## Exploding Can: A Stoichiometric Ratio

Author(s): Tom Schuster
Discrepant Event - Teacher's Guide
SED 695B; Fall 2005

Can You See the Flame?

There Goes the Lid! KaBoom!

The Explosion in Slow Motion
 Principles illustrated Exothermic Reactions Stoichiometric Ratios Organic Molecules (Hydrocarbons) Combustion Reaction Rapid Gas Expansion (explosions) What's Happening!?!?!?! This is an exciting demonstration that will get you and your students excited about chemical reactions. Materials are minimal. You will need: a paint can with lid (available for about \$2 at a hardware store) a large nail or awl to punch 2-0.5cm holes (one in the center of the lid and one about 2cm up from the bottom on the side of the can) about 3dm of hose that can be attached to a gas jet gas source, like a gas jet match Procedure: Cover both holes with masking tape Attach the gas hose to the gas jet Place the lid (top surface down) on the countertop Invert the can and fill it with gas from the hose. Fill it until you can smell the escaping gas Turn off the gas and quickly set the can on the lid. Press the can with the palm of your hand so that the lid sits tightly into the can (i.e., secure the lid to the can) Invert the can and place it in a location where students can observe it, but that it is at least 2m from the nearest student light a match Remove the both pieces of tape and simultaneously light the gas that is escaping from the hole in the lid Observe the flame. It will diminish in size and may even appear to go out. Within 6-10 minutes (depending on the amount of gas and the size of the holes), the lid will hit the ceiling as the combustion reaction occurs! THIS WILL BE STARTLING AND VERY LOUD! Explanation: The gas burns at the top of the can where the gas can react with the oxygen in the atmosphere. As the gas escapes and burns, it is replaced with the atmospheric gases (including oxygen) through the hole located at the bottom of the can. Once the oxygen entering the can reaches the correct stoichiometric ratio, the reaction goes to completion very quickly. Only then can the explosion occur. This is a standard combustion reaction; it produces carbon dioxide and water. It is also highly exothermic (exergonic) as it produces a large amount of heat. In our homes, it is important that we carefully regulate gas flow and that oxygen is not permitted in gas lines. Standards Grade 8 5. a. Students know reactant atoms and molecules interact to form products with different chemical properties. c. Students know chemical reactions usually liberate heat or absorb heat. Chemistry 3. a. Students know how to describe chemical reactions by writing balanced equations. 7. b. Students know chemical processes can either release (exothermic) or absorb (endo-thermic) thermal energy. 8. a. Students know the rate of reaction is the decrease in concentration of reactants or the increase in concentration of products with time. b. Students know how reaction rates depend on such factors as concentration, temperature, and pressure. Questioning Script Prior knowledge & experience: Gas can be dangerous. Applying a flame to a gas source often results in fire and sometimes an explosion. Students are familiar with the move Backdraft and are intrigued by explosions. Students might be familiar with propane torches, gas barbeques, and gas appliances. Root question: Why is there a several minute delay before the reaction happens? Target response: The ratio of the gases must be 1:2 (methane: oxygen) in order for the explosive reaction to occur. Common Misconceptions: Gas explosions are relatively uncommon, yet our homes have many gas appliances and often pilot lights. Why is natural gas a safe way to cook, heat our homes, and dry our clothes? Relationship to Everyday Life: This reaction is exactly what happens in a car's pistons. When properly timed and mixed, the fireing of the correct ratio of fuel and oxygen produces the greatest explosive reaction. If carburation is done correctly, cars run most efficiently. References & Links: Methane Explosion Warmed the Prehistoric Earth, Possible Again Methane Explosions and volcanoes