Combustion of Cellulose: Teacher's Notes

Activity 3: Combustion Products of Cellulose Nitrate

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.

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Full Disclaimer

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 and cellulose nitrate
To relate the difference in combustion products to the elemental composition of the materials

National Science Education Standards: Content Standards

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

Content Standard A: Science as Inquiry
Safety must be an integral component of their investigations.
Content Standard B: Physical Science
- Structure and Properties of Matter
- Chemical Reactions

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 nitrate - this can be prepared by the instructor before hand (see the demonstartion reference below) or can be purchased as flash paper at selected hobby shops or stores specializing in supplies for magicians.
Fireplace lighter or Bunsen burner. Note: Do Not use these methods of ignition with the cellulose nitrate. See the note below for details of constructing an electrical ignition source.

Method

Have the students place the cellulose sample on a nonflammable surface (in a hood if available).
Have the students ignite the sample. After the sample has begun to burn, have the students remove the ignition source.
Have the students place the cellulose nitrate sample (smaller than the size of a small cotton ball) on a nonflammable surface (in a hood if available).
Have the students ignite the sample. Use Extreme Caution! The sample will almost instantaneously ignite and burn completely within a second or two.

Questions

What did you list as the combustion products after observing the initial combustion of cellulose? What observations or prior knowledge led to these conclusions?
How do the observations of combustion products from nitrocellulose differ from those of cellulose? What are some of the possible nitrocellulose combustion products that you would not expect to produced by the combustion of cellulose? Why?
Based on the elements found in cellulose nitrate, what might some of the products be?

The answers to the heat of combustion calculations are here.

Extensions

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.