Calorie Content of Fuels

In this exercise, students determine the energy content of methanol and paraffin wax (candles) as models for comparison with the calorie contents of carbohydrate and fat, respectively.

Heat sources are small candles in watch glasses and alcohol lamps made from 50-mL flasks with a wick passing through a glass tube in a rubber stopper.  The masses of the heat sources are determined to the nearest 0.01 g before and after heating as described below.

Energy contents are determined by warming 100 mL of water an aluminum drink can hung by a paper clip from a ring on a ring stand. Students are cautioned to keep flames close to the bottom of the can, but not touching so that no soot is deposited. In order to compensate for errors due to heat gain from, and subsequent loss to, the room air, the water is first cooled to around 15 degrees Celsius below room temperature. It is then heated to around 15 degrees above room temperature before stopping. From the temperature change and the mass of water, the amount of heat gained by the water can be calculated. Division by the change in mass of the heat source gives the heat content of the fuel.

Finally, students are asked (1) how their determined heat contents compare with those of carbohydrate and fat, and (2) to describe possible sources of error in the exercise (an appropriate question since students' values invariably come out to less than half of the values tabulated in references.).

Revised 9/21/06