Expt 001 -- Under-Water Fireworks

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

Chemical Concepts

1. 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."
2. Halogens have seven electrons in their outermost level; thus, they only need one more to form a stable octet. This gives them a high electron affinity.
3. Activation energy is the energy required by reactant particles so that they might collide with enough force to initiate a reaction. Many reactions require high temperatures before they can begin, but some reactions, like those between halogens and unsaturated hydrocarbons, have activation energies low enough that the reactions occur spontaneously at room temperature.

Background

• Calcium carbide reacts with water to form acetylene.
  • CaC₂ + 2 H₂O --> C₂H₂ + Ca(OH)₂
• Chlorine gas reacts spontaneously with the acetylene in the bubbles with the release of energy. One might predict the addition of chlorine across the triple bond similar to the reactions observed in dilute solutions, but chlorine extracts the hydrogen ion to form HCl on contact. Black soot also forms.
  • C₂H₂ + Cl₂ --> 2 HCl + 2 C (black soot)
• Whereas many exothermic reactions, such as the combustion of hydrogen or methane, require a spark to initiate the process, this particular reaction between acetylene and chlorine has a low enough activation energy that room temperature is "hot enough" for the reaction to occur spontaneously.

Safety

• Work in a well ventilated laboratory. Do not attempt this demonstration without good ventilation. Use a hood for the demonstration if possible.
• See additional safety notes for teachers below.
• Chlorine is toxic and noxious. Acetylene gas is flammable and toxic.
• Place a fire extinguisher nearby.
• Wear goggles.
• Calcium carbide reacts with any source of water including eyes. Use care.

Procedure

1. Cut a length of glass tubing about 10 cm longer than the height of the graduated cylinder, then use it to assemble the set-up shown below.
2. Fill the graduate with tap water to within 1-2 cm of the top. Under the hood, place 100 mL of NaClO solution in the flask and carefully pour in 20 mL of 3 M HCl. Caution: these two react to form chlorine gas, especially when the flask is swirled or shaken. Insert the 1-holed stopper assembly and place the glass rod into the cylinder.

   !!!Click here to See Movie. Click |> or <| to step the slides forward or back.

3. Swirl the flask slightly until 2-3 bubbles of gas bubble up out of the tube. Bubbles should not be emitted in a steady stream; bubbles should only form when you swirl the flask. See safety below.

   !!!Click here to See Movie.

   If insufficient chlorine is being generated, swirl again. If there is still too little chlorine, try adding 10 more mL of HCl. Swirl. Add an additional 10 mL of HCl only if there is still no chlorine being generated. Only a few bubbles from each swirl are required.

4. Move the set-up into a visible location, but make sure there is adequate ventilation to carry away the excess chlorine gas. Drop 2 pea-sized pieces of calcium carbide into the water. Note the immediate generation of acetylene gas. Safety demands that you limit the amount of calcium carbide used.

   !!!Click here to See Movie.

5. Swirl the flask gently and maneuver the glass tube along the bottom of the graduate 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.

   !!!Click here to See Movie. Note many flashes were missed at 1/30 second per picture. The sound is recorded but the light flash was complete between shutter clicks.

   The reaction is so fast that many explosions were missed by the video camera until the shutter speed was slowed to 0.10 seconds.

   !!!Click here to See Movie. Shutter speed slowed to 0.10 sec to catch more explosions.

   !!!Click here to See Movie. Click |> or <| to step the slides forward or back.

   Tipping the cylinder a little can facilitate the reaction, for it causes the bubbles to travel up the inside surface, increasing the likelihood of bubbles colliding with one another.

6. Turn on the lights to observe any products.

   !!!Click here to See Picture.

7. As soon as the reaction is over, add base to the bleach solution, and move the entire apparatus to a hood.

Questions

1. In which phase is the chlorine acetylene reaction occurring?
2. Describe a major difference between this reaction and the reaction of hydrogen and oxygen.
3. What side products are present? Describe any precipitate that forms.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

BeckerDemos 001 Under-Water Fireworks

Watch the movies.

Describe the changes you observe.

Answer the questions.

