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it is Experiment 10 on Bomb Calorimetry, its main purpose is to calibrate a bomb calorimeter and then use the calibrated bomb calorimeter to determine the calorie content of a cashew.
Typology: Lab Reports
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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
Which rearranges to give:
Plugging in the known heat of combustion for benzoic acid gives:
o
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:
Bucket
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
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.
Post-Lab Questions: