Oxygen Bomb Calorimetry

You will be measuring the heat of combustion, DH°_c, for each of the two esters synthesized in the prior portion of the experiment.

The stoichiometry of the combustion is:

C₈H₁₄O₂ + 21/2 O₂     --->      8CO₂ + 7H₂O

The difference between the two compounds should be (to a first approximation) the strain energy in the cyclopropane ring.

Q.  Which of the two compounds will give off the most heat when combusted?  Why?

Diagram of the calorimeter apparatus.
 

The bomb	The bomb and its associated water jacket, stirrer and thermometer

The general procedure is to:

• accurately measure a small amount of ester into the combustion capsule,
• attach a weighed piece of iron fuse wire,
• seal the sample in the bomb,
• flush the bomb twice with O₂ and pressurize to 30 atm,
• place the bomb into 2.000 L water at 24°C
• record the water temperature at 30-second intervals for 5 minutes,
• ignite the sample by passing electric current through the fuse, record the temperature to 0.01 every 10 sec until the temperature rise starts to slow down, then
• record the water temperature at 30-second intervals for at least 20 minutes after you pressed the button.
• Finally, measure how much of the fuse wire was consumed (by mass difference), and whether there was any soot produced.  Wipe the combustion capsule with a kimwipe and reweigh it - the mass should come back to the starting mass for the empty crucible.  If not consult the instructor.

You should watch the slide presentation on use of the bomb calorimeter to become familiar with details of the procedure.  We will try to get this info onto the Web page at some time in the future.  For now, you may follow the links below to what we have.

Use of the Bomb Calorimeter
Finding the calorimeter constant (combustion of methylsalicylate)
Analysis of Data
Propagation of Error

Sample data:  Excel format

Temperature correction spreadsheet files are posted here.

The thermometers you use are calibrated on the assumption that the entire mercury column is submerged in the liquid.  Since you do not do this, contraction of the glass between the surface of the water bath and the top of the mercury introduces a systematic error.  Proper use of the T correction in these files will remove that error.

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