Curriculum-

• This demonstration is an effective illustration of the role the activation energy plays in the rate of a reaction. Use this demonstration after students have seen a hydrogen-oxygen explosion. (002, 003, or 050)
• In second year chemistry courses, use this demonstration when discussing unsaturated compounds. For contrast, you may wish to show the same apparatus bubbling some oxygen through the water instead of chlorine. (CAUTION: BE SURE TO GENERATE ONLY SMALL QUANTITIES OF ACETYLENE AND OXYGEN AND BE SURE THAT NO OPEN FLAMES OR POSSIBLE IGNITION SOURCES ARE NEARBY). Although acetylene and oxygen react quite explosively, nothing happens when their bubbles collide because the acetylene-oxygen reaction has a much higher activation energy.

Activity-

• Demonstration - Teacher Only
• This demonstration is dangerous in the wrong hands. Follow all safety instructions.

Safety-

• Chlorine is toxic and noxious. Acetylene gas is flammable and toxic. Work in a well ventilated laboratory. Use a hood for the demonstration if possible.
• Wear goggles. Remember that calcium carbide reacts with any water including eyes.
• If a hood is not available, 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 for the chlorine to exit into a beaker filled with sodium thiosulfate solution. Tape the tubes in place. This should filter out most of the chlorine.
• Use only 2 small pieces of calcium carbide. The acetylene gas produced is flammable and explosive.
• Bring only a small amount of calcium carbide to the lab bench. Any accidental water spill is dangerous if a large container of acetylene is in the classroom.

Time-

Teacher Preparation: 5 minutes

Class Time: 10 minutes

Materials-

• 1 g calcium carbide (CaC₂)
• 100 mL Clorox® bleach (5% NaClO solution)
• 30 mL of 3 M HCl (1 liter -- Add 250 mL of 12 M HCl, hydrochloric acid solution to approximately 450 mL of water. After the solution equilibrates, dilute to 1000 mL with water.)
• 40 mL of 3 M NaOH (1 liter -- Dissolve 120.0 g of NaOH, sodium hydroxide in approximately 500 mL water. Add the solid to the water slowly stirring constantly to avoid spattering and caking. If the solid is added too rapidly, the solid cakes into one large piece which dissolves much more slowly than the small pellets. Dilute to 1000 mL with water.)

Optional:

• 200 mL of 3 M sodium thiosulfate (Dissolve 148.9 g of Na₂S₂O₃•5 H₂O, sodium thiosulfate pentahydrate in water. Dilute to 200 mL with water.)
• 1- or 2-L Pyrex graduated cylinder
• thin glass or plastic tube (3-5 mm OD)
• plastic tubing (2-4 mm ID)
• 250 mL flask, with 1-holed stopper to fit

Disposal-

• As soon as the demonstration is complete, add base to the bleach solution to stop the formation of chlorine gas. Place the flask in a hood. The solution may be neutralized and flushed with water after class.
• Place the cylinder in a hood until the calcium carbide is dissolved. Dispose of the solution in the sink.
• Store any unused calcium carbide tightly covered in a dry location. Check the MSDS statement for the most current recommendations.

Observations-

Carbon is a product; some black soot forms at the top of the cylinder. It wipes clean quite easily, however, with a paper towel. If tap water is used, calcium compounds usually precipitate.

Answers-

Q1. In which phase is the reaction occurring?

A1. The reaction takes place between bubbles in the gaseous phase.

Q2. Describe a major difference between this reaction and the reaction of hydrogen and oxygen.

A2. The hydrogen and oxygen reaction requires a spark to ignite it. The activation energy of this reaction is so low that ignition occurs at room temperature.

Q3. What side products are present? Describe any precipitate that forms.

A3. In the movies a white precipitate forms from the reaction of calcium ions with the tap water. You may sometimes observe carbon black.

Reference-

This demonstration was originally developed by Walter Rohr of Eastchester, New York. The first time I saw it presented was at Chem Ed '91 when Rob Lewis performed it as part of the Weird Science performance. I have tried to create colored fireworks by incorporating various dissolved ions with vibrant flame tests (such as copper and lithium), but I have not had much success. The only original contribution offered here is the chlorine trap.

Shakhashiri, B. Z., in his "Chemical Demonstrations", Volume 2 (p 227., University of Wisconsin Press, 1985), discusses the reactions in this experiment.

Key Words 1-

rate of reaction, kinetics, unsaturated, activation energy,