Underwater Fireflies - Light Display Produced as Gas Bubbles Collide Underwater
Acetylene gas and chlorine gas are simultaneously bubbled up through a large graduated cylinder filled with water. As bubbles of the two gases collide, a momentary, bright flash of light occurs.

Equipment and Materials: 1 g of calcium carbide, CaC2 10 mL of 6 M hydrochloric acid, HCl 100 mL of commercial bleach     (sodium hypochloritesolution) 1-L or 500 mL graduated cylinder 250 mL Erlenmeyer flask 1-hole rubber stopper     (to fit the Erlenmeyerflask) Glass or plastic tubing (3-5 mm OD) Flexible plastic tubing (2-4 mm ID)

Chemical Concepts:

1. Some hydrocarbons have two or three pairs of electrons shared between two adjacent carbon atoms. These bonding arrangements are known as double or triple bonds, and the regions where they occur are said to have high electron densities. Organic molecules with double or triple bonds are said to be “unsaturated”.
2. Chlorine has seven electrons in its outermost shell so it needs only one more to give it a stable octet. This gives chlorine a high electron affinity.
3. Because of this high electron affinity, and the high density of electrons around the double or triple bond, chlorine will often “attack” (break open and connect into) a double or triple bond in a hydrocarbon. This type of reaction is called an addition reaction.
4. Activation energy is the energy required by reactant particles so that they might collide with enough force to initiate a chemical reaction. Reactions frequently require high temperatures to initiate them, but this one has a low activation energy and will occur spontaneously at room temperature.

CAUTION:
When hydrochloric acid is mixed with sodium hypochlorite solution, toxic chlorine gas is evolved! Ample ventilation must be provided! This demonstration must be done under a fume hood.

Procedure:

1. Read all the instructions carefully before performing this demonstration. Chlorine gas is very toxic and hazardous! This demonstration must be done under a fume hood and with the greatest of care.
2. Cut a length of glass tubing about 10 cm longer than the height of the graduated cylinder. A short bend at the bottom end similar to a candy cane would be a plus. Cut a second piece of glass tubing about 10 cm in length and insert into the rubber stopper. Connect the two pieces of glass with flexible plastic tubing.
3. Fill the graduated cylinder to within 1 or 2 cm of the top. Do not leave space above the water for dense chlorine gas to collect at the top of the cylinder. Place the long section of glass tubing into the graduated cylinder allowing the candy cane style bend to rest on the bottom.
4. Working under the fume hood, pour 100 mL of commercial bleach into the 250 mL Erlenmeyer flask, then pour in 10 mL of 6 M HCl. Caution: These two chemicals will react immediately to form chlorine gas, especially when the flask is swirled or shaken. Use a high quality flask to contain the reaction. Swirl the flask slightly until 2 or 3 bubbles rise up out of the tube. Do not reopen the flask. Do not use a flask smaller than 250 mL. Only use exact quantities of chemicals as mentioned above.
5. Arrange the students and the setup for maximum visibility by the entire class. Drop 2 or 3 pebble-sized lumps of calcium carbide into the graduated cylinder of water. Be prepared for the immediate evolution of acetylene gas.
6. Swirl the flask gently and maneuver the glass tubing along the bottom of the graduated cylinder in a manner that increases the likelihood that the bubbles of chlorine will collide with the bubbles of acetylene. Turn down the classroom lights as much as possible to improve the visual impact of the reaction. Reactions will probably last from 30 to 45 seconds.
7. Leave the setup under the hood until the reaction producing chlorine gas has completely stopped and any excess chlorine has been carried away.

Tips:

1. Always use fresh commercial bleach for this demonstration.
2. Tilting the graduated cylinder a few degrees can enhance the effect by forcing bubbles to travel up the inside surface, increasing the opportunity for them to collide with one another.

Discussion:
When bubbles of the two gases collide, the electrophilic chlorine attacks the triple bond and two competing reactions occur. The predominant reaction is the diatomic chlorine molecule adding across the carbon-carbon triple bond producing dichloroethene. Further addition of chlorine will produce tetrachloroethane. In a competing reaction, chlorine abstracts the hydrogen from the acetylene to produce HCl and carbon. The carbon is visible as black soot, which appears near the top of the cylinder.

Many exothermic reactions, such as the combustion of hydrogen or methane, require a spark to initiate the process. This particular reaction between chlorine and acetylene has a low enough activation energy that room temperature is sufficient for the reaction to occur.

Chemical Reactions:
Chlorine generation:
NaOCl(aq) + 2 HCl -> Cl₂(g) + NaCl(aq) + H₂O(l)

Acetylene generation:
CaC₂(s) + 2 H₂O(l) -> C₂H₂(g) + Ca(OH)₂(aq)

Addition reaction:
2 C₂H₂(g) + 3 Cl₂(g) -> C₂H₂Cl₂(aq) + C₂H₂Cl₄(aq)

Abstraction reaction:
C₂H₂(g) + Cl₂(g) -> 2 HCl(aq) + 2 C(s)

Safety Precautions:
Commercial bleach (sodium hypochlorite solution) is a corrosive liquid; causes skin burns; evolves chlorine gas when heated or mixed with acid; toxic by ingestion; avoid contact with organic materials.

Hydrochloric acid is highly toxic by ingestion or inhalation; severely corrosive to skin and eyes.

Calcium carbide is corrosive to eyes and skin; exposure to water or moisture evolves highly flammable acetylene gas.

This entire demonstration, including setup and cleanup, must be performed under a fume hood.

Wear chemical splash goggles, chemical-resistant gloves, and chemical-resistant apron.

Acknowledgements:
Walter Rohr of Eastchester High School in Eastchester, NY first developed this fascinating demonstration. It was presented by Rob Lewis at Chem Ed ’91 as a part of the Weird Science performance. Excellent versions can also be found in a number of high quality classroom demonstration manuals.

For more lab safety tips, visit The Laboratory Safety Institute (LSI) at: http://www.labsafetyinstitute.org

More Teaching Tips from Quantum^® coming soon!