Gay-Lussac's Law

          When gas molecules are trapped in a rigid container and exposed to burst of heat, their kinetic energy rises quickly. If the molecules are moving faster they will collide with the walls of the container more frequently and forcefully. These increased collisions will result in more force being applied to the walls of the container. In a rigid container, the area of the walls to which the collisions and forces are being applied does not increase (unlike an expandable container like a balloon) so the pressure (force/area) increases. As the gas molecules move faster and faster and the pressure builds, the lid of the container is blasted to the ceiling. The direct relationship between the temperature and pressure of gases held at constant volume was first proposed in the late 1700's by the French scientist Joseph Gay-Lussac. It is the rapid motion and increased collisions from heated gas molecules that cause the most spectacular explosions. But like any action-packed Hollywood movie, a good explosion needs a good source of heat. The chemical reactions of dynamite, TNT, etc. create rapid exothermic reactions which are able to quickly increase the speed of any gas molecules which happen to be near the reaction.
          For this small-scale explosion, I use burning lycopodium powder to create the heat I need. I have built an apparatus that mimics a grain elevator explosion. This demonstration is excellent for teaching the effect of the surface area of the reactants on the speed of a reaction. In order to burn something, the combustible reactant must be exposed to oxygen. If you build a fire with a large log, only the cellulose molecules on the surface can burn leaving 90% on more trapped on the inside. This makes for a slow combustion. If you cut the log into smaller and smaller pieces, you expose more and more of the cellulose to oxygen so more burning can take place at the same time - increased reaction rate. If I prepare an experiment to produce hydrogen gas from zinc metal and hydrochloric acid, I find it interesting how students will always choose the largest chunk of zinc they can find. Most of their zinc atoms are well-hidden from the acid and the reaction proceeds slowly. Zinc dust is much more dramatic, but may be too fast. Lycopodium is a combustible substance whose powder is composed of extremely fine particles. I begin the demonstration by filling a metal spoon with the powder and holding the edge of the pile in the flame of a Bunsen burner. The pile burns slowly and begins to blacken, but the students are not impressed. However, with a flick of your wrist, you can take advantage of the tremendous surface area of lycopodium powder. As the powder spreads out into the air, a large flame cloud develops as the oxygen molecules make their way between all the little particles. Now I want to recreate this flame cloud inside a rigid container with trapped air.
          My "tower" consists of two paint-type cans. They did not actually have paint in them when I got them, but they have the type of lid that seal tightly like a paint can. The kind you need a screwdriver to pry off. I don't remember what was in them, but I got them from a lawn and garden center. I removed the bottom of one of the cans so I could attach the two together in one tall stack. Do not try welding them. I tried that. I went to the hardware store and found a special epoxy type glue to attach them. So far so good. And they have been through a number of explosions. Next, I drilled a hole near the bottom of the stack. The hole is large enough for a #3 rubber stopper. I inserted a 4 inch section of glass tubing through the stopper. Insert the stopper so that the narrow end is coming out of the can. To the outside end of the glass tubing I attach a 4' length of rubber tubing. The tricky part was finding something for the other end of the tubing. I needed some kind of cup that hold lycopodium powder yet had a hole in the bottom to allow the powder to be dispersed inside the can. It took awhile, but I found the perfect solution. The plastic caps to the old mimeograph fluid containers have a hole that fits the glass tubing, and when turned upside down, they hold a nice pile of lycopodium powder. Fill the cap about 2/3 with powder and reach down and attach it to the glass tubing. Place a candle at the bottom and light it with match using tongs. Seal the lid down as tight as possible. I take the end of the 4' rubber tubing and retreat under the lab table directly beneath the set-up. With a quick burst of molecules from my lungs, I disperse the lycopodium powder inside the can tower. The speeding gas molecules take care of the rest. The lid of the tower will put a dent in your ceiling and the flame will rise 2-4' out the top. It is pretty spectacular. If you want to see the action, go this way....... (23 K)

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