Combustion
of Methanol
Introduction:
In many chemical reactions a catalyst is needed in order to
get the reaction going. When MnO2 is added to hydrogen peroxide,
a very exothermic and rapid chemical reaction takes place.
This reaction liberates water, oxygen and heat, as shown below.
Without the MnO2 catalyst, this
reaction does not occur at an appreciable rate. This is a very
visual reaction that is carried out in a two liter plastic
bottle.
Another very visual reaction is one involving the combustion
of methanol vapor/ air. This reaction again liberates water,
but as the reaction below shows, carbon dioxide is also given
off. Without a spark or flame, this reaction does not go because
of the high activation energy.
These two reactions can be discussed in terms
of the activation energy and its effect on the rate of a chemical
reaction. This concept can be related to many practical observations
such as catalytic converters in automobiles exhaust systems
and the fact that most things around us ate unstable with respect
to combustion. In order to get these reactions started, either
some energy or a catalyst is required.
Materials:
- Small flask of methanol
- Large plastic bottle
or plastic tube
- Matches and meter stick
- Waste container
Procedure:
Place about 25 to 30 mL of methanol into the plastic bottle
or tube and swirl it in order to saturate the air in bottle
with the methanol vapors. Pour off the methanol into the
waste container. Slowly bring a lit match to the open end
of the bottle. The methanol vapor that is present in the
bottle will ignite and burst into a blue flame. Vast amounts
of energy are given off as water and carbon dioxide are
produces. This reaction works best when the lights are
dim.
Helpful Hints:
- Both of these reactions liberate enough heat
to melt a two liter pop bottle. With the methanol reaction,
be sure that flame is out before handling the bottle (the
flame is often hard to see).
- You may need to use the mortar
and pestle to grind up the MnO2 to expose fresh surface
and increase surface area of the catalyst.
- Neither of these
reactions will occur at a noticeable rate without the catalyst
or the flame to overcome the activation energy.
- For audiences
with more chemistry background, it may be helpful to discuss
the differences between a catalyst and activation energy
with the help of a potential energy diagram (a catalyst
lowers the activation energy, in combustion energy is supplied
to get over the high energy barrier).
Safety:
- In the decomposition of H2O2 smoke and steam pour from
the mouth of the bottle very vigorously. Be sure that
the bottle is not pointed toward anyone.
- When doing the
methanol demo, the plastic bottle should never be used
as a "rocket".
- Make sure that the
bottle is secure on either the floor or a table,
safely away from any students.
- Always use safety goggles
when doing these demos and be sure to provide safety goggles
for any volunteers that may be helping.
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