Underwater Fireworks

Introduction

Chlorine gas is bubbled up along with acetylene gas through a large graduated cylinder filled with water. Where the bubbles of the two gases collide, an instantaneous, bright flash of light occurs.

Concepts

• Electron affinity
• Activation energy

Materials

Safety Precautions

Perform this demonstration in an operating fume hood. Sodium hypochlorite solution (bleach) is a corrosive liquid and causes skin burns. It generates toxic chlorine gas when reacted with acid and is toxic by ingestion. Avoid contact with organic material. Hydrochloric acids is highly toxic by ingestion or inhalation; severely corrosive to skin and eyes. In this lab sodium hypochlorite is reacted with hydrochloric acid to generate small amounts of very dilute halogen solutions for use by the students. This step should only be performed by the teacher and in the amounts indicated. Follow the directions carefully and work in an operating fume hood. Calcium carbide is corrosive to eyes and skin; exposure to water or moisture evolves flammable acetylene gas. The reactions in this demonstration release harmful chlorine gas and flammable acetylene gas. 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. Allow the solutions to ventilate in an operating fume hood for 12–24 hours. The chlorine water in the graduated cylinder and bleach/hydrochloric acid solution in the Erlenmeyer flask may also be reduced and neutralized according to Flinn Suggested Disposal Method #12a.

Prelab Preparation

1. Cut a length of glass tubing about 10 cm longer than the height of the graduated cylinder. Cut a second piece of glass tubing about 10 cm in length and insert it into a 1-holed rubber stopper. Attach the two pieces of glass tubing with flexible plastic tubing about 10 to 20 cm in length. Insert the long glass tube into the graduated cylinder as shown in the figure on the next page.
2. Fill the graduated cylinder with distilled or deionized (DI) water to within 1 to 2 cm of the top to prevent gases from collecting at the top of the cylinder.
3. Set up a support stand and clamp to hold the 250-mL flask at the appropriate level so that the rubber stopper can easily be connected and removed when the long glass tube is inserted into the graduated cylinder (see Figure 1). Make sure the setup is located in an area with plenty of ventilation to carry away excess chlorine gas, or in a fume hood.
   {14137_Preparation_Figure_1}

Procedure

1. Working in an operating fume hood or in a very well-ventilated room, place 50 mL of bleach solution in the 250-mL flask and carefully pour in 15 mL of 6 M HCl. Caution: Bleach and hydrochloric acid solution will react to form chlorine gas, especially when the flask is swirled or shaken. Quickly connect the 250-mL flask to the 1-holed rubber stopper and clamp the flask in place. Do not use a flask smaller than 250-mL. Do not reopen the flask. Use only the exact quantities of each chemical as mentioned above.
2. Swirl the flask slightly until 2 to 3 bubbles of gas rise up out of the tube in the graduated cylinder.
3. Drop 2 to 3 pebble-sized pieces of calcium carbide into the water in the graduated cylinder. Note the immediate generation of acetylene gas.
4. Swirl the flask gently and maneuver the glass tube along the bottom of the graduated cylinder to cause the bubbles of chlorine to collide with the bubbles of acetylene. Turn down the lights to enhance the visual impact of the reaction. The reaction will last approximately 30 to 45 seconds. If the calcium carbide is consumed but chlorine gas is still being produced, additional calcium carbide pieces can be added to the cylinder.
5. Move the setup back to the fume hood when the reaction is complete to effectively degas the solutions.

