Combustion of Cellulose: Teacher's Notes

Activity 1: Flammability of Cellulose and its Derivatives

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 examine and compare the combustion of cellulose, cellulose acetate and cellulose nitrate.
• To formulate ideas about the similarities and differences of the combustion of the three cellulose derivatives

National Science Education 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 explanations for the differences in combustion of cellulose and its derivatives. They are also asked to formulate hypotheses about combustion products. This ties into the Content Standard B.
  Safety must be an integral component of their investigations.
• Content Standard B: Physical Science
• Structure and Properties of Matter
  Students learn about the structures of polymers, specifically, cellulose and its derivatives. In their observations, students will note the different phases of matter before and after combustion.
• Chemical Reactions
  Combustion is a chemical reaction that releases large amounts of energy in the form of heat and light. Different materials combust at different rates; students are asked to consider how the structural differences between materials affect the chemical reaction of combustion.

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 acetate - tuxedo taffeta available at fabric stores
• Cellulose nitrate - this can be prepared by the instructor beforehand (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 notes below for details on how to construct an electrical ignition source.

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 acetate sample on a nonflammable surface (in a hood if available).
4. Have the students ignite the sample. After the sample has begun to burn, have the students remove the ignition source.
5. 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).
6. Have the students ignite the sample. Use Extreme Caution! The sample will almost instantaneously ignite and burn completely within a second or two.

Questions

1. Which sample burns the fastest? Why do you think this is the case?
2. Which sample do you think releases the most heat? Why?
3. Other than the speed of combustion, what differences did you observe?

Notes on 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
• two 9V batteries or a 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, a flame 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.