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?