University of Virginia Physics Department The Energy Content of Fuels A Physical Science Activity

Adapted from Experiment 9: Energy Content of Fuels from Physical Science with CBL published by Vernier Software.

2003 Virginia SOLs

• PS.1
• PS.6
• PS.7

Objectives

Students will

• use the TI graphing calculator, CBL system, and temperature probe;
• use a balance to measure changes in mass;
• determine the internal energy change of water;
• compare energy contents for various fuel types.

Motivation for Learning

The Burning Candy Bar

Materials

• Bunsen burner or source of flame like a heat source for a chafing dish.
• large piece of aluminum foil, enough to cover the table below the Bunsen burner. This will just make things easier when you clean up later on.
• fun size candy bar, preferably Snickers
• pair of tongs

Procedure

1. WEAR SAFTY GOGGLES FOR THIS EXPERIMENT.
2. Place the heat source on the aluminum foil so that it can collect any melted candy bar that falls off
3. Light the heat source. Be careful because many of the heat sources have a blue flame that is difficult to see. You may not think the flame is there, but it is! WATCH CAREFULLY!!
4. Using the tongs, place the candy bar in the flame of the heat source. Tilt the burner slightly so that the candy bar doesn't drip onto the flame opening (this can be hard to clean off). The candy bar will fuel the flame and create additional flame size and heat. This shows that the candy bar has energy in it.
5. You do not need to burn up the candy bar completely. Make sure to turn off the heat source and make sure the bar is completely extinguished when you finish.

Background Information

Energy content is an important property of both food and matter used for heating. The energy that your body uses (sleep, walk, talk) comes from the food that you eat, like the candy bar. The energy that is created when a fuel is burned is an important quantity, and we would like to be able to measure the efficiency of fuel. The energy content is the amount of heat produced by the burning of 1 gram of a substance and is measured in joules per gram (J/g). Heat is a form of energy (actually energy flow), and it is normally measured in calories. We know that 1 cal = 4.186 J.

Sometimes it is difficult to directly measure the amount of heat something produces. We can make the process easier by burning an amount of the fuel to heat water. The energy lost by the fuel can then be calculated by finding the heat gained by the water as measured by the change in temperature of the water. We have already discovered in the Heat Transfer of Hot and Cold Water Activity that the heat gained by water can be represented by

Q = DT * m * c

where DT is the change in temperature, m is the mass, and c is the specific heat capacity constant of water. In this experiment, we will practice using a calorimeter, and measuring heat transfer, then relating this to the energy content of the fuel that heated it.

In this activity, two different types of fuel are explored and compared. Candle wax (paraffin wax) and isopropyl alcohol. Typical averages for this experiment for the energy content of these fuels were 23 - 25 kJ/g for the paraffin wax and about 14 kJ/g for the isopropyl alcohol. The isopropyl used was rubbing alcohol bought commercially and is therefore 70% isopropyl and the other 30% is water. A correction was made therefore to find the energy content of 100% isopropyl. The accepted value for the energy content of paraffin wax is 42 kJ/g. An accepted value for isopropyl was not found because it is not commonly used as a fuel, however assuming the percentage loss is the same, an extrapolated value for the energy content is 17 kJ/g. Some other accepted energy contents for comparison are:

Student Activity

To print out the Student Copy only, click here.

Materials

• CBL System
• TI-83 graphing calculator
• Temperature probe
• 100-mL graduated cylinder
• Tin can, approximately 300 mL
• Stand and 2 clamps
• Stopper with slit
• Stirring rod
• Cold water
• Electronic scale (the accuracy should be at least 0.1 g)
• Two types of fuel:
  1. A regular candle
  2. Isopropyl alcohol in a 30 mL beaker with a wick  
     (If you are using 70% isopropyl (rubbing alcohol), you need to make a correction in the data found to find the pure isopropyl energy content. This can be done by dividing the value found by 0.7)

