Heat of combustion


Procedure:

You will use a Parr bomb calorimeter to measure heat of combustion of The procedure for preparing the samples and carrying out the measurements is very similar to that described by SGN but you should instead follow the procedure described in the Parr manual that comes with the equipment (follow this procedure rather than the one described in SGN). The manual can be found in a folder on the shelf above the instrument. A scanned copy in jpeg format can be downloaded here, page by page (each page is 200 kB, page 1 is skipped)

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    Read the description of the experiment in SGN to get theoretical background and discussion of the analysis.

    It is important that the instruments warm up for at least 30 minutes before measurements are made. Turn on the paper recorder and set the knob on the calorimeter to the "zero" position (switch it from the "off" position). Set the temperature knob on the calorimeter to 20 degrees. Set the adjustment of the recorder to 1 volt while you set the zero. Set the voltage on the paper recorder to 0.1 V before taking the measurements so that a temperature rise of 1 degree spans nearly the full width of the paper. If during the measurement the pen goes all the way to the right, increase the setting on the calorimeter (for example from 20 to 21 degrees) until the pen moves away from the right edge of the paper.

    Weigh the wire on the balance with high precision (0.1 mg) befor and after the combustion (do not measure the length, that is not precise enough). It is convenient to use a clamp on a stand to hold the lid of the bomb while the wire is thread between the contacts. The part of the wire that burns forms metal oxide which appears as black spherical particles. That needs to be removed before weighing.

    You need to weigh 466 g of distilled water to put into the Dewar of the calorimeter. This constitutes the heat bath. Make sure the distilled water is warmer than 20 degrees Centigrade, otherwise the initial temperature of the water bath cannot be measured (it tends to be colder if it has just been produced and has not been sitting around for long enough). Use the less precise Mettler PM4000 balance. The water needs to be replaced for each measurement, its temperature rises by about a degree during the combustion. Dry the Dewar with paper before weighing in the water.

    The samples should be made into pellets (tablets) to make it easier to handle them. The instructions for preparing the pellets is given here. The amount of benzoic acid should be around 0.07 g (not more than 0.08 g). Put the cylinder for making the tablet as well as the funnel on the course balance beside the tablet press. Weigh in about 10% more than you want the tablet to weigh to account for some losses as the tablet is made. After forming the tablet by pressurizing the sample, the larger cylinder is then turned around to press the tablet out. Check the weight of the pill roughly and scrape it if it is too heavy. The tablet then needs to be weighed on the more precise balance in the crucible of the bomb (zero the balance with the empty crucible in it). Do not touch the tablet or leave it lying around after measuring its weight. Place it in the bomb right after it has been weighed. The amount of naphthalene should be about 0.040 - 0.045 g (not over 0.05 g). The samples of the "squeezy powder" should weigh around 0.10 g.

    Read carefully the instructions for working with gas cylinders in SGN (in the chapter on "Miscellaneous Procedures") before coming to the lab. Practice opening the oxygen cylinder before the bomb is closed. The pressure should be set to 35 psi (correct for the zero setting). It is important to continue recording the temperature well after the combustion reaction has taken place, for at least 4 minutes, so that a straight baseline has become clear.


    Analysis:

    Use the method described in the manual (see figure 5) to determine the temperature rise from your measurements. The part of the manual that explains the procedure is reproduced here. A similar method is described in SGN (see fig. 3b) and could be used as well. For each one of the three measurements of benzoic acid, obtain an estimate of the heat capacity of the instrument. Show detailed calculations for one of the three. How important is it to include the heat of combustion of the wire? Use an average over your measurements of benzoic acid to get the best estimate of the heat capacity of the instrument and use that in the analysis of subsequent measurements (the naphthalene and the food supplement).

    From each of the measurements of naphthalein and the food supplement, calculate the change in internal energy upon combustion. Compare the values you get for napthalene with tabulated values (one source of this information is the CRC handbook). Compare the values you get for the food supplement with the nutritional statement on the jar. Also calculate the change in enthalpy upon combustion assuming the ideal gas law is obeyed.

    Use the measured heat of combustion as well as known values for the formation energy of the combustion products to estimate the formation energy of naphthalene. Write a balanced chemical equation to illustrate the relationship between the various quantities.

    Estimate the order of magnitude of the error in each aspect of the measurements and calculate how big an error in the results is caused by these. Estimate the error introduced by using the ideal gas law in the calculation of the enthalpy change. You can use the van der Waals equation with constants given in chapter one in the text book by Silbey et al.