Mini-Lab 3-05: Pop Rockets
Purpose:
Can you determine the efficiency of this “rocket”?
Think about this:
ΔG° is the free energy of a reaction, which is the maximum amount of energy available
to do work. It is the combination of both heat and entropy.
Raising an object away from the center of the earth requires energy (work is done on the
object) because of the gravitational field attracting it. You can calculate the work to raise
an object to a certain height by: Energy = (mass in kg)(9.8 meters/sec2)(height in meters)
Safety
1)
2)
3)
4)
5)
Don’t eat or drink anything in the lab.
Always wear eye protection.
Wear protective clothing (lab coats, etc.).
Don’t play around – treat the lab with respect.
Make sure the rocket is pointing straight up and that no body parts are
in the way when you fire it.
Questions
Put two drops of methanol into a film canister and put the top on tightly. Warm it
with your hands to vaporize the methanol. Point the canister straight up and push
the igniter button.
1) If all the free energy from the complete combustion of 2 drops of methanol (747
joules) was used to raise the film canister straight up, how high would it go?
2) Perform the experiment several times and determine the height reached by the
film canister to the best of your ability.
Average height (meters): __________________________
3) How much energy is required to lift the canister to that height?
Average energy (joules):___________________________
4) What percentage of the total energy available is being used to raise the canister?
This is also called the “efficiency” of the system.
Efficiency: ________________________
WAIT! Do not write down an answer to the Final question until your Instructor
tells you to.
FINAL: Why is the efficiency so low? Where is the rest of the energy going?
Instructor’s Page
3-05: Pop Rockets
Source: Activity designed by Ron Perkins, Educational Innovations, Inc.
Concepts: free energy, work, energy transfer
Materials: plastic film canisters, piezo-electric gas grill igniters, 1 to 2 feet of lamp cord,
electrical tape, meter sticks, methanol, some 2x4 lumber, balances or scales for
determining the masses of the film canisters
More Safety: The main danger to keep an eye out for is students creating methanol fires,
which are mainly colorless and hard to see. To avoid this, make sure the students ONLY
use two drops of methanol. Two drops will result in a fuel-rich combustion anyway, so
adding more will NOT result in higher rocket flights. What it will do is to increase the
chance of having a puddle of flaming methanol after the rocket lifts off.
Hints: The film canisters, igniters, etc., are sold as a kit by Educational Innovations, but
if you have the film canisters available, you can just by the igniters, wire and tape from a
hardware store. A source of methanol is the automotive product called “Heet”. Strip
about 2” of one end of the lamp cord and affix to the igniter. Strip about ½” of the other
end and insert through a slit in the film canister top. To achieve higher flights, and to
protect fingers from getting smacked by the canister lid at ignition, drill a ¼” hole in a
2x4 piece of wood, and insert the wire through the hole before putting it through the
canister lid (see figures below). This gives a launch pad for the “rocket” and will result
in as much as 10 meters of height when the methanol is ignited. When holding the wire
and igniting the methanol, the canister will fly a couple of meters into the air. When
using the launch pad, they may fly as high as 10 meters!
Discussion ideas: An astounding result of this activity is the fact that, if 747 Joules were
used to lift a film canister (about 4 grams) straight up, it would travel to a height of more
than 19,000 meters! Having the students determine this first will set an expectation of
great height, which will not be true when they do the experiment. Of course, the question
of where the energy goes comes down to analyzing what is really happening. The lid fits
tightly to the canister, and a lot of energy is expended in separating the two. In addition,
once the canister separates from the lid, some of the gas molecules escape horizontally
and do not contribute to lifting the canister. Only the portions of the molecular vectors
that are moving straight down contribute to lifting the canister. A great way to illustrate
this is by igniting the methanol without seating the canister firmly into the lid. The result
will be a small “poof” and hardly any motion by the canister at all. The relatively low
height of a few meters that the canister travels is also a clue that perhaps all the methanol
is not combusted. This is, in fact, true (see the advanced question and answer below).
The idea that energy is transferred primarily by physical interaction of molecules and
atoms is one that cannot be stressed enough. The canister moves because the molecules
of gas are moving out of it – Newton’s third law of motion. Would this work in space?
Of course, there would be no oxygen in space, but if you had a separate source of oxygen,
it would work just fine. This is why rockets work in space – because they carry their own
source of oxidizer. Another obvious transfer of energy is the fact that the film canister
heats up significantly. The energy is being transferred to the molecules of polymer that
make up the film canister, increasing their rate of vibrational motion and causing them to
feel “hot”. “Sound” is another energy transfer process. Get students to think about what
sound really is – air molecules bumping against one another and eventually against your
inner-ear. So, the main learning point is that energy always goes somewhere; it doesn’t
just disappear.
On the next page, there is a cartoon depiction of the launch process. You may want to
project this for the students to illustrate how to assemble and launch the rockets.
On the last page, there is an advanced chemistry question that you could use in place of,
or in addition to, the Final question if you have a more advanced group of students.
Cut about a ½” slit in the canister top:
1) Slip top over the wires that you have passed through the hole in the 2x4 wood:
2) Put two drops of methanol (“Heet”) into the canister and seal the canister onto
the lid. Warm it with your hands for about 30 seconds to vaporize the methanol.
3) Now, push the igniter button!
Here is an advanced chemistry question that you could pose as an alternative to the Final
Question above (if you have a more advanced group of students):
If you assume 1 atm of air pressure and that air contains 21% oxygen (by volume), is the
methanol undergoing complete combustion in the canister? Here is some more
information you will need:
- two drops of methanol is about 0.10 mL
- methanol has a density of 0.79 g/mL
- the film canisters have a volume of approximately 33.2 mL
Solution: The short answer is: “no”.
This solution requires the student to be able to write and balance the chemical equation
for the combustion of methanol, and also to use the ideal gas law to relate pressure,
volume, temperature and moles of a gas.
Balanced equation:
2CH3OH(g) + 3O2(g)  2CO2(g) + 4H2O(g)
Two drops of methanol (0.10 mL) is about 0.0025 mole of methanol:
 0.79 g  1mol 
  0.0025mole
(0.10mL)

 mL  32.04 g 
At 1 atmosphere of pressure, the canister contains about 0.00037 mole of O2:
 1.0 L  0.21L _ O2 
  0.00697 L _ O2
(33.2mL)

 1000mL  1.0 L _ air 
PV = nRT (assume 298 K, which is standard temperature, or measure and use the
actual room temperature)
PV
(1atm)(0.00697 L)
n
 0.000285moles _ O2
L  atm 
RT

 0.08206
(298K )
K  mol 

This amount of oxygen will only combust 0.00019 mole of methanol, which is about
7.5% of what is in the canister.


0.000285mole _ O2  2moles _ CH 3OH   0.00019mole _ CH 3OH
 3moles _ O2 
A follow-on question is: how much energy is actually being released by the incomplete
combustion of the methanol?
Solution: Since only about 7.5% of the methanol is combusted, only about 7.5% of the
energy is being released: 56 Joules.