The Effervescent Oscillator

Introduction

Oscillating reactions typically involve eye-catching color changes, but here’s one where the oscillations are between the generation and cessation of gas bubbles. A colorless solution will effervesce intensely, then the bubbling will decrease, with the cycle repeating itself for several minutes.

Concepts

• Oscillating reactions
• Effervescence

Materials

Safety Precautions

Sodium nitrite solution is highly toxic by ingestion. The acidified ammonium sulfate solution is 0.2 M in sulfuric acid; it is corrosive to skin and eyes and slightly toxic by ingestion and inhalation. Though only a small amount of nitrogen dioxide gas is produced, it is extremely toxic and poisonous; therefore, the demonstration should be performed only in an operating fume hood or in a well-ventilated area. This reaction is exothermic and will generate a considerable amount of heat; use only a Pyrex reaction vessel. Handle the hot reaction vessel with care. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. 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. Dispose of the resulting solution according to Flinn Suggested Disposal Method #12b.

Procedure

Caution: Perform this demonstration only in an operating fume hood or a well-ventilated area.

1. Add 25 mL of the acidified ammonium sulfate solution to a 100-mL Pyrex graduated cylinder or tall form beaker.
2. Add 25 mL of the sodium nitrite solution to the 100-mL Pyrex graduated cylinder or tall form beaker. Stir the two solutions to mix thoroughly.
3. Observe the bubbling oscillations that will begin within a few seconds. The solution will oscillate between a “cloudy” look (due to excessive gas generation) to a relatively clear look with smaller and fewer bubbles. The oscillations should continue for about four minutes. The reaction is exothermic and will cause the reaction vessel to become quite hot.

Teacher Tips

• The rise and fall of the foam is most easily observed if the reaction is performed in a tall vessel, such as a graduated cylinder or tall form beaker.
• The oscillations become more distinct after about a minute. To see the oscillations most easily, watch the interface between the liquid and the foam.
• To make the reaction easier to observe for the entire classroom, place the reaction vessel on an overhead projector.
• It is important to stir the two solutions well at the beginning of the reaction so that they are thoroughly mixed.

Answers to Questions

1. Describe what happened in this demonstration.

   Acidified ammonium sulfate solution and sodium nitrate solution were mixed together in a beaker. The solution started bubbling so vigorously that it developed a cloudy, foamy look. After a little while, it stopped bubbling as much. The solution oscillates between those two states for several minutes.

2. Write a balanced chemical equation for the reaction that occurred in this demonstration between ammonium ions and nitrate ions.

   NH₄⁺(aq) + NO₂^–(aq) → N₂(g) + 2H₂O(l)

3. Looking at the reactants and the products, determine what substance is responsible for the foaming. Hint: It’s a supersaturated gas.

   The dissolved nitrogen gas reaches a level of supersaturation and therefore produces the bubbles.

4. As more dissolved nitrogen gas is produced in the reaction, that nitrogen diffuses into the supersatured nitrogen gas bubbles. Knowing this, why do you think oscillations in the bubbling occur?

   The solution becomes less and less supersaturated as the dissolved nitrogen is diffused into the bubbles. Thus, more nitrogen starts dissolving, and the bubbling stops. Then the solution again becomes supersaturated with nitrogen gas and the foaming resumes.

Discussion

The reaction between ammonium ions, NH₄⁺, and nitrite ions, NO₂^–, produces nitrogen gas, N₂ and water according to Equation 1:

The concentration of dissolved nitrogen gas reaches the supersaturation level within a few seconds. Supersaturation causes tiny bubbles of nitrogen gas to form throughout the solution. While these bubbles emerge, nitrogen gas is still being formed according to Equation 1. The diffusion of additional dissolved nitrogen into the nitrogen gas bubbles causes them to become larger. This increases the surface area of the bubbles, making diffusion of additional dissolved nitrogen into the nitrogen gas bubbles even more rapid. The larger bubbles rise to the surface of the solution giving the appearance of foam.

The solution loses its supersaturation in nitrogen as more dissolved nitrogen diffuses into the nitrogen gas bubbles. Once the solution is no longer supersaturated in nitrogen, the small nitrogen gas bubbles redissolve and the formation of foam ceases. This causes the solution to appear clear.

The cycle continues until the reagents begin to become exhausted such that the production of nitrogen is no longer rapid enough to cause supersaturation. Once this occurs, the oscillations cease and the reaction is over.

Nitrogen dioxide, a toxic, brown gas, is given off during the reaction. It is produced from the reaction between nitric oxide gas, NO, a byproduct of the reaction, and oxygen in the air according to Equations 2 and 3.

References

Bowers, P. G.; Rawji, G. J. J. Phys. Chem. 1977, 81, 1549.

Ealy, Julie B.; Ealy, James L. “Close up on Chemistry,” video manuscript, ACS, 1991.

Kaushik, S. M.; Noyes, R. M. J. Am. Chem. Educ. 1986, 63, 76.

Morgan, J. S. J. Chem. Soc. Trans. 1916, 109, 274.

Showalter, K.; Noyes, R. M. J. Am. Chem. Soc. 1978, 100, 1042.

Smith, K. W. Ph.D. Dissertation, University of Oregon, 1981.

Smith, K. W.; Noyes, R. M.; Bowers, P. G. J. Phys. Chem. 1983, 87, 1514.

Smith, K. W.; Noyes, R. M. J. Phys. Chem. 1983, 87, 1520.