Potassium Permanganate And Glycerin
Age
Elementary School, Middle School, High School
Format
Hands-on, Stage Show
Materials
Potassium Permanganate Glycerin Medicine droppers (x2) Spatula (two would be useful, but one works) Watch glasses (plastic dishes will melt) Chemical waste container Poster
Safety Precautions
Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience.
Make sure students don't lean over the watch glass. This demo involves an exothermic reaction, so the glassware can be hot, so be cautious about handling it after the reaction has taken place. Make sure you have a fire extinguisher available.
Potassium Permanganate should not be put down the drain. Make sure you have a safe way to dispose of the chemicals - i.e. have a hazardous waste container on hand. Also, this stuff stains, so the demonstrators should wear gloves.
Preparation
Set up the poster. Pour some glycerin onto a watch glass and place to the side. (You can use the medicine droppers to suck up the glycerin from here.) Set up two more watch glasses that will hold your potassium permanganate. It may be a good idea to set those on top of two more glass dishes to prevent the heat from the reaction from damaging the table you are working on.
Demonstration
Using the spatula, place a scoop of potassium permanganate onto each of the empty watch glasses. In one, make a dimple in the middle of the pile. Use the medicine droppers to place drops of glycerin simultaneously onto the two piles of potassium permanganate. In the pile with the dimple, place the drops directly into the depression. In the other, place the glycerin to the side, just touching the edge of the potassium permanganate. The reaction will take a few seconds, but eventually the mixtures will produce a white smoke followed by a bright burst of flame. Wait until the chemicals have finished reacting (You may need to add a few more drops of glycerin to get the ‘on the edge’ reaction going.) then use the spatula to scrape the products of the reaction off of your watch glasses and dispose of them in the hazardous waste container.
What to Say
Here we have two chemicals—we’re going to combine them and observe the reaction between them. The powder is potassium permanganate, (place some onto the watch glasses) and this clear, gooey liquid is called glycerin or glycerol. (gesture to other watch glass) What we’re going to observe is something called a redox reaction. That means that in the reaction, one of the chemicals, the reducing agent, in this case; glycerol, is doing okay, but it has a lot of electrons it’s juggling, and would be pretty glad if it could get rid of some of them. The other chemical, potassium permanganate, is called an oxidizing agent, which means that it would be more than happy to grab up some extra electrons. When they mix a reducing-oxidation reaction will take place; Chemists shorten that and call it a redox reaction. (Probably because it’s easier to say and sounds way cooler.)
You’ll observe that the reaction between these two chemicals will also be exothermic. If you look at the different parts of that word, you can understand what it means: “exo” means out (think exoskeletons on insects) and “thermic” means it involves heat, so an exothermic reaction is one that releases a lot of heat. (Add the glycerin to the KMn04.) We’re going to set up two dishes to watch this experiment. You’ll notice that in one dish, we put a little dimple into the pile of potassium permanganate, and placed the glycerol directly in the middle, while in the other one we put the glycerol off to the side a little. Which one do you think will react first? (Get answers, ask students why they think that, then watch to see what happens—hopefully the dimpled pile will react first.) What we were looking at with the two different reactions was to see how the surface area of where the two substances come into contact affects how fast they react. In the pile with the dimple, the glycerol was cradled by the potassium permanganate, they were touching all around, but in the other, they were just touching a little on the side. The experiment where they were touching more reacted faster.
A useful metaphor to understand what happened is this: imagine that there are two groups of people, one with candy and one without, and the people with candy want to give it to the people that don’t have it. Now, if the two groups just rush together in a big mob and hand over the candy all at once, it will get done a lot faster than if they stood in a single file line and gave pieces of candy to each other one by one. The experiment where the reactants were just touching on the side was like that single file line: if there isn’t much area to for electrons to get passed between the glycerol and potassium permanganate, the reaction isn’t going to happen very fast. That’s why more surface area makes the reaction proceed more quickly.
Why It Is
Any chemical reaction that results in atoms having their oxidation numbers changed is called a redox (reduction-oxidation) reaction. Oxidation describes either the loss of electrons or hydrogen, or the increase of oxygen or oxidation state of a molecule. Reduction describes the gain of electrons or hydrogen or the loss of oxygen or decrease of the oxidation state of a molecule. It is not entirely accurate, but the reaction can be roughly described by saying that the Oxidizing agent (potassium permanganate) “wants” more electrons and accepts them from the Reducing agent (glycerol) which wants to give them up. The reaction that takes places is:
14 KMnO4(s) + 4 C3H5(OH)3(l) à 7 K2CO3(s) + 5 CO2(g) + 16 H2O(g) + Mn2O3(s)
Real Life Examples
Potassium permanganate, KMnO4, has many uses, both everyday and scientific. It is used for water treatment to remove iron and hydrogen sulfide, which makes a rotten egg smell, from swimming pools and waste water. Dilute solutions of it are used in disinfectants for treating canker sores, hand sanitizers, and for treating fungal infections of the hands and feet. It is also used in chemistry for both quantitative and qualitative tests, such as redox titrations. KMnO4 was used in the past as a component of flash powder, which is used in fireworks, explosives, and old fashioned photography. This is no longer the case, as there are other compounds that are more stable and reliable for these purposes than potassium permanganate.
Glycerol, C3H5(OH)3, is also called glycerin or glycerine.It is relatively common, and you migh recognize the name from the ingredient lists of many foods and toiletries. In food, glycerol is used as a solvent and sweetener, as well as a preservative and an agent for keeping food moist. It is also used to produce nitroglycerin, an explosive. In pharmaceutical products, glycerol is used primarily as a lubricant to improve smoothness. It is found in cough syrups, toothpaste, shaving cream, soaps, and hair care products.
Redox reactions are important in many biological processes. Ex: the oxidation of glucose and reduction of oxygen to water ins cellular respiration.
The reaction in this demo was apparently used to start a fire once on the TV show Survivor Man, but it is not a common practice.(A lighter would probably be way more reliable and it won't turn your hands purple!) A forum discussing this: http://episteme.arstechnica.com/eve/forums/a/tpc/f/770002407831/m/857005467831