Summary

In this demonstration, students will witness a chemical reaction create an explosion inside of a pumpkin. They will write balanced equations for the reactions that take place and will complete stoichiometry and thermochemistry calculations.

Grade Level

High School

NGSS Alignment

This demonstration will help prepare your students to meet the performance expectations in the following standards:

• HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.

Objectives

By the end of this demonstration, students should be able to:

• Write balanced chemical equations from the description of the reactions that take place.
• Calculate how much product is formed and how much energy is involved in the reactions that take place.

Chemistry Topics

This demonstration supports students’ understanding of:

• Chemical reactions
• Combustion
• Balancing equations
• Heat of combustion

Time

Teacher Preparation: 30 minutes

Lesson: 30 minutes

Materials

• Pumpkin
• Knives for carving the pumpkin
• Calcium carbide, about 1 g
• 20 mL Water
• 100-mL plastic beaker
• Piezoelectric igniter
• Electronic balance
• Scoopula
• Weigh boat
• Safety goggles

Safety

• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
  
• An operational fire extinguisher should be in the outdoor work area for this demonstration.
  
• Keep acetylene away from open flames as it is highly flammable.
  

Teacher Notes

• This demonstration should be performed outside in a well-ventilated area.
• When it comes time to light the pumpkin, make sure students are at a safe distance away from the pumpkin(s) (at least 15 feet away) and are wearing goggles.
• You will have the best results using a medium- to large-sized pumpkin with simple shapes carved in it. Make sure that if the pumpkin is especially thick, remove excess pumpkin in the carved area until it is only about half an inch thick.
• This demonstration uses calcium carbide, CaC₂, to produce acetylene gas, C₂H₂, when it is placed in water. This reaction follows the equation:

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

When ignited, the acetylene gas then reacts with oxygen in the air according to the equation:

2C₂H₂ (g) + 5O₂ (g) --> 4CO₂ (g) + 2H₂O (g)

This reaction releases a lot of energy very quickly when the triple bond between the carbons in acetylene is broken.

• Since the calcium carbide produces the very flammable acetylene gas when exposed to water, be sure to seal the container tightly and store it in a cool, dry place. See the complete SDS.
• After the calcium carbide has completely reacted, you will be left with a basic solution of calcium hydroxide. Neutralize it with hydrochloric acid and test for a neutral pH with pH paper before pouring the liquid down the drain with lots of excess water. Calcium hydroxide is only slightly soluble in water, so if you are left with any precipitate in the beaker, refer to local waste disposal regulations for appropriate disposal methods.
• You may want to practice this demo before presenting it to students to get the timing right. You want to allow a reasonable amount of acetylene gas to build up in the pumpkin before igniting it or the resulting explosion will not be enough to blow the carved pumpkin pieces out, but you don’t want it to be too forceful an explosion. (Another problem that might occur with too much acetylene is that the interior of the pumpkin doesn’t have enough oxygen left and so the ratios of acetylene to oxygen are not ideal and the force of the explosion is not enough to blow out the carved pieces.) In general, the larger the pumpkin, the longer you should wait for it to fill with acetylene gas.
• DO NOT scale up this reaction – it could put demonstrator and students in danger! Keep a fire extinguisher on hand as well.
• Do not perform this experiment near sources of flame or flammable substances. Use a piezoelectric igniter to light the acetylene gas in the pumpkin. Piezoelectric igniters are preferable because they use materials that generate an electric spark in response to mechanical deformation, such as being struck by a small hammer-like mechanism. There is no fuel in these igniters that could catch fire if the acetylene explosion is larger than expected. Alternatively, you could light a long fuse, such as those used in model rockets, as they do in this video from the Royal Institution (which also explains some of the chemistry as well as other uses of acetylene as a fuel in mining lamps).
• The student document contains questions that require students to know how to write formulas from compound names and to predict products of reactions (particularly combustion reactions). There are also quantitative questions that require students to use stoichiometry to calculate the number of moles of acetylene gas produced by the first reaction (you could also ask for mass and/or volume), and then use the heat of combustion of acetylene (-1300 kJ/mol at 25°C) to calculate the amount of heat energy released by the explosion. You could also add questions about limiting reactants if you wanted. If you have not covered these topics yet, you can remove the questions.

Procedures for demonstration:

Preparation (Inside)

1. Cut a circle around the stem of the pumpkin so you can remove it. If the inside is really thick, slice away the excess until you have about one half of an inch of inside remaining. Set the lid aside.
2. Clean out the inside of the pumpkin and discard the seeds.
3. Cut out the design that you want carved in the pumpkin. If the insides of these cut pieces are too thick, slice away the excess until you have about one half of an inch of inside remaining.
4. Cut out a small 1-2” opening on the back of the pumpkin near the bottom and throw it away. This is where you will light the pumpkin.
5. Replace all other pieces, including the top of the pumpkin.

Explosion (Outside)

6. Measure out 1 g of calcium carbide on an electronic balance.
7. Place a 100-mL plastic beaker containing 25 mL of water on the bottom of the pumpkin.
8. Add the 1 g of calcium carbide to the beaker of water and quickly replace the lid on the pumpkin.
9. Within 30-45 seconds, put the wires of the piezoelectric igniter in the hole on the back of the pumpkin and light the inside.
   1. If the pumpkin does not ignite, continue trying every 10 seconds or so up to two minutes of total elapsed time. If it does not light by then, remove the beaker and place it in a fume hood. Once it has finished reacting, dispose of it as described in the teacher notes above. Try steps 6-9 again with 2 g of calcium carbide in 50 mL of water. (DO NOT scale up beyond this!)