Hydrogen-Powered Soda Bottle Rockets

          The students love making hydrogen-powered soda bottle rockets. We use plastic 2 L soda bottles with the labels torn off. We produce the hydrogen by reacting zinc with 3M hydrochloric acid and and collect it "over water". The important question is, "How much hydrogen do you collect?". If you let the students decide, they will want to fill the entire bottle with hydrogen. The more the better, right? Well, not exactly. The recipe calls for oxygen and if the bottle is full, only the hydrogens at the mouth of the bottle will find a partner. It is better to pre-mix the molecules in the ratio that nature wants to react them. Have the students exercise their brains by determining how much zinc will be needed to produce the best explosion. This prediction will require some elegant back-calculating. Assuming that the atmosphere will provide us 1 oxygen molecule out of each five we let in the bottle, the students must determine what ratio of hydrogen to air in a 2L bottle will provide a 2:1 ratio of hydrogen:oxygen. The answer should be approximately 570 mL of hydrogen and 1430 mL of air. The bottle should be filled to about 29% of capacity with hydrogen. Now they must use gas laws and stoichiometry to predict how much zinc would produce this much hydrogen. They should mass out more than the predicted amount because the reaction slows considerably as the zinc is depleted. When they have filled their bottles to the desired level, they can mark it with tape and determine the volume later by filling to the marked line with water and measuring the water in a graduated cylinder. This will allow them to calculate a molar volume if that is a lesson of interest. The bottle can now be lifted from the water reservoir allowing the remaining water to drain out and be replaced with air. The bottle should be kept upside down and capped until they are ready to make water. I only let one team synthesize water at a time. One chemist should hold the bottle in an extended arm while the other chemist uses an extended arm and pair of tongs to hold a lit match at the opening of the bottle. Take a look at some matches that were used - and split right down the middle. Note where the bottle is aimed before firing. Anyone close to the reaction should protect themselves from sound waves with cotton or some other means. This is especially true if the bottles are launched in the classroom. Also, we have determined that classroom launches can greatly reduce the lifespan of ceiling tiles and the covers of light fixtures. Most of my launches have been indoors. Some stairwells make for nice indoor launch sites. If matches are used, outdoor launches will only work when there is little wind. However, outdoor launches will demonstrate just how far these bottle rockets can fly! You can even have a distance contest. Aiming straight up, we have sent bottles above our two story school building. It makes for a nice learning moment to calculate just how little hydrogen fuel there really was in that loud sound producing, high-flying bottle. Are you ready to see the pictures yet? Follow me...... (27 K)

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