Summary

In this demo, students will witness a cracking reaction. They will verify the products by exposing the products to side reactions.

Grade Level

High school

Objectives

By the end of this lesson, students will

• Witness a cracking reaction.
• Verify the products are different than the reactants based on two tests.

Chemistry Topics

This lesson supports students’ understanding of

• Organic chemistry
• Cracking
• Alkenes

Time

Teacher Preparation: 45 minutes
Lesson: 1 class period

Materials

• Pasteur pipet (sealed)
• Test tube
• Delivery tube
• Rubber tubing (to connect Pasteur pipet with test tube)
• Bunsen burner
• Medical paraffin
• Mineral wool
• Aluminum oxide powder
• Spirit burner with fuel and wick
• 0.002-M potassium permanganate in a small test tube (75 × 100 mm)
• 0.002-M bromine water in a small test tube (75 × 100 mm)
• Matches
  

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Always be aware of an open flame. Do not reach over it, tie back hair, and secure lose clothing.
• An operational fire extinguisher should be in the classroom.
• Take caution when handling Br₂. Wear rubber or nitrile gloves at all times.

Teacher Notes

• See Chemistry Solutions article “A Cracking Good Reaction” for more details.
• To carry out the demo, follow these instructions:

1. Seal the glass Pasteur pipet by heating the tip in a Bunsen burner flame. Allow to cool. Broken ones from the biology department can be used.
2. Use a plastic 3-mL pipet to add about 0.5 mL of medical paraffin down the sealed pipet.
3. Insert some mineral wool into the pipet so the liquid paraffin is all absorbed by the wool. Use a wooden splint or a kebab stick to push the wool down. Superwool 607 is a nontoxic mineral wool. Do not use cotton wool.
4. Insert some alumiinum oxide powder into the sealed pipet. Do not pack it too tightly.
5. Set up the sealed pipet as shown in Figure 1. Support the sealed pipet at its wide end around the silicone tubing with a clamp.

6. Place a small test tube containing 0.002-M acidified potassium permanganate solution in position so the bubbling gas can pass through it.
7. Light the spirit burner with ethanol as fuel. Place the spirit burner so that the flame is at the junction of the mineral wool and aluminum oxide. It may take a little time for the temperature to be high enough. The cracking starts to work as you see black carbon forming on the catalyst.
8. You may hold the burner and move it to the wool end to increase the bubbling at times.
9. The purple permanganate ion goes brown (MnO₂) and ultimately colorless (Mn²⁺).
10. Replace the permanganate test tube with a test tube with 0.002-M bromine water. If 0.02-M bromine water is used, do this experiment in a fume hood.
11. Once the gas is steadily bubbles through the test solution in the small test tube, quickly remove the test tube and light the gas as it emerges from the delivery tube. You may need to darken the room to see the tiny flame of burning cracked gas. This does not always work. You can also bubble the gas through a small container of water into an inverted small test tube of water and collect the gas. You may obtain enough to light.
12. Disposal: place the used glass pipet and contents in the glass waste container.