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Experiment 10: Bomb Calorimetry
Equipment: Bomb calorimeter Fuse Wire Thermometer for Bomb Calorimeter Pellet press Consumables: Benzoic acid Oxygen cylinder Cashew (or M&M's)
Purpose- To calibrate a bomb calorimeter and then use the calibrated bomb calorimeter to determine the calorie content of a cashew.
Background: One of the most important types of reaction studied by means of calorimetry is combustion. We can measure the amount of heat from a combustion reaction by measuring the effect the heat produced from the reaction has on a container surrounding the reaction chamber filled with water. This apparatus is called a bomb calorimeter, and is composed of an insulated container of water in which there is a metal "bomb" (reaction chamber), a stirrer and a thermometer (Fig 1). The bomb is a strong metal container that can withstand the high pressures associated with the rapid combustion of a material and transmit the heat to its surroundings very quickly. Inside the bomb is a sample holder, an electrical ignition system to start the reaction and enough oxygen gas to achieve complete and rapid combustion.
Theory : To calculate the heat of combustion, the sample is placed in the sample holder and sufficient oxygen is pumped into the chamber to allow for complete combustion. The reaction chamber is then immersed in the water, and the initial temperature checked over a length of time to ensure that the temperature is stable before beginning the combustion reaction. After the combustion reaction is performed, the final temperature is recorded. The increase in temperature corresponds to the amount of heat generated (evolved) by the combustion reaction.
To calculate the heat of combustion from the increase in temperature of the water, we need to first find the relationship between the temperature change we measure in the experiment and the heat produced from a reaction. This is the heat capacity (Ccal) of the whole calorimeter, including the bomb, water, stirrer, thermometer and calorimeter casing. The process of finding Ccal is called calibration. We do these by combusting a sample of benzoic acid which produces a known amount of heat and observing the temperature change of the apparatus.
Thermometer
O 2 Inlet
Contact Wires Sample Holder Bomb (Reaction Chamber)
Stirrer
Figure 1
For example, it is known that the combustion of 1.000 g of benzoic acid (C 7 H 6 O 2 ) will produce -26.38kJ of heat. If we combust 1.000 g of benzoic acid in a bomb calorimeter, and observe a temperature rise of 5.022 o^ C, the heat capacity of the calorimeter would be given by
q evolved = - Ccalorimeter
Which rearranges to give:
-q evolved = C Calorimeter
T
Plugging in the known heat of combustion for benzoic acid gives:
- (-26.38)kJ = C (^) calorimeter = 5.253 kJ/ OC
o
C
This relationship means that for the apparatus in this example if we ever see a 1 oC temperature increase when doing a reaction, the reaction produced 5.253 kJ of heat. If you observe a 2 oC temperature increase in this apparatus, the reaction produced 10.56 kJ ( 2 x 5.253 kJ) etc.
The first part of the experiment is to come up with the relationship between heat and temperature change, and then we will use this relationship to find the heat produced by the combustion of a cashew (calorie content of a cashew).
Procedure:
- Fill the calorimeter water bucket with 2.00 Liters of distilled water. The amount of water must be reproducible (so that the heat capacity of the calorimeter will be constant), but it is not necessary that it be exactly 2.00 Liters. The initial temperature of the water should ideally be 1-2 oC below that of room temperature. PLACE the water bucket in the calorimeter.
- Obtain (or make using a pellet press) a benzoic acid pellet. The pellet should weight close to, but not over, 1 gram.
Bucket
Benzoic Acid Cashew
Mass of Benzoic acid, g
Mass of cashew, g
Calculated heat of combustion for benzoic acid, kJ
Initial length of string, cm
Calculated heat of combustion for benzoic acid, cal
Final length of string, cm
Initial length of string, cm
Length of string burned, cm Final length of string, cm
Calculated heat for the string burning, qstring cal
Length of string burned, cm
Final temperature, OC
Calculated heat for the string burning, qstring cal
Initial temperature, OC
Total heat produced in bomb, q, cal
T, oC
Final temperature, OC Total heat for cashew and string, q, cal
Initial temperature, OC Heat for combustion for cashew only burning qcashew, cal T, OC Heat for^ combustion for cashew only Burning in Cal/g Note: 1000 cal= 1 Cal
Ccalorimeter , cal/ O^ C Calories/gram from package Percent error
Temperature readings
Benzoic Acid Cashew Time (minutes) Temperature, OC Time (minutes) Temperature, OC
- .5. - 1.0 1. - 1.5 1. - 2.0 2. - 2.5 2. - 3.0 3. - 3.5 3. - 4.0 4. - 4.5 4. - 5.0 5. - 5.5 5.
- Ignition 6.0 Ignition 6. - 6.5 6. - 7.0 7. - 7.5 7. - 8.0 8. - 8.5 8. - 9.0 9. - 9.5 9.
- 10.0 10.
- 10.5 10.
- 11.0 11.
- 11.5 11.
- 12.0 12.
- On your printed graph, draw a vertical line such that the area of the upper (area A) and lower triangles (area B) have approximately the same area. Extend the vertical line to the x-axis to determine the value of x. Solve the two equations on your graph for the temperature (y) by substituting x into both equations. Determine T values to the nearest .01 oC by subtracting these two values. From the graphs determine the change in temperature for benzoic acid and the cashew.
- For the calibration experiment with benzoic acid, you will need to calculate the heat produced from the combustion of benzoic acid. Keep in might that 1.000 gram of benzoic produces -26.38 kJ of heat. You will also need to calculate the heat produced from the burning string. This can be done given that every cm of burning string produces 3.59 cal.
- For the cashew experiment, you will use the Ccal found in the calibration experiment and the T to from the combustion of the cashew find the total heat produced. Taking into account the heat from the burning string again, you can find the heat from the combustion of the cashew. You will compare your result to the know Calorie content of the cashew from the food package.
Post-Lab Questions:
- How is a dietetic calorie (Cal) related to a chemical calorie (cal)
- Using your data, what percent error would you expect in the internal energy if the heat from the combustion of the fuse wire were ignored?
- From the packaging of the cashew and your book, determine the percentage of calories that come from sugar, fat, carbohydrate, and protein in the cashew sample.
- Bomb Calorimeters are also commonly calibrated by combusting naphthalene. If a 2.000 g sample of naphthalene produces a 7.81 OC temperature increase in a calorimeter, what is the Ccal of the calorimeter. The heat of combustion of naphthalene is -43.6 kJ/g.
- How does your value for the caloric energy of a cashew (or other food) compare to the label information? Give three reasons why there might be a difference between the two.