Student Worksheet PDF

14137_Student1.pdf

Teacher Tips

• This kit contains enough chemicals to perform the demonstration seven times: 30 grams of calcium carbide, 140 mL of 6 M hydrochloric acid solution and 500 mL of 5% sodium hypochlorite (bleach) solution.
• Always use fresh sodium hypochlorite. Do not scale up the reaction.
• Tipping the cylinder a little can facilitate the reaction, for it causes the bubbles to travel up the inside surface of the cylinder, increasing the likelihood that the bubbles will collide with one another.
• If ventilation is a problem, use the following chlorine trap: Place a plastic bag over the mouth of the graduated cylinder, and secure it in place with a rubber band. Poke a hole through one corner of the bag for the glass tube delivering the chlorine gas and one hole in the other corner for a length of tubing to deliver any unreacted chlorine into a beaker filled with 50% aqueous solution of sodium thiosulfate, Na₂S₂O₃. This should filter out most of the chlorine. To make a 50% w/v aqueous solution of sodium thiosulfate: Add 50 g of sodium thiosulfate to approximately 50 mL of distilled water. Stir to dissolve the solid (some heating may be required). Then dilute the solution to a final volume of 100 mL with distilled water.
• Do not use plastic flexible tubing longer than 20 cm. Longer tubing creates more resistance for the flowing chlorine gas and may prevent the gas from flowing through the tube smoothly. This may cause a pressure buildup in the flask that may pop the rubber stopper off the top of the flask and release harmful chlorine gas.
• Tap water may be used in this demonstration as long as the water is not too hard (hard water ions may interfere with the reaction). If you suspect that your school’s water is hard, distilled or deionized water should be used.
• To reduce the amount of chlorine gas lost after adding the HCl to the bleach, attach the flask as quickly as possible to the 1-holed rubber stopper, and do not swirl the flask until it is connected.
• Smaller sized graduated cylinders or hydrometers may be used. A smaller cylinder will cause the bubbles of chlorine and acetylene to collide more frequently, but it may not be as visible from a distance. Do not use a cylinder smaller than 250-mL.

Answers to Questions

1. Describe what happened in this demonstration.

   A graduated cylinder was set up with a glass tube that was connected to a flask containing sodium hypochlorite and hydrochloric acid. The reaction in that flask generated chlorine gas, which was bubbled through the water in the cylinder. Calcium carbide was then added to the cylinder, which produced acetylene gas. When the bubbles of chlorine collided with the bubbles of acetylene, the reaction emitted a bright flash of light.

2. Write two balanced chemical equation showing:
   1. The generation of acetylene (C2H2) from calcium carbide and water

      CaC₂(s) + 2H₂O → C₂H₂(g) + Ca(OH)₂(aq)

   2. The generation of chlorine from sodium hypochlorite and hydrochloric acid

      NaOCl(aq) + 2HCl(aq) → Cl₂(g) + NaCl(aq) + H₂O(l)

3. Acetylene is a hydrocarbon containing triple bonds between carbon atoms. Chlorine is a halogen with seven electrons in its outer energy level. Explain why chlorine reacts with acetylene the way it does.

   Chlorine needs another electron to form an octet, resulting in a high electron affinity. The triple bonds in acetylene obviously have a high density of electrons, so the chlorine tries to break apart one of those bonds for itself, creating a fast, intense reaction.

4. Define activation energy.

   Activation energy is the threshold of energy reactants must reach in order to cause that certain reaction.

Discussion

In some hydrocarbons, two or even three pairs of electrons can be shared between two adjacent carbon atoms. These multiple sharings are known as double or triple bonds, and the areas where they occur are said to have high electron densities. Hydrocarbons with double or triple bonds are referred to as “unsaturated.” Halogens have seven electrons in their outermost level. Thus, they only need one more electron to form a stable octet. This gives them a high electron affinity. Because of this high affinity for electrons, and the high density of electrons around a multiple bond, halogens will often “attack”—break open and connect onto a double or triple bond in an unsaturated hydrocarbon.

When chlorine and acetylene gas mix, the electrophilic chlorine attacks the triple bond in acetylene and two competing reactions occur. The predominant reaction is chlorine adding across the carbon–carbon triple bond to produce dichloroethylene. Further addition of chlorine will produce tetrachloroethane. In a competing reaction, chlorine abstracts the hydrogen atom from acetylene to produce HCl and carbon. The carbon is visible as black soot which appears near the top of the cylinder. The reactions for the demonstration are the following:

Activation energy is the energy required by reactant particles so that they might collide with enough force to initiate a reaction. Many reactions, even exothermic reactions such as the combustion of hydrogen or methane, require high temperatures or sparks to initiate the process. This particular reaction between acetylene (an unsaturated hydrocarbon) and chlorine (a halogen) has a low enough activation energy that room temperature is “hot enough” for the reaction to occur spontaneously.

References

Becker, Bob; Twenty Demonstrations Guaranteed to Knock Your Socks Off!; Flinn Scientific, Batavia, IL, 1994; pp 33–35.