The Energy Content of a Peanut

Experiment 72: The energy content of a peanut

TEACHER NOTES

RATIONALE

The subject of this experiment is the release of chemical energy from food. The student applies his or her understanding of calorimetry to measuring the energy content of a particular item of food. The use of datalogging increases reliability and reduces drudgery in this simple exercise. It would also be suitable for use in Year 9 or 10 in a food-testing unit.

Some datalogger manufacturers have this experiment as a preset file, automatically configuring the interface to suitable sampling rates. Excellent results can be expected when compared with traditional manual methods.

If interfaces are limited, this experiment is also suitable as a demonstration or extension activity following traditional methods.

Concepts: energy, heat transfer, specific heat, insulation, heat of combustion, heat of reaction

Time: 30 minutes

Suitable interfaces: 1–3 × Analogue sensor connections supporting sampling rates between 1 second per sample and 10 minutes per sample. Standard graphing functions should be available with the supported software.

MATERIALS

The materials required per group of 20 students, working in pairs, are listed below. Adjust the quantities depending on the number of interfaces available. A more fully automated solution is possible with voltage and current sensors.

• interface (sampling rate = 10 seconds per sample, sampling time = 20 minutes)
• steel can
• packet of peanuts, cashews, other nuts, dry biscuits or potato chips
• temperature sensors (1 per interface; more would allow immediate comparisons between different fuels)
• See page 188 of Heinemann Chemistry Two Teacher’s Resource Book, third edition, or the following student notes, for the remainder of the equipment.

General notes for getting started

• Consult your interface’s manual for instructions on where to find the software features that are referred to in the experiment.
• When editing the experiment include some screen captures or copies from the manual of particular screens and features to assist students who are new to datalogging.
• Laminate copies of the ‘Quick Start’ guides for use as mouse mats; students will find most features they need by themselves.
• Make sure the appropriate connecting cables are supplied for your interface and computer combination. Only put out the correct cables so that students are not confused.
• Students should be familiar with the concepts of heat of combustion, heat of reaction (H) and specific heat. While a calorimeter is not used for this experiment, students may find it useful to understand the principles of a calorimeter.
• An aluminium foil guard around the candle flames will decrease heat-loss.
• The energy contents of a wide variety of foods are given in Nutritional Values of Australian Foods by Ruth English and Janine Lewis, Australian Government Publishing Service, 1991.

Student practical follows.

Experiment 72: The energy content of a peanut

PURPOSE

To measure and compare the thermal energy released during the combustion of a peanut.

THEORY

The energy that a particular food supplies to the body is almost the same as the energy released when that food is burnt. In this experiment the energy content of a peanut is estimated by the amount of heat absorbed by water when the peanut is burnt to heat that water. Water absorbs 4.18 J g-1 for a rise in temperature of 1°C. Refer to Heinemann Chemistry Two, third edition, Chapter 23, for further discussion on the measurement of the energy content of food.

SAFETY PROCEDURES

1.        Follow all instructions for using the equipment in this activity.

2.        Wear gloves and use tongs when handling the cans after heating.

3.        Wear safety glasses and a laboratory coat for this experiment.

EQUIPMENT

• peanut, cashew, dry biscuit or potato chip
• interface
• temperature sensor/s recording in the range –10–100°C
• 250 mL measuring cylinder
• steel can/s
• tongs
• retort stand and clamp
• bench mat
• electronic balance
• safety glasses

PROCEDURE

In this activity, the temperature sensors take the place of a standard thermometer to automate the data collection process. If you have access to more than one sensor you could measure a number of foods at one time for a direct comparison of the energy from different foods. Set up a graph for each sensor/food.

PART I: Computer Setup

1. Connect the interface to the computer following the manufacturer’s instructions, turn on the interface, and finally turn on the computer.
2. Connect one temperature sensor to Analog Channel A/1.
3. Configure the datalogging software as follows:

**Consult your interface’s manual for instructions on where to find these features.**

• Set up a graph of Temperature vs Time to show your results. You may also like to add digital or metered displays to monitor the specific value of temperature.
• The Sampling Options… for this activity are: Sampling rate = 10 seconds and
  Stop Condition = Time at 480 seconds (8 minutes).
• No calibration of the sensors is required since temperature sensors produce a voltage that is directly proportional to the quantity being measured.

PART II: Data Recording

1. Place the food in a flame-proof container and weigh both the food being tested and the container. Record the mass. Also record the brand and the energy content reported on the packet.

1. Set up a steel can about 10 cm above the food using a retort stand and clamp.

1. Add 100 mL of water to the can. Set the temperature sensor in the can and use the MONITOR function of your datalogging software to check the temperature until a stable reading is noted. At this point start recording the data.

1. After about 45 seconds, ignite the food with a Bunsen burner, quickly move the water over it and heat the water. Use the temperature sensor to stir the water continuously to assist in even heating of the water. Be careful not to touch any of the can’s surfaces.

1. Stop recording once the water temperature has increased by about 20°C or when the software automatically stops recording at 8 minutes.

1. Use the graph and the software’s analysis functions or a table of data to identify and record the starting and maximum temperatures, and the time between beginning and ending the heating. Weigh and record the final mass of container and food. Calculate the mass lost by the food.

1. If you are able, repeat the above steps for additional fuels. Additional sensors would allow you to test a number of foods at one time.

QUESTIONS

1. Calculate the change in temperature of the water.
2. Calculate the thermal energy absorbed by the water in the can when the peanut was burnt.
3. Use your answer to question 2 to calculate the energy released per gram of peanut burnt.
4. Compare the result you obtained for the energy content of the peanut with that specified on the packet. Account for any difference.
5. How could you calibrate your equipment to obtain a more accurate result?

ENDING THE ACTIVITY

Equipment Clean Up

• Check with your instructor about putting away the equipment for this activity.

Computer Shutdown

When you have finished, you have several options

1.        You can select Quit/Exit from the File menu to end the activity.

2.        You can select Save or Save As… from the File menu to save your data for this activity, including the data you have recorded.

• If you are using a manufacturer’s preset experiment the document may be ‘locked’, so you should give your experiment a new filename.

• Calculator-based systems will automatically save the data on the calculator. Consult your manuals for instructions on downloading the data to a computer for analysis and printing.