g) = 0.0; CO2(g) = –94.05; H2O(l) = –68.32; H2O(g) = –59.6, then fHo of your target compounds can be determined from Eq.1, using ∆Hrxn (Hocomb) from your experiment. Apparatus 2 of 7

Parr Bomb Calorimeter (clean), ignition power supply; pellet press; one 18-30o C thermometer graduated to 0.01oC; timer; oxygen cylinder; 4-liter beaker (S); 2000 ml graduated cylinder (S), fuse (S), Bomb Calorimeter Manual (M). Tweezers, timer (can use your own, or computer clock, or timer from stock room seconds are sufficient. Chemicals Two standard hydrocarbon pellets for calibration. Hydrocarbon solid and liquid samples. Fusewire having a known heat of combustion per unit length.

Procedures: During two weeks you should perform following experiments: - Two runs of the standards to calibrate the calorimeter system heat capacity. - Two runs of hydrocarbon solid sample. - Two runs of an empty capsule which can hold the liquid sample. - Two runs of hydrocarbon liquid sample. - Two runs of food sample. Detailed description: 1. Clean and dry the Parr type calorimeter and a suitable ignition circuit apparatus. 2. Weigh standard pellet accurately – 4 decimal places if possible (0.75 -0.95 grams is an ideal mass range for all types of pellets, gels and liquid samples). 3. Place the pellet in the cup like holder inside the stainless steel bomb case. 4. Weigh a 10 cm piece of ignition wire. (10 - 12 cm for liquid samples). Cannot be longer than 10 cm. 5. Connect each end of that wire to one of the two terminals (rods with holes in them) inside the stainless bomb case. A tight connection is needed for electrical contact. 6. Bend the wire in such a way that it physically touches the pellet - fairly firmly. (This good physical connection is helpful to ignite – start the combustion process). Suggestion: Place the cup in the lowest position in the holder, then bend the wire to position just barely touching the pellet. Now raise the cup about ¼ to ½ cm in the holder, so the pellet touches the wire more firmly. Keep in mind, wire should not touch the cup. 7. Place 0.25 ml (use drops from your calibrated eye dropper) of H 2O (liquid) in the bottom of the reactor. 8. Place the cup/wire/pellet holder assembly in the reactor vessel, screw on the top holder and tighten the top by hand. (Please be sure to screw on the top). 9. Carry the reactor to the fill station, (Check the top is screwed down) connect reactor to the O2 cylinder (compressed gas cylinder) and flush the case assembly two times and final loading of O 2 as below: Ask TA for help for the first run. a) Fill it with about 10 atm oxygen; b) Then vent, refill 10 atm - and vent again.

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c) The final fill (3rd) is to pressure of 25 atm. (not exceeding 25 atm!) 10. Check the pressurized bomb for leaks at this point by submerging in the H 2O. (If leaks occur, consult your instructor. Use Calorimeter water not distilled. 11. Place 1800 ml of water (we provided a plastic container), at room temperature in the calorimeter can (use bottle provided – fill to very top each time.). New water is needed for each run. Every run you need to add exactly the same amount of water. To have exact amount each time is important for accuracy. 12. Place the can within the adiabatic jacket (it sits on three legs) - the bomb is immersed in the water (the water in the can must cover the bomb). Room temperature water is needed and provided in marked containers; please do not use Distilled Water. Check the label on the water station. Make sure you take the water from "tap water at room temperature for calorimeter experiment". 13. Set up computer thermometer – Read temperature every 5 seconds (12 times min -1, 300 sec). Scale 20 – 30oC. Ask TA for help to work with logger pro for the first run. 14. Connect the two insulated igniter wires. (It does not matter which wire is on which electrode). The outside wire connections should be attached to the appropriate length indicator on the igniter (10 cm) and the ground terminal. (Connections should be firm – not loose – this is an low voltage electrical connection). 15. Mount top cover with the stirrer closest to the stirrer motor. Turn stirrer by hand to see that it turns freely. 16. Mount the stirrer belt (motor to stirrer wheel), start the motor and verify stirrer wheel is turning. Attention: The heavy wall bomb case should be away from the stirrer motor in the bucket, but still about 2 cm from the wall of the steel calorimeter water bucket – the thermometer will need to go in this space. 17. Start monitor temperature every 5 sec (with Computer thermometer) for ~ 5-6 minutes total (1.5 – 2 minutes before ignition to have good temperature slope (T change with time) pre-ignition slope. If not using the Computer temperature monitor, start temperature readings, every 20 seconds, for two minutes before ignition to obtain a baseline. Continue temperature measurements every 20 second. 18. a) A temperature rise (about 2 - 4°C), indicates successful ignition. b) Continue temperature readings every 15 - 30 seconds for about 2- 3 minutes to a get second base line. c) The difference in temperature between the two base lines (perpendicular) is your ΔT. (Ideally base lines should be parallel. The baseline will have no slope if water in the can is at room temperature. If water is at a different temperature than the room, then there will be a positive or negative slope to each line. 19. Upon completion of the run: a) Remove the bomb from the can. Carefully release the pressure by slowly opening the valve on top of the bomb (the heavy wall calorimeter bomb can be in the mount on the table next to O 2 for added safety). 4 of 7

b) Only when vented - open the reactor, remove the sample holder and place it in the mounting bracket for re-assemble with a new sample. c) Weigh the remnants of the ignition wire to determine the mass that has combusted (initial mass minus final mass). d) Clean the inside with a paper towel and prepare it for the next run. 20. For a solid sample, ask TA to illustrate how to make pellet for the first sample. 21. For a liquid sample you need to weigh the capsule, then add the liquid to the capsule

in hood – half fill the capsule only) close, and weigh the (capsule + liquid) again. You need to know mass of empty capsule and mass of liquid for calculations.

Solid/liquid/food Sample Runs - Two runs (duplicates) on one solid hydrocarbon – make two pellets – weigh each pellet, run calorimeter expt. - Two runs of empty, weighed, capsule to determine heat capacity of the capsule material per gram. - Two runs of liquid hydrocarbon samples – fill capsule in hood – tie ignition wire in a knot around capsule. - Two runs of your food. Chocolate will be the best choice. Treat it as the solid sample and run. (Other food may also be okay, like peanut, chips, French fries, etc...) Notes:  If gas bubbles escape, the assembly ring may require tightening, or the gaskets may need to be replaced.  It is important to have the same amount of water for each run, because this quantity of water is part of the system heat capacity; you need to keep Cp constant once it is determined.

Data analysis (1) Perform experimental data in the table 1: Table 1. Experimental data Sample Mass of sample Initial mass of (g) wire (g)

Final mass of wire (g)

Mass of burned wire (g)

T°C

(2) Plot temperature versus time and find temperature rise ΔT for all runs. If your curve after combustion decreases slowly with increasing time, use the maximum temperature that you get from the plot.

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Fig. 1. An example of temperature versus time data plot (3) Determine the average heat capacity (Cp, cal/°C) of the calorimeter assembly using the known Hcomb of standard and wire Hcomb (1400 cal/g). Check Hocomb of wire on the wire packing. Example of calcultions: If you use naphthalene as a standard, H of naphthalene = 9560 cal/g, then Hcalorimeter = Cp  T Hcalorimeter = Hstandard + Hwire Hstandard = (9560 cal/g)  mstandard Hwire = (1400 cal/g)  mwire burned (4) Calculate the heat of combustion of your solid sample using this Cp value. Heat of combustion is a positive value. Hcalorimeter = Hsample + Hwire Hcalorimeter = Cp  T Hsample = (Cp  T) – Hwire

(cal)

(5) Calculate Hsample in cal/g. (6) For liquid sample: calculate the heat of combustion of empty capsule from experiments with empty capsule Hcapsule = (Cp  T) – Hwire (cal) (7) Find average Hcapsule (8) Calculate the heat of combustion of your liquid sample. Use average Hcapsule and mass of capsule that you used in experiment with liquid sample. Hsample = (Cp  T) – Hwire – (Hcapsule)  mcapsule (cal)

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(9)

Find mass of liquid sample used (take into account the mass of capsule before loading of liquid). Calculate Hliquid in cal/g.

(10) Find the number of moles of samples burned. Calculate the molar enthalpy of combustion in kcal/mol for all compounds. Pay attention to the sign of the value. (11) Calculate enthalpy of formation fHo(298) using a balanced reaction and the individual fHo of the components in the reaction. Important: equation must be balanced to have 1 mol of sample! Please note, other stoichiometric coefficient can be fractional. –ΔHcombustion (heat of combustion) = ∆Hrxn (enthalpy of combustion) = ∑Hf Products – ∑Hf Reactants

Error Analysis Error analysis should include: (i.) the Cp or specific heat capacity of the calorimeter (ii.)

ΔHcomb

(iii.)

ΔfHo(298)

Report: 1. 2. 3. 4.

Table 1. Average heat capacity of calorimeter Cp. Show calculations. Temperature versus time data plots. Average molar enthalpy of combustion ΔHcomb of target compounds in kcal/mol. Your target compounds – solid sample, liquid sample. Show calculations. Do not calculate ΔHcomb of standards! 5. Enthalpy of combustion of food sample in kcal/g units. Compare experimental data with nutrition information written on the wrapper. 6. Average enthalpy of formation ΔfHo(298) target compounds in kcal/mol. Show calculations. Show balanced reactions for combustion of the samples, remember, that equation must be balanced to have 1 mol of sample. 7. Compare your results with literature data (units should be the same!). Calculate uncertainties and show these values in the table 2. Table 2. Results ΔfHo(298), kcal/mol Literature Sample ΔHcomb, kcal/mol Literature ΔfHo(298) ΔHcomb

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