Combustion of Cellulose: Teacher's Notes

Activity 2: Incomplete 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 observe and compare the combustion products of cellulose in air and cellulose in an inverted beaker.
• To have students discuss the fire triangle and how it relates to the combustion of cellulose.

National Science Education Standards: Content Standards

This activity fulfills the following within the Content Standards: 9-12

• Content Standard A: Science as Inquiry
  Students use evidence (provided largely by their observations of video or the procedures outlined below) to propose possible products of cellulose combustion. Previous conceptions may be challenged; they may already "know" that combustion results in CO₂ and water, but the reaction observed produces solid products as well.
  Safety must be an integral component of their investigations.
• Content Standard B: Physical Science
  • Structure and Properties of Matter
    As students observe the products of combustion, they will note that the reaction products are chemically very different from the reactants, including their different phases of matter. The structure of cellulose is reviewed.
  • Chemical Reactions
    Combustion is a chemical reaction that releases large amounts of energy in the form of heat and light.
    In comparing two combustion reactions, students will consider what is needed for combustion to occur, and in this way, will apply critical thinking of the design of a chemical reaction. For example, they will observe a reaction where there isn't enough of one reactant available, and the reaction doesn't go to completion.

Materials

• The students may use the observations from the video clips provided. If you want the students to perform the experiments, the materials listed below will be needed.
• Cellulose - this sample can be ordinary cotton balls - these need to be real cotton, not the synthetic cotton balls
• Cellulose in an inverted beaker - the students should be made aware of the fact that burning materials in a confined space (or a closed system) can be very dangerous. In this case, the total energy released is small.

Method

1. Have the students place the cellulose sample on a nonflammable surface (in a hood if available).
2. Have the students ignite the sample. After the sample has begun to burn, have the students remove the ignition source
.
3. Have the students place the cellulose sample on a nonflammable surface (in a hood if available) and cover with a large beaker (at least 1-liter).
4. Have the students ignite the sample. After the sample has begun to burn, have the students remove the ignition source. The students need to carefully watch the combustion and the appearance of condensed water vapor on the inside of the beaker.

Questions

1. What elements are found in cellulose? What could they form when they combine with oxygen? Hint:Look at the structure of cellulose again.
2. What do you think the products are when cellulose is burned?
   Hint: In many ways, combustion is similar to metabolism. What "products" of metabolism do you exhale?
3. What different product(s) are produced when the cellulose is burned in the beaker?
   Hints:Look carefully at the combustion in a beaker. What is the "fog" that appears on the beaker? Don't forget about that black stuff that's left behind.
4. Did all of the cotton ball burn in both reactions? What do you think is responsible for the difference?
   Hint: Go back and look at the two combustion reactions again. Think about the fire triangle.

Extensions

1. Have the students find a chemical test for the presence of carbon dioxide and design an eperiment to test for the production of carbon dioxide in the combustion of cellulose.
2. Based on the structure of cellulose acetate, what can the students predict about the possible combustion products of cellulose acetate? Have the students examine the combustion of cellulose acetate and compare the compustion products to the combustion of cellulose and nitrocellulose.
3. Have the students comapre their observations of the combustion of cellulose as a cotton ball to their experiences of watching wood burn (like a campfire). Why does wood (basically cellulose) burn differently than their observations of the cotton ball burning?

Notes:

The Igniter

• Flame vs. Battery-Powered Nichrome Wire (described below): For the open flame ignition source, we used a long-handled butane lighter. The disadvantages to the open flame were safety concerns and difficulty in reproducibility of flame height/strength. Regarding safety, the advantage to the igniter is that the person doing the igniting can easily be further away from the experiment. The main advantages of the lighter were cost and the ability to light the difficult-to-light samples. Regarding cost, we found that the 9V batteries did burn out relatively quickly.
• Battery Igniter -- Materials Needed:
• nichrome wire
• alligator clips
• insulated copper wire
• 2-9V batteries or 12V lantern battery (should have longer lifetime)

Additional Safety Considerations

Never ignite anything in a sealed or closed container. You've probably noticed that some of the video demos were done underneath a 1-liter beaker. The only advantage to using a beaker is to be able to videotape frame-by-frame in an atmosphere that is more quickly depleted of oxygen and more readily shows the combustion products. Please use our videos to show these effects, as the safety hazards far outweigh any observations that might take place outside the video. Specific hazards that we observed with the nitrocellulose were: 1) flames shot out from under the lip of the beaker, resulting in a singed experimenter, and 2) a few samples unpredictably did not ignite, but filled the beaker with copious amounts of toxic, flammable gases (including the red-brown NO₂). Please do not put yourself and your students at risk, and conduct all experiments in open, still air, and NOT in an enclosed container.

Demonstration References

Synthesis of Cellulose Nitrate
Chemical Demonstrations: A Handbook for Teachers of Chemistry, Vol. 1; Shakhashiri, B.Z., Ed.; The University of Wisconsin Press; Madison, 1983; p. 43.