An easy steam engine (Hero's engine) is constructed from a soda can and a swivel. Burning fuel heats the engine to produce steam. The chemical energy in the fuel is converted to a frantic rotation of the can illustrating the conversion of chemical energy to thermal energy to mechanical energy.
Energy is conserved.
Chemical energy may be converted into kinetic energy.
The energy of reaction is the difference in potential energy stored in chemical bonds before and after reaction.
Kinetic energy may be used to do work.
Keep a fire extinguisher and/or a soaking wet towel nearby.
Using an unopened soda can, turn the ring-shaped opening tab 180º so it overlaps the oval-shaped seal. Then carefully bend it so it extends straight upward without breaking the seal to provide a convenient handle. If the seal around the mouth should break, start again with a new can.
This step can get messy and should be done in a sink or even outdoors! Lay the soda can on its side, and firmly press a pin into the can. Note: depending on the can's orientation, soda may squirt out of the hole. Pivot the pin to one side in order to distort the hole so that it opens tangential to the side of the can. Hold the can hole-side down over a sink or cup and gently shake to facilitate the draining of the soda.
When the pressure in the can, no longer squirts soda out, make a second hole directly opposite the first. Pivot the pin to distort the hole. (Important: the holes must both open tangentially in the same direction -- both clockwise or both counterclockwise.) Empty all soda out of the can. Either place the can in a sink with one hole down for a few minutes or blow in the top hole to force the liquid out the bottom hole.
Fill the can 1/8 to 1/4 full with water. This may be done by any of a number of ways; perhaps the simplest is just to place the can on its side in a pan containing 2-3 cm of water, with one hole on bottom (submerged), and let the water leak in over the course of 10-15 minutes. Placing something over the can to weight it down into the water more facilitates this filling process. (Submerging the can completely does not work as well.)
One end of the swivel opens like a safety pin. Clip this end of the swivel to the tab of the can. The source of fuel may be a Bunsen burner or sterno.
To use a Bunsen burner, set up a ring stand and burner. Attach the ring to the other end of the swivel with a paper clip. Establish a cool flame on a Bunsen burner and place it under the can, adjusting the ring height if necessary.
If you are not in a laboratory with gas lines, an inexpensive stand may be made by bending a hanger. Slip the other end of the swivel directly onto the hook of the hanger. The bottom of a second soda can may be cut off and inverted to serve as a container for the fuel. Prepare sterno in a beaker by mixing 20 mL of ethanol and 5 mL of saturated calcium acetate solution to form a gel. Transfer to the bottom of the soda can.
Once the water inside starts boiling and the steam starts spurting out the pin holes, the can begins spinning quite rapidly, and continues for several minutes. (To increase the duration of the spin, use more water, but expect a proportionately longer heat-up period.)
Keep a fire extinguisher and/or a soaking wet towel nearby.
Wear safety goggles.
In the past, ethanol was used as a fuel, however on one occasion the coat hanger stand began vibrating so strongly that it spilled the burning ethanol, spreading flames. The gel (sterno) prepared from ethanol and calcium acetate prevents spills.
If you do not have a gas line, use a small container of sterno as the fuel. Make the sterno in front of the students; it is an impressive demonstration itself! Mix 20 mL of ethanol with 5 mL of a saturated calcium acetate solution in a beaker.
The bottom of a soda can may be cut off, inverted, and used to contain the fuel.
If you are doing demonstrations outside the laboratory, you may bend a coat hanger to form a stand.
When the water boils the steam exits at an angle tangent to the surface of the can. The whole device spins frantically. When a hanger is used as the stand, an additional vibration of the hanger begins.
This activity demonstrates in a very inexpensive way the conversion of chemical energy in the form of methane (or ethanol) and oxygen into the thermal energy of the flame which in turn can be converted into the mechanical energy of the spinning can. This, in turn, might be used to do useful work -- perhaps some sort of spool placed over the can could coil up a thread as it spun, which in turn might lift an object off the floor.
Q1. Write a chemical equation for the complete combustion of the fuel, ethanol (C2H5OH), a component of sterno.
A1. 2 C2H5OH + 7 O2 --> 4 CO2 + 6 H2O
Q2. Is the reaction exothermic or endothermic?
Q3. Describe the steps converting the energy from the chemical reaction to the mechanical energy of the rotating can.
A3. The energy from the combustion boils the water in the can, forming steam. Because steam occupies more space than the liquid, jets of steam exit from the holes in the can pushing the can in the direction opposite to the way the holes are facing. The chemical energy is changed to thermal energy during combustion; subsequently, the thermal energy is converted to mechanical energy by changing water from the liquid state to the gaseous state. Work is accomplished by directing the steam.
In a recent K-12 science workshop, a participant, Jayne Norman, shared a neat outdoor activity she had just learned about: poke a few tangential nail holes in an empty soda can, hang it by a string from the end of a stick, dip it into a bucket of water to fill the can, lift it out and watch it spin as the water leaks out in a manner quite similar to a rotary lawn sprinkler. I was immediately reminded of a device known as a Hero's engine; Ron Perkins (Greenwich H. S., Greenwich, CT) had shown me how to make one years ago -- a blown glass bulb half-filled with water, suspended by a swivel and thread over a candle flame. When the water boils and the steam exits through a spout bent tangential to the bulb, the whole device spins frantically!
The next day it occurred to me that perhaps a Hero's engine could be made from a soda can, and, working together with workshop organizers Ed Ginoza and Don Chaney, a wonderfully easy engine was developed.