In this demo, students will witness a decomposition reaction of gun cotton; a smokeless flame results.
By the end of this lesson, students should be able to
- Understand combustion.
- Recognize a chemical change took place.
- Explain why the reaction is exothermic.
This lesson supports students’ understanding of
- Chemical change
- Decomposition reaction
- Exothermic reaction
Teacher Preparation: 45 minutes (but be sure to prepare the gun cotton more than 24 hours before carrying out the experiment so it can dry)
Lesson: 10 minutes
- 400 mL of conc. H2SO4 (18 M)
- 200 mL of conc. HN03 (16 M)
- Ice in a large plastic tray
- 3-L beaker
- 2 Tongs
- Stir plate
- Stir bar
- 1-M NaHCO3 (aq)
- Ceramic tile or fire-proof table
- Always wear safety goggles when handling chemicals in the lab.
- When working with acids and bases, if any solution gets on your skin immediately rinse the area with water.
- Handle concentrated acids with extreme care.
- Conduct the preparation for the demo in a fume hood.
- Mixing the acids in an ice bath is necessary as it is an exothermic process. The ice bath controls the released heat.
Preparation (do this part in the hood)
- Slowly pour the acids into a beaker, which contains a stir bar and that is submerged in an ice bath. Allow the solution to stir until the beaker is cold (about 20 minutes). Check gloves often because the vapors can destroy them.
- To make gun cotton, separate cotton into strips about 4-5 inches long, and 2 inches wide (less than 1 g). Using tongs, grab a pieces of cotton and submerge them in the solution. Use another set of tongs to squeeze out the air bubbles, slowly. If red gas starts to form, quickly shut the hood and stand back until it subsides.
- Soak the cotton in the beaker for up to 30 minutes. Using tongs, wring the acid solution into the beaker (more cotton can be prepared with the remaining acid solution). Rinse the acid-soaked cotton in three water samples (about 200 mL each). To assure there is no residual acid in the cotton, immerse it in a solution of NaHCO3(about 100 mL). If the solution bubbles, immerse the cotton in another water sample. Check for residual acid by immersing in another NaHCO3 (aq) sample. Rinse with water after each NaHCO3rinse.
- Allow the cotton to dry overnight. Once it is dried, you have gun cotton.
What’s going on?
Gun cotton is nitrocellulose cotton (C6H7N3O11). Cotton balls, which are 90% cellulose (C6H7O5), are placed in concentrated nitric acid. The hydroxyl groups on the cellulose are replaced with nitro groups.
Using an extinguished match, ignite the gun cotton on a flame-proof surface or ceramic tile. It should go up in a smokeless flame immediately. Despite the result of a flame, this is actually a decomposition reaction, not a combustion reaction.
Adapted from: Shakashiri Vol. I, 1.17, pg. 43