Summary

In this demonstration, glycerol is oxidized by using potassium permanganate as a catalyst. This gives off a tremendous amount of heat, light, and gas as a volcano of purple flames emit. It is an excellent way to introduce exothermic reactions and discuss spontaneous reactions.

Grade Level

High School

AP Chemistry Curriculum Framework

This demonstration supports the following units, topics, and learning objectives:

• Unit 4: Chemical Reactions
  • Topic 4.7: Types of Chemical Reactions
    • TRA-2.A: Identify a reaction as acid-base, oxidation-reduction, or precipitation.
• Unit 6: Thermodynamics
  • Topic 6.1: Endothermic and Exothermic Processes
    • ENE-2.A: Explain the relationship between experimental observations and energy changes associated with a chemical or physical transformation.

NGSS Alignment

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

• Scientific and Engineering Practices:
  • Analyzing and Interpreting Data

Objectives

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

• Determine which species has been oxidized and which has been reduced by evaluating a chemical equation.
• Distinguish between an exothermic and endothermic reaction.
• Understand the purpose of a catalyst.
• Successfully record meaningful observations.

Chemistry Topics

This demonstration supports students’ understanding of

• Energy & Thermodynamics
• Endothermic & Exothermic Reactions
• Spontaneous Reactions
• Oxidation & Reduction
• Classifying Reactions

Time

Teacher Preparation: 30 minutes including practice

Lesson: 15-20 minutes including discussion

Materials

• Evaporating dish (ceramic)
• Glycerol (3 mL)
• Potassium permanganate solid (15 g)
• Pipet
• Spatula
• Fume hood or safety shield

Safety

• Always wear safety goggles when handling chemicals in the lab.
• Students should wear proper safety gear during chemistry demonstrations. Safety goggles and lab apron are required.
• Potassium permanganate is a strong oxidizer and a dangerous fire risk, keep away from other materials including skin, eyes, and rubber.
• This demonstration should be carried out in the fume hood with the sash closed as sparks may be produced. Alternatively, using a polycarbonate shield to surround the reaction would also be beneficial.
• It’s recommended to wear gloves when carrying out this demonstration.
• The reaction takes between 20 and 60 seconds to get going.
• Always use caution around open flames.Keep flames away from flammable substances.
• An operational fire extinguisher should be in the classroom.

Teacher Notes

• For more information about how demos are used in my classroom, see the article, Magic Monday: Inspiring Students to Observe and Question in Chemistry, published in the March 2019 edition of Chemistry Solutions.
• I use this demonstration to discuss endothermic and exothermic reactions, types of reactions, oxidation/reduction and thermodynamic favorability depending on the level of the class.
• The original demonstration is found in Shakashiri, B. Z. Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin: Madison, 1983; Vol. 1, pp 83–84.
• The only information I provide to the students is to tell them what chemicals are used in the reaction, then I begin.
• Teacher Demonstration Procedures:

1. Put on Goggles and have the students put on goggles.
2. Place an evaporating dish in the fume hood.
3. Place about 15 grams of KMnO ₄ in a pile the shape of a volcano in the evaporating dish.
4. Using a spatula make an indentation in the top of the volcano.
5. Carefully drop about one mL (using piper) into the indentation.
6. The reaction takes about 30 seconds to begin.
7. Step back as flames and smoke are given off.

• During the demonstration I ask students some guiding questions to help them consider what they might be observing:
  • What type of compound is potassium permanganate: Typical response: ionic.
  • What type of compound is glycerol? Typical response: an alcohol.
  • Why are there flames? Typical response: the temperature is so hot that the gases ignite.
  • Why does the flame look purple/violet? Typical response: potassium burns purple.
  • Is he reaction endothermic or exothermic? Typical response: definitely exothermic.
• Students complete the observation worksheet as they watch the reaction. Sample student observations/Answer Key has been provided.
• After the demonstration we discuss the observations and I provide explanations by providing the chemical equation for the reaction.
• The complete reaction mechanism is not known, although the primary reaction involves generation of white potassium carbonate, K₂CO₃, and black manganese (III) oxide, Mn₂O₃.

14KMnO₄(s) + 4C₃H₅(OH)₃(l) → 7K ₂CO₃(s) + 7Mn₂O₃(s) + 5CO ₂(g) + 16H₂O(g) + heat

• Potassium permanganate is an oxidizer, what is the definition of oxidizer?
  • Sample Answer: a species that causes another element to oxidize by removing its electrons. Oxidizers are necessary for combustion.
• Which species is oxidized and which is reduced?
  
  • Sample Answer: The carbon (oxidation number from 0 to +4) and the manganese is reduced (oxidation number from +7 to +3).
• Other than oxidation-reduction, what other type of reaction that we have studied is taking place?
  
  • Sample Answer: the combustion of an alcohol to produce Carbon Dioxide and water.
• Is the reaction endothermic or exothermic?
  
  • Sample Answer: The reaction is exothermic as heat is generated.
• What is the purpose of using the potassium permanganate when combustion of glycerol is already exothermic?
  
  • Sample Answer: The potassium permanganate speeds up the process.