Prelab Report

by My Name

Student ID #

Heats of Combustion

Teaching Assistant: TA Name

Chem 374-005

Group #6

10/31/96

Lab Partner: Their Name

Introduction & Theory

Heats of combustion is the amount of heat energy released by a substance when it is completely burned. To study heats of combustion, a calorimeter is employed to determine the change is temperature when a given amount of the sample is burned. By measurement of the change in temperature of the sample in comparison with a known reference substance, the number of calories (an energy unit) released by the combustion of the sample may be determined.

Goal

The goal of this experiment is to determine the heat of combustion of naphthalene through the use of a bomb calorimeter.

Apparatus

A bomb calorimeter is used, with the outer jacket left empty to serve as an insulating layer. The calorimeter has a stirrer powered by an electric motor, an optional thermometer to measure "ambient" temperature in the jacket, a precision thermometer (graduated to at least 0.02C) with range from 19 to 35C, bomb, pail, jacket, insulating table, cover, 110 volt power supply, ignition unit and an ignition lead that passes from outside the calorimeter to the top of the bomb. The bomb (which sits inside the pail) is made of a nickel alloy with leads running into it such that an electrical charge may be used to ignite the sample.

A bomb assembly stand is required, along with a compressed oxygen cylinder with appropriate fittings, large pail of water, 5 grams benzoic acid, 5 grams sample (naphthalene or other substance), pellet press, spatula, 0.004 inch diameter iron wire (50 cm), and an optional continuity tester.

























Figure not available

Figure 1 - Bomb Calorimeter













Figure not available

Figure 2 - Parr bomb

Experimental Observables

In this experiment, the temperature change in the water bath surrounding the bomb will be measured. Ambient temperature will have an effect on the results (as ambient air fills the insulating jacket), however ambient pressure should not be a factor.

Experimental Method

Samples to be tested are pelletized, fused to an iron wire, and fastened inside a Parr style bomb, which is in turn placed into a calorimeter. After the temperature of the bomb and its contents are in equilibrium with the calorimeter, the sample pellet is ignited by an electric current through the iron wire, and the temperature increase measured. From the temperature increase the amount of energy released by the combustion of the sample is determined, the heat of combustion may then be calculated from this quantity and the mass of sample used.

Experimental Procedure

Approximately 0.5 g samples (0.8 g for benzoic acid) of material are pressed into pellets, fused to a accurately known mass of iron wire (~5 cm) with a 1.5 volt dry cell, and the combined mass is then determined accurately. By subtraction, the amount of sample is then known.

The bomb should be inspected to ensure it is clean and dry, and that the terminals are clean. The pellet should be placed inside the bomb, over the pan and between the terminals. The wire should be wrapped around the terminals, and care should be taken so that the wire does not touch anything except the terminals. Assemble the bomb and screw the top on only hand tight. Slowly fill the bomb with oxygen to 380 psi, then vent to expel atmospheric gases, and fill to 380 psi again. Never fill to more than 450 psi. Possible leaks (1 bubble every 10 seconds or less is of no consequence) should be checked in a large pail of water, with common causing being poor seating of the bomb top, and poor condition of the O ring and closure surfaces.

The pail and bomb should be dried if necessary, then place the bomb in the pail and the pail in the center of the calorimeter. Connect the ignition lead securely to the top of the bomb. Using a 2-L volumetric flask, fill the pail with exactly 2-L of water at 25 C (distilled water is preferred). Ensure that the flask is completely drained.

The lid should be placed on the calorimeter, and the precision thermometer should be gently but securely clamped as low the temperature scale will allow. Test the ignition circuit for continuity, and turn on the stirrer. With all switches off and the power supply unplugged, attach the ignition circuit to the calorimeter. Then plug the power supply into a 110-V source.

Begin taking time and temperature readings every 30 seconds, tapping the thermometer before each reading. If feasible, estimate to the thousandths of a degree. Continue taking these readings throughout the run. When the readings change at the rate of approximately 0.001 K min -1, the bomb may be ignited by briefly turning the ignition switch on then off. Notice the time required for the temperature change to become slow and steady, and continue taking readings for this time fourfold at least.

Remove the bomb, release the pressure, and open the bomb. Remove and weigh any unburned wire and "globules" that can not be crushed. Subtract this mass from the initial mass of the wire to determined the mass of wire that burned. If soot is present, the amount of oxygen was insufficient and the entire run must be discarded and done again. Dry the assembly.

Experimental Precautions

Preliminary Calculations

The heat capacity of the entire calorimeter must be determined by combustion of a sample with a known heat of combustion. The change in energy would be equal to the heat of combustion of the substance. The heat capacity may then be determined as follows:

Once the heat capacity of the calorimeter has been determined, the heats of combustion of unknown substances may be determined. The heat released by a reaction may then be determined with the following formula:

The net change in energy for the reaction may then be found by subtracting energy introduced to the system by the electrical charge, burning iron, etc.

Safety 1

Care must be taken to avoid contact with the terminals while the equipment is plugged in. Exercise caution when heating exposed wire, as it heats extremely rapidly, possibly causing burns or igniting flammable items in the surroundings. In the event of a shock, turn off or unplug the power supply. Never try to remove a person from the source of the shock while power is still applied. Check vital signs and administer CPR as necessary. Monitor victim for signs of clinical shock (faintness, chills, disorientation) and treat as necessary. Treat electrical burns as you would any other burn; use no salves or ointments. Seek medical attention if shock is severe or has any lasting effects.

Do not pressurize the bomb above 450 psi, as this presents an explosion hazard. In the event of explosion related injuries, apply direct pressure to the areas bleeding heavily. Do not attempt to remove deeply imbedded fragments. Do not apply a tourniquet unless trained in their use to avoid unnecessary amputation of limbs.

Mercury and mercury vapor are highly poisonous. Inhalation of vapor may lead to fever, nausea, vomiting, diarrhea, headache, chest pain, and possibly death. Skin contact may lead to a rash or allergic reaction, and if extensive may also cause the same effects as inhalation of vapor. Affected persons should be removed to fresh air and contaminated clothing removed. Necessary first aid techniques should be performed. Seek medical attention immediately.

May irritate mucous membranes if inhaled. If ingested, may cause abdominal pain, nausea, vomiting or diarrhea. Dilute with several glasses of water. Seek medical attention. In the event of contact with eyes, flush with water for at least 20 minutes and seek immediate medical attention.

May irritate mucous membranes if inhaled, as well as cause headache, or nausea; remove victim to fresh air. If ingested may cause extreme gastric discomfort, remove by gastric lavage or emesis; seek medical attention. In the event of eye contact, flush with water for at least 15 minutes and seek immediate medical attention.

References

  1. Safety data taken from MSDS database on the Internet at "http://hazard.com/msds".
  2. D. P. Shoemaker, C. W. Garland, J. W. Nibler, Experiments in Physical Chemistry, 6th ed., chap. 6, experiment 6, The McGraw-Hill Companies (1996).