What's In A Name? Part 1: Comparing KCl(aq) and aqeous potassium chlorate reactions with aqueous silver nitrate; Part 2: Comparing hot KCl vs potassium chlorate reaction with sugar ("Gummi Bear")
Part 1: Potassium chloride is added to one glass cylinder containing water. Potassium chlorate is added to another. An eye-dropper of aqueous 1.0 M silver nitrate solution is added to each cylinder. Silver chloride precipitates in the cylinder containing KCl(aq), but not in the KClO3(aq).
Part 2: In the second part of this demo, a test tube containing KCl (s) is heated and a small piece of "Gummi Bear" is added. Nothing happens, no reaction occurs. Next, a test tube containing KClO3(s) is heated, potassium chlorate undergoes a phase change, it melts. Further heating will cause some of the KClO3 to undergo a thermal decomposition reaction. When the piece of "Gummi Bear" is added to the hot liquid KClO3, the sugar interacts with the hot KClO3 causing thermal decomposition, the sugar reacts with the oxygen gas. A violent reaction is observed: the "Gummi Bear" bursts into flame as it is oxidized, releasing light and sound.
This demonstration is useful when the rules of inorganic nomenclature are being introduced. It emphasizes the importance of the difference in chemical properties of the anion suffix. The contrast in chemical reaction is made between the chloride anion, Cl-, and a polyatomic ion containing chlorine, ClO3-, the chlorate anion.
This demonstration illustrates the amount of energy available in carbohydrates, such as sugar which is a major component of candy.
A video of this reaction is available on-line at the following URL:
-Lee Marek, University of Illinois at Chicago: http://www.chem.uic.edu/marek/cgi-bin/vid7b.cgi
In order to help all students understand what occurs in this demonstration, instructors should show some of the accompanying Power Point slides during the performance of this demonstration.
Learning Objectives
1. There is a difference in chemical properties of the chloride, chlorate, and perchlorate anions.
2. The chloride anion is not an oxidizing agent, it does not contain oxygen. The chlorate anion, which contains oxygen, is an oxidizing agent, it causes something to be oxidized.
3. KCl, potassium chloride has a higher melting point compared to KClO3, potassium chlorate. This is due to Coulomb's Force Law. K+ and Cl- have opposite charges and have a force of attraction between the cation and anion, which is called an ionic bond. There is less distance between K+ and Cl- compared to the distance between K+ and ClO3-, therefore the force of attraction is greater in KCl. It requires more energy to separate the KCl units from its lattice compared to the KClO3 lattice. This results in KCl having a higher melting point compared to KClO3.
4. A small amount of sugar, a carbohydrate, reacts with an oxidizing agent to release a large amount of energy.
5. A balanced chemical equation represents what occurs during the reaction of sucrose with oxygen gas.
C12H22O11(s) + 12O2(g) → 12CO2(g) + 11H2O(l) + heat
Part I: Most silver salts are insoluble. The nitrate and chlorate salts are exceptions, so the chloride ion will precipitate silver ion, whereas the chlorate ion will not.
Part II: Gummi Bears are mainly sugar. Hot KCl(s) does not react with sugar. However, sugars and other organic compounds, are violently oxidized by the hot KClO3. The thermal decomposition of potassium chlorate produces potassium chloride and an excess of oxygen, which is sufficient to ignite the Gummi bear. The heat produced continues to decompose the potassium chlorate resulting in a very rapid combustion reaction.
2KClO3(l) → 2KCl(s) + 3O2(g)
C12H22O11(s) + 12O2(g) → 12CO2(g) + 11H2O(l) + heat + light + sound
The products of this reaction are KCl, CO2, and H2O. ∆H° = -5645 kJ/mol-rxn
- 2 10g samples of KCl in 25 x 200 mm stoppered test tubes
- 2 10g samples of KClO3 in 25 x 200 mm stoppered test tubes
- 2 500 mL glass cylinders filled most of the way with distilled water
- 2 long stirring rods
- dropper bottle containing 0.1M AgNO3
- propane torch
- test tube clamp
- 2 250 mL Erlenmeyer flasks
- 1 Gummi Bear
- 3 Petri dish bottoms (or tops)
- spatula
- forceps
- large bell jar
Performance mechanics in front of an audience
Part 1
- Hold up a test tube of each substance and ask students to observe their physical appearance.
- Pour one of the test tubes of each of the substances into the glass cylinders. (The KCl into one and the KClO3 into the other.) Stir to dissolve.
- Add a dropperful of AgNO3 solution into each of the cylinders.
- A precipitate forms in the cylinder containing the KCl. There is no visible reaction in the other.
Part 2
- Using the tongs, hold a test tube containing KCl in the burner flame for a minute or so. Insert the test tube into an Erlenmeyer flask sitting on an inverted Petri dish bottom. The Petri dish serves to insulate the table top from heat.
- Cut a Gummi Bear in half and drop it in the test tube. Nothing happens other than the Gummi Bear melting and sizzling a little.
- Repeat the procedure using KClO3. Heat it until the powder in the tube is completely melted. Be careful when you drop the Gummi Bear in because this time it will burst into flame.
- Carefully place the bell jar over the flaming reaction mixture to contain the fumes.
- Silver nitrate and potassium chlorate are strong oxidizers. Avoid contact with flammable materials. If melted potassium chlorate comes into contact with any flammable material it will spontaneously burst into flame.
- Goggles and proper protective equipment should be worn while performing the demonstration. This demonstration can be performed in a lecture hall, but the reaction should be performed in a (portable) hood or under a bell jar, after the Gummi Bear is added.
- Be careful handling the test tube and flask after the violent oxidation reaction. The test tube will be extremely hot! Let the apparatus cool before handling.
- Wash immediately with soap and water if any chemical gets on your skin. Flush eyes immediately with water if any chemical gets in your eyes. Contact the emergency staff.
- Make sure that the demonstration area is clear of flammable substances. Students should not be close to the reaction. Have a fire extinguisher close at hand in case of fire.
1. B.Z. Shakhashiri. Chemical Demonstrations: A Handbook for Teachers of Chemistry; Wisconsin; 1983; Volume 1; p. 77-78
2. Woodrow Wilson National Fellowship Foundation Website: http://www.woodrow.org/teachers/ci/1988/gummybear.htm
Warning: The ethical ramifications of research on Gummi animals are very controversial. Many Gummi researchers have received formal complaints from the SPCGA (Society for the Prevention of Cruelty to Gummi Animals) and a few have had their labs ransacked by the GLF (Gummi Liberation Front). This scientist believes that Gummi research must continue as long as there is hope that Gummi research may lead to cures for human diseases such as GDS (Gummi Deficiency Syndrome). At the same time, every reasonable measure must be taken to ensure that Gummi suffering is kept to a minimum.
Disposal: Allow test tube and reaction products to cool. Soak in water and scrub to remove residue. Residues can be flushed down the drain with water
