Alcohol Cannon Apparatus

Introduction

Use this apparatus kit to construct a safe “alcohol cannon” to launch soft foam balls. The alcohol cannon is an exciting way to demonstrate the controlled combustion of isopropyl alcohol—and to “launch” your students into a discussion of chemical reactions!

Concepts

• Combustion
• Thermochemistry
• Gas laws
• Exothermic reactions

Materials

Safety Precautions

Isopropyl alcohol is a flammable liquid and a dangerous fire risk. Combustion of isopropyl alcohol may produce a potentially violent explosion. Follow the directions exactly with respect to the amount of alcohol and the type of ball used. Warn observers to anticipate the noise and demonstrate the proper way to “cup” or cover ears. Do not add more alcohol than called for in the instructions. Scaling up this demonstration will present dangerous conditions to both the demonstrator and the observers. Always recap the alcohol bottle after adding alcohol to the cannon and move the bottle far from the demonstration area. Never leave an open bottle of alcohol in the vicinity of the demonstration. Perform this demonstration outdoors or in a well-ventilated area, such as a large gymnasium. Make sure there are no flames in the area. Do not attempt to launch any object heavier or harder than the foam ball. In addition, instruct students to stand clear of the cannon to avoid the ball as it launched. Wear safety goggles and hearing protection while performing the demonstration. Wash hands thoroughly with soap and water after performing the demonstration. 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 remaining in the apparatus after the reaction has ceased may be disposed of using Flinn Suggested Disposal Method #26b. The alcohol cannon and foam ball may be stored for future use.

Prelab Preparation

1. Inspect the assembled PVC “cannon” apparatus. Remove any possible debris from the ignition hole and barrel.
2. Remove the end cap and check for debris. Tighten the end cap so it is hand-tight and then loosen the end ¼ to ½ of a turn. Caution: After the reaction the threads will swell—if the end is not loosened slightly it will be very difficult to remove. Note that the end cap will not tighten all the way in; some threads will be showing.
3. Inspect the foam ball and practice pushing it into the cannon.
4. Familiarize yourself with the components of the alcohol cannon (see Figure 1)—end cap (a), assembled cannon (b) and 7" foam ball (c).
   {12147_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. Take note of all obstacles including ceiling height if applicable.

Procedure

1. Review all of the safety precautions and go through a mental checklist to make sure all precautions are followed.
2. Fill a pipet with 2 mL of isopropyl alcohol.
3. Holding the pipet, reach your arm into the open end of the cannon as far as possible and squirt the 2 mL of isopropyl alcohol into the cannon (see Figure 2).
   {12147_Procedure_Figure_2}/div>
4. Quickly compress and stuff the 7" foam ball into the open end of the alcohol cannon. Most of the ball should be completely inside the pipe.
5. Refill the pipet with 2 mL of isopropyl alcohol.
6. Squirt the 2 mL of isopropyl alcohol into the ignition hole of the alcohol cannon.
7. Recap the alcohol bottle and move it far from the demonstration area. Never leave an open bottle of alcohol in the vicinity of the demonstration.
8. Spin and rotate the cannon for 3–4 minutes to promote evaporation of the isopropyl alcohol.
9. Aim the cannon in the predetermined safe area and hold securely. Make sure no one is in the path of the projectile.
10. Remind observers to “cup” their ears.
11. Place the safety lighter near the ignition hole and ignite.
12. A thump should send the ball flying 20–30 feet.
13. To repeat procedure, remove the end cap to allow air to flow freely through the alcohol cannon, replenishing the oxygen supply for a second launch. Repeat steps 1–11.
14. Use paper towels to clean and dry the alcohol cannon for future use.

Teacher Tips

• Always practice the demonstration before performing it in front of an audience. It is helpful to become familiar with the demonstration as not to be startled by the noise when conducting it in front of the students.
• The apparatus may be reused many times. Isopropyl alcohol and 7" foam balls may be purchased separately from Flinn Scientific, Inc.
• The 7" foam ball 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 7" foam ball to be launched is placed tightly into the end of the alcohol cannon.
• Take into account temperature changes if this demonstration will be performed outdoors. If the temperature is too cold, the isopropyl alcohol will not vaporize well and the demonstration does not work well.
• Never completely seal the alcohol cannon apparatus.
• Never use methyl alcohol for this demonstration. The high volatility of methyl alcohol means that it has the potential for the most violent combustion of any alcohol.
• The alcohol must be completely vaporized inside the tube.
• To perform the demonstration again on the same day or class period, replenishing the air in the empty apparatus (no alcohol) is required—as the demo will not work immediately after the first launch. Unscrew the cap to promote air entry. Ensure that the area, in the room or outside, is clear and vigorously twirl and turn to circulate air back into the long and relatively narrow PVC tube.
• Do not burn alcohol in a pure oxygen environment since the heat of combustion of the isopropyl alcohol/pure oxygen mix¬ture is significantly higher than the isopropyl alcohol/air mixture. Do not fill the apparatus with any gas other than air.
• A Flinn Scientific piezo sparker (Catalog No. AP6286) may also be used to ignite the cannon. • A video of this demonstration, Giant Alcohol Cannon, presented by Jeff Bracken, is available for viewing as part of the Flinn Scientific “Teaching Chemistry” eLearning Video Series. Please visit the eLearning website at http://elearning.flinnsci.com for viewing information. This video is part of the Combustion of Alcohols and Energy in Combustion Reactions video packages. We decided to use 99% isopropyl alcohol instead of ethyl alcohol, which is used in the video. Isopropyl alcohol is much safer.
• Have students perform calculations to determine the amount of gases produced based on the starting amount of isopropyl alcohol.
• For example, if 4.00 mL of isopropyl alcohol (density = 0.78 g/mL) is used:
  {12147_Tips_Equation_1}
  From the balanced equation,
  {12147_Tips_Equation_2}
  Seven times more gaseous product is formed than isopropyl alcohol originally added. The pressure inside the cannon increases dramatically and the volume of gas expands. No wonder the ball launches!

Discussion

Low-boiling alcohols vaporize readily, and when alcohol is placed in air, a combustible mixture of alcohol and air is produced. A butane safety lighter held by the ignition hole of the cannon provides the activation energy needed for the combustion of the alcohol/ air mixture.

Only a small amount of alcohol is added to the cannon and it quickly vaporizes to a heavier-than-air vapor. The alcohol vapor and air are all that remain in the cannon. Alcohol molecules in the vapor phase are farther apart than in the liquid phase and present far more surface area for reaction; therefore the combustion reaction that occurs is very fast.

Since the burning is so rapid and occurs in the confined space of the cannon with a small ignition hole, as the products form the pressure builds and the ball is launched.

The equation for the complete combustion reaction of isopropyl alcohol shows that 2 moles of isopropyl alcohol combine with 9 moles of oxygen to produce 6 moles of carbon dioxide and 8 moles of water:

2(CH₃)₂CHOH(g) + 9O₂(g) → 6CO₂(g) + 8H₂O(g)

References

Jeff Bracken credits chemistry teacher Kathleen Holley who presented this demonstration at the ACT-2 Conference, Belton, Texas, June, 2001. This demonstration also appears in Chemistry Demonstration Aids That You Can Build! written by Jeff Hepburn, Bruce Mattson, Mary Alice Kubovy and Joe Lannan published by Flinn Scientific (Catalog No. AP9320).