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

 

Objectives

Students will

(For information on how to obtain the calculator programs necessary for this activity, click here.)

Motivation for Learning

The Burning Candy Bar

Materials

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:

Fuel Type
Energy Content (kJ/g)
Kerosene
46.3
Paraffin Wax
42.0
Ethanol
26.8
Isopropyl
24.04
Methanol
19.9

School systems have varying safety requirements; if your school will allow students to find the energy content of these other fuels, you may want to include them in the procedure. 

Student Activity

To print out the Student Copy only, click here.

Materials



Procedure



 

Data Sheet

To print out the Data Sheet only, click here.

 
Trial #1
Trial #2
1. Fuel Used    
2. Initial Mass of Fuel (g)    
3. Final Mass of Fuel (g)    
4. Dm (line 3 - line 2)    
5. Mass of Empty Can (g)    
6. Mass of Can + Water (g)    
7. Mass of Water (6-5) (g)    
8. Initial Water Temp (C)    
9. Final Water Temp (C)    
10. DT (line 9 - line 8) (C)    

Data Processing

Calculate the heat gained by the water. Qwater = DTwater *mwater * c Where specific heat capacity of water is c = 4.18 J/gC.

 

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

Data sheet and questions to be completed during the activity.