Combustion of Cellulose: Teacher's Notes

Activity 4: Calculating Heat of Combustion &
Activity 5: Comparing Heats of Combustion

Safety: First and Foremost

• If you choose to do any of the testing as a demonstration or as hands-on activities, it's essential to follow all safety precautions to the utmost. Students may need to be reminded that burns are painful and can be disfiguring, and trying any of these experiments (especially the cellulose nitrate, even as commercial flash paper sold in magic stores) on a larger scale can be fatal. It is our hope that in providing video footage of the more dangerous demonstrations, the need for individuals to do these in person will be nil, and thus students will be able to see the chemistry and learn from it without any risk whatsoever.

• Wear safety goggles at all times.
• Provide a safety shield for protection of the students and of you.
• Perform all experiments and demonstrations in a well-ventilated area, in open, still air. (That is, if you do this outdoors, a breezy day can be unpredictable and hazardous.)
• Never ignite anything in a sealed or closed container.

Objectives

• To have the students calculate the heat of combustion of the three cellulose derivatives.
• To have the students discuss the possible sources of the differences in the heat of combustion of the three cellulose derivatives.

National Science Education Standards: Content Standards

• Content Standard A: Science as Inquiry
  
  • Use Technology and Mathematics to Improve Investigations and Communications
    Students calculate heats of reaction to help investigate the differences between the combustion of different materials.
  • Formulate and Revise Scientific Explanations and Models Using Logic and Evidence
    As a result of their calculations, students may need to revise their explanations and/or their hypotheses.
    Also, they will see that they can balance a combustion reaction without a predicted reactant (oxygen) and product (water).
    
• Content Standard B: Physical Science
  • Structure and Properties of Matter
    From their calculations, students will see that differences in molecular structure affect their heats of combustion.
  • Chemical Reactions
    Students study a few combustion reactions more closely, and balance reactions before doing their calculations.

Materials

• The students will require a calculator in order to do the calculations.

Method

• The method is described on the web pages.

Activity 4

In order to balance the reactions, assume that the products of the cellulose acetate combustion are carbon dioxide and water. Assume that the combustion of cellulose nitrate produces only carbon dioxide and ammonia (NH₃). Yes you'll see that it's possible to balance the reaction without producing water (or adding oxygen!).

Calculate the heat of combustion for each reaction in kJ/mole and in kJ/gram.

• The answers to the heat of combustion calculations are here.

Activity 5: Comparing Heats of Combustion

1. Compare the values of the heat of combustion for each of the three forms of cellulose.
2. Which form of cellulose did you think would release the most heat? Do your calculations agree with your prediction?
3. What are some reasons for wanting to report the heat of combustion in kJ/gram?

Extensions

1. Calculate the heat of combustion for some common fuels: gasoline (assume hexanes), natural gas (methane), coal (assume 90% carbon).
2. Compare and discuss the heat of combustion (in kJ/gram) for these fuels. What are some of the advantages and disadvantages of using each fuel?
3. Find or calculate the energy released per gram of uranium used in a nuclear fission reactor. What are the advantages and disadvantages of using nuclear fission compared to the combustion of coal, gasoline and natural gas?