Zinc Pyrotechnics

Introduction

A spectacular “fireworks” display is initiated by adding just two drops of water to a mixture of dry chemicals.

Concepts

• Oxidation–reduction reactions
• Autocatalysis
• Decomposition reactions
• Catalysis (optional—see Tips section)

Materials

Safety Precautions

Ammonium nitrate is a strong oxidizer; may explode if heated under confinement or at temperatures of 250 °C; toxic by ingestion and inhalation; skin, eye, and respiratory irritant. Zinc dust can be a dangerous fire risk; may form explosive mixture with air. If allowed to get damp in a confined bottle, heat will be generated and the mixture may even possibly ignite. Ammonium chloride is toxic by ingestion. Zinc oxide fumes, one of the products of the reaction, can be toxic and severely irritating. Do not premix the ingredients before the demonstration. They can react prematurely and, if in a closed container, explode violently. This reaction should only be done in an operating fume hood or possibly out-of-doors due to amount of smoke produced. This reaction also produces a great deal of heat so it is imperative that an insulating material such as the ceramic fiber square be placed under the beaker. Do not scale this reaction up. The demonstrator should wear chemical splash goggles, a chemical-resistant apron and chemical-resistant gloves. Students should wear chemical splash goggles. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The solid products of this reaction may be disposed of in a landfill according to Flinn Suggested Disposal Method #26a.

Procedure

1. Remove all combustible materials from the fume hood. Place the 50-mL Pyrex beaker on a ceramic fiber square or other heat-resistant surface inside fume hood.
2. Add 8 g of ammonium nitrate, 8 g of zinc dust, and 1 g of ammonium chloride to the beaker; mix with the stirring rod.
3. Using the medicine dropper or a Beral pipet, add two drops of distilled water to the beaker and quickly lower the hood shield. Do not add more than two drops.
4. Observe the reaction and “fireworks!”

Student Worksheet PDF

12258_Student1.pdf

Teacher Tips

• This demonstration can be modified to provide a dramatic example of catalysis. Prepare two 50-mL Pyrex beakers with the ammonium nitrate and zinc dust, as described, but only include ammonium chloride in one of the beakers. Add the two drops of water first to the beaker without ammonium chloride. It will produce smoke and vapor, but will probably not produce flame. After the reaction is complete, add two drops of water to the second beaker. The reaction will proceed much more quickly and will produce smoke, sparks, and flames—the “eruption.”
• This kit provides enough materials to perform this modification five times.

Answers to Questions

1. Describe what happened in this demonstration.

   Solid ammonium nitrate, zinc dust and ammonium chloride were mixed together in a small beaker in a fume hood. Two drops of water were added to the beaker. Almost immediately, the mixture in the beaker produced intense sparks and flames as well as a white smoky cloud.

2. The exact mechanism of this reaction is unknown, but it is believed to begin with the heat produced by the addition of water to zinc melting the solid ammonium nitrate.
   1. Write a balanced chemical equation showing the volatilization of liquid ammonium nitrate into nitrous oxide and water.

      NH₄NO₃(l) → N₂O(g) + H₂O(g)

   2. The heat from the volatilization reaction causes the zinc to dissolve in the ammonium nitrate. Write a balanced chemical equation for this step.

      Zn(s) + NH₄NO₃(s) → N₂(g) + ZnO(s) + 2H₂O(g)

   3. Then, a cloud of white and yellow particles is produced. What do you think this cloud is?

      The cloud is solid zinc oxide, produced when the zinc dissolved in the ammonium nitrate. The energy from that reaction caused the zinc oxide to disperse into a cloud.

3. Looking over the reaction as it is described above, notice that one of the solids in the original beaker is nowhere mentioned. Propose an idea for what the purpose of the ammonium chloride was.

The ammonium chloride may have served as a catalyst for any of the steps in the reaction, most likely the first. It may be why the reaction happened almost immediately after the water was added to the mixture in the beaker.

Discussion

The mechanism of this reaction is complex. It is believed that the heat produced by the addition of water to the zinc dust melts the ammonium nitrate. Once melted, ammonium nitrate volatilizes, producing N₂O and H₂O. This reaction is catalyzed by chloride ions:

The energy change for the volatilization of ammonium nitrate to nitrous oxide and water is reported to be –23 kJ/mol. As the volatilization proceeds, the heat produced causes the zinc to dissolve in the molten ammonium nitrate. This leads to the rapid oxidation of the zinc, possibly according to the reaction:


The energy change for this reaction has been calculated to be –466.5 kJ/mol. This tremendous amount of energy produced causes the dispersal of zinc oxide as a white cloud and yellow/white particulate matter.

References

Cotton, F. A. and Wilkinson, G. Advanced Inorganic Chemistry: A Comprehensive Text; John Wiley and Sons: New York, 1980; p 417.

Shakhashiri, B. Z. Chemical Demon stra tions: A Handbook for Teachers in Chemistry; University of Wisconsin: Madison, WI; 1989; Vol. 1, p 51–52.