Procedure

1. Put on your safety glasses. NOTE: Goggles must be worn throughout this laboratory.
2. Plug in the temperature probe to the CBL Unit and connect the CBL to calculator with link cable.
3. Run the Vernier Program PHYSICS on the calculator.
4. Select SET UP PROBES from menu.
5. Select 1 PROBE.
6. Select TEMPERATURE PROBE.
7. Select USE STORED from calibration menu.
8. To set up the calculator for data collection:
   • Change the data collection speed by selecting COLLECT DATA from the main menu, then select TIME GRAPH from the data collection menu.
   • Enter 5 as the time between samples, in seconds
   • Enter 95 as the number of samples
   • Press enter, then select USE TIME SETUP to continue.
   • Select LIVE DISPLAY
   • Enter 0 as the minimum temperature (Ymin)
   • Enter 60 as the maximum temperature (Ymax)
   • Enter 5 as the temperature increment (Yscl). Hit enter and you are now ready to log temperature.
9. Arrange the setup as shown in the figure above. Attach the tin can to the stand so that the fuel can rest underneath it.
10. Mass the tin can alone. Use the graduated cylinder to measure out the amount of water to put in the tin can (100 mL for the candle and 200 mL for the alcohol). Pour into the can and then mass the tin can with the water inside. Enter values on the data table.
11. Place the temperature probe through the slit stopper and clamp above the tin can so that the probe falls into the water in the can but does not hit the bottom or the sides.
12. Mass the fuel and its container using the digital balance. Make sure you are very precise because the change in mass will be very small. Write down the mass in the data table. If the candle drips, be sure to scrape the dripped portions up and mass them also, because this fuel was not burned and therefore does not need to be part of the mass of fuel used.
13. Place the thermometer in the tin can and wait about 45 seconds. This equilibrates the thermometer to the surrounding water.
14. Start logging time by hitting enter on the calculator and then light the fuel source.
15. Continually stir the water so there are no spots that are hotter than others.
16. WHEN THE TEMPERATURE REACHES 40 °C, blow out the flame and CONTINUE TO STIR. Soon after this the temperature will start to decrease and you will have your maximum temperature reading
17. After the calculator is done logging data, the CBL will read DONE. To determine the initial temperature of the water and the maximum temperature of the water, hit ENTER to display the graph and trace through the graph by using the left and right arrow keys. Enter the maximum and minimum water temperatures in the data sheet.
    
    
    
    
18. Measure the mass of your fuel sample and enter it into the data sheet.
19. To repeat the measurement, hit ENTER and select YES when asked if you want to repeat the experiment. Perform steps 8 - 16 using the second fuel sample.

Data Sheet

To print out the Data Sheet only, click here.

Data Processing

Calculate the heat gained by the water. Q_water = DT_water *m_water * c Where specific heat capacity of water is c = 4.18 J/g°C.

We now have the amount of heat energy that each fuel gave to the water. We want to know how much heat the fuel produces for each gram burned, so divide the heat increase of the water by the change in mass of the fuel during burning.

Questions:

1. Which fuel has the greatest energy content?
   
   
   
   
   
   
   
2. What other factors must we consider when choosing to use a particular fuel source?
   
   
   
   
   
   Answers to Questions:

1. The candle will have a greater energy content than the isopropyl. See background information for accepted values and for sample data.
2. The energy content is a great way to perceive the benefits of a fuel source, but we need to look at the particular situation to truly understand which fuel is more beneficial to use. For example, when sending a rocket into space, the mass is very vital, so a fuel that has a great energy content is important. However, to heat our homes or to create electricity at power plants, we need not be as concerned with the energy content, and can focus more on the environmental influences each fuel source has or which fuel is more readily available.

Students with Special Needs

Some students may need help with the flame or with setting up and using the calculator CBL system. Adjust the activity to the needs of your students.

Click here for further information on laboratories with students with special needs.

 

Assessment