Decomposition
of H2O2 (a.k.a Genie in a Bottle)
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:
- Hydrogen peroxide (H2O2) 30%
- Manganese dioxide
(MnO2)
- Mortar and pestle
- Clean two liter bottle
Procedure:
There are a few way in which this demo can be presented.
The first way is to present the story of Aladdin's magic
lantern. For this method, place a small amount of MnO2
in a kem wipe and tie the kem wipe together with a piece
of string. Hang the string from the opening of the bottle
and secure the string with a rubber stopper so that the
kem wipe and MnO2 rest just above the level of hydrogen
peroxide. As you tell the story of Aladdin's magic lamp,
the rubber stopper can be removed to allow the kem wipe
to fall into the hydrogen peroxide. Steam and smoke will
vigorously pour from the mouth of the bottle. Then you
can go on and explain the reaction.
The second method that can be used for groups with more chemistry
background is to set up by explaining the function of a catalyst
within chemical reactions. Be sure to stress that a catalyst
is not used in the reaction, it only supplies an alternate
pathway that does not require as much energy to overcome.
Mention that since the catalyst is not used, it can be recovered
when the reaction is complete.
Without the catalyst, the decomposition of hydrogen peroxide
will occur, but the reaction time is on the orders of years
to go to completion. By using the MnO2, the reaction time is
speeded up so that it occurs instantaneously. |
