Knock Your Socks Off Apparatus

Introduction

Use this apparatus kit to construct a safe “carbide cannon”—the projectile is a sock! The carbide cannon is an exciting yet controlled way to demonstrate the reaction of calcium carbide with water and the explosive nature of acetylene.

Concepts

• Combustion
• Organic chemistry
• Oxidation

Materials

Safety Precautions

Combustion of acetylene produces a loud noise and is a potentially violent explosion. Warn observers to anticipate a loud noise and demonstrate the proper way to “cup” or cover ears. Keep the apparatus nearly vertical when loading. DO NOT overload the apparatus with calcium carbide. Scaling up this demonstration will present dangerous conditions to both the demonstrator and the observers. When calcium carbide is exposed to water or moisture it evolves flammable acetylene gas, which is corrosive to eyes and skin. Perform this demonstration outdoors or in a well-ventilated area. Make sure no flames are in the area. Do not attempt to launch any object heavier or harder than a sock. Wear safety goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all laboratory safety guidelines. 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 residue in the apparatus may be disposed of by Flinn Suggested Disposal Method #26b after the reaction has ceased. Any unreacted calcium carbide must be immediately moved to a vented fume hood and disposed of by Flinn Suggested Disposal Method #25.

Prelab Preparation

1. Remove the tape and mailing cap from the end of the cannon.
2. Inspect the cannon. Remove any possible debris from the ignition hole and barrel.
3. Determine the item to be launched (i.e., a sock, and practice folding the sock so that the material fits into the barrel snugly but not too tight).
4. Familiarize yourself with all of the carbide cannon components (see Figure 1).
   {12782_Preparation_Figure_1}
5. Before firing the cannon for the first time, determine an appropriate location to perform the demonstration to ensure overall safety. Clear the area of all potential obstacles as necessary.

Procedure

1. Place the material to be launched (i.e., a sock, into the barrel snugly). Make sure the sock is not stuffed down too far into the barrel or has too tight a fit. The sock should come out easily with one hand.
2. Unscrew the end cap and place a maximum of 0.5 g calcium carbide into the hollow square cavity of the end cap.
3. Holding the cannon almost vertically, thread the end cap with the calcium carbide in it onto the adapter end of the pipe until it is hand tight. Do not overtighten. Note: The end cap will not tighten all the way in—some threads will be showing.
4. After the end cap is tightened hand tight, loosen the end ¼ to ½ of a turn. Note: After the reaction the threads will swell—if the end is not loosened slightly it will be very difficult to remove.
5. Still holding the cannon almost vertically, add one thin-stem pipet full (about 1–3 mL) of water through the angled ignition hole directly into the hollow square cavity of the cap. The hydrolysis reaction of calcium carbide will begin immediately.
6. Wait about 15 seconds for the reaction to produce acetylene.
7. Aim the cannon in the predetermined safe area and hold securely. Make sure there are no people in the path of the projectile.
8. Place the tip of a safety lighter into the ignition hole to ignite the acetylene gas. Upon lighting the acetylene, the sock will be launched with a loud bang.

Teacher Tips

• Practice this demonstration before performing it in front of an audience. It is helpful to become familiar with the demonstration as to not be startled by the noise when conducting it in front of the students.
• The apparatus may be reused many times. Calcium carbide can be purchased separately from Flinn Scientific, Inc.
• The sock must be removable with only a small amount of pressure, otherwise the gas pressure from the reaction could build up and be dangerous.
• Double check to make sure that the item to be launched is not placed too tightly into the end of the carbide cannon.
• Never completely seal the carbide cannon apparatus.
• Acetylene can burn producing a steady flame. If the carbide cannon exhibits a flame outside the ignition hole, this flame can be blown back into the cannon, which will allow the reaction to occur as expected.
• Do not burn acetylene in a pure oxygen environment since the heat of combustion of the acetylene/pure oxygen mixture is significantly higher than the acetylene/air mixture. Do not fill the apparatus with any gas other than air.
• A video of this demonstration, Knock Your Socks Off, is available for viewing as part of the Flinn Scientific “Teaching Chemistry” eLearning Video Series. See the eLearning website at http://elearning.flinnsci.com for viewing information. This video is part of the Reactions of Calcium Carbide—Combustion of Acetylene video package.

Discussion

Calcium carbide, CaC₂, is primarily used to produce acetylene gas. Acetylene gas, also called ethyne, is a product of reacting calcium carbide with water in a semi-closed container. When the reaction proceeds long enough (10 to 15 seconds), the air–acetylene mixture can be ignited. A practical application of this reaction was utilized in miners’ lamps for many years. Water dripped onto the calcium carbide, producing acetylene gas which was then ignited (see Equation 1). These lamps were commonly used in slate, copper and tin mines. Since acetylene gas is highly flammable, the carbide lamps were not used in coal mines, where they would be a serious hazard. Most carbide lamps have been replaced today by electric LED lamps. Acetylene consists of two hydrogen atoms and two carbon atoms attached by a triple bond (see Figure 2). Acetylene is often found as the fuel in torches as it burns brilliantly in air with a very sooty flame. It has a very high heat of combustion (1300 kJ/mole) and burns with a very hot flame.

The reaction of calcium carbide and water produces acetylene, a flammable gas (Equation 1). When a flame source is brought up to the ignition hole, the combustion of the acetylene occurs inside the carbide cannon apparatus. The resulting rapid expansion of the gaseous products gives the cannon its power (Equation 2).