Controlled explosion of a methane

Controlled explosion of a methaneair mixture
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A large tin fitted with a press-on lid and a glass chimney is filled with methane. The gas is
lit at the top of the chimney. After a while the flame burns down the chimney and, as the
methane-air mixture in the tin changes in composition, an explosive mixture is reached
and the lid of the tin is blown off with loud bang.
Lesson organisation
This simple demonstration can be used for fun, such as at Open Days, or to provide an
entertaining illustration of the effect of the composition of a mixture of an inflammable gas
and air on its explosive properties. A link can be made to domestic gas explosions.
A darkened room will heighten visibility of the explosion flame.
The time for carrying out the demonstration should be about 5 min.
Apparatus and chemicals
Eye protection for the demonstrator
Safety screens
•
A large tin with a press-on lid (see note 1)
Glass tubing, 2 - 3 cm in diameter, 30 - 50 cm long
Epoxy resin adhesive (e.g. Araldite)
Length of rubber tubing
Boss, clamp and stand
Methane (Natural Gas) supply (Extremely flammable)
Wooden splints
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Technical notes
Methane (Natural Gas) (Extremely flammable) Refer to CLEAPSS® Hazcard 45A
1 A catering size (500 – 750 g) instant coffee tin works well. Make a small hole about 1 cm
in diameter in the base of the can. Make a larger hole halfway up the side of the tin to
take the glass tubing. Use epoxy adhesive to glue the glass tube in place – see the figure
below.
flame
glass chimney
epoxy resin adhesive
coffee tin
lid
CH4
Procedure
•
a Put the lid on the tin, place it on its side and use the glass chimney to clamp it in position
so that the lid is facing away from the class – see the figure above. Place a safety screen
between the tin and the class.
b Insert the rubber tube attached to the gas source into the hole in the base of the tin
and fill the tin with methane. Allow at least a minute to ensure that the methane has
displaced all the air in the tin.
c Turn off the gas supply and remove the tubing. Without delay use a lighted splint at arms
length to ignite the gas emerging from the chimney. It will initially burn with a yellow
luminous flame. This will change to a blue flame as more air is drawn into the tin.
d After a short while the flame will start to descend the chimney. As it reaches the bottom,
the gas mixture in the can will explode, blowing the lid off the can. The explosion is fairly
gentle and the lid must not be too tightly in place or it will not be blown off.
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Teaching notes
The reaction is the combustion of methane to form carbon dioxide and water:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l), ΔH = - 890 kJ mol–1
The similarity of the flame to the flames of a Bunsen burner with the air hole open and shut
should be pointed out. The flame descends the chimney because the combustion reaction
is using up gas faster than the gas is rising up the chimney.
The energy released appears as heat, light, sound and kinetic energy (of the flying lid), similar
to the situation in an internal combustion engine. Methane-air mixtures have quite narrow
explosive limits (4 -17 mol %), whereas hydrogen-air mixtures are explosive over a much
broader range (4 -77 mol %).
The source of the energy produced by the reaction could be discussed in terms of the
breaking and making of bonds.
This demonstration will not work with other domestic gases, such as propane and butane,
because they are denser than air. This difference also affects how these gases behave in the
event of leaks. Methane (natural gas) would concentrate near the ceiling or move upstairs
and could escape through open high windows. Propane and butane would sink, building up
at floor level, and could migrate down stairs or into cellars.
Many mining disasters, especially coal mining, have been caused by explosion of methaneair mixtures. Methane levels have to be constantly monitored. Canaries and the Davy safety
lamp have been used in the past.
Reference
This experiment has been reproduced from Practical Chemistry:
http://www.practicalchemistry.org/experiments/intermediate/energy-changes-and-fuels/
controlled-explosion-of-a-methane-air-mixture,295,EX.html
Useful resources
Related experiment:
Experiment 66: Controlled explosion of a hydrogen-air mixture
News report of methane explosion in Ukranian coal mine in 1999.
http://uk.youtube.com/watch?v=LTsfyv8tT8g
Photo and some details of a volcanic methane explosion.
http://volcanoes.usgs.gov/images/pglossary/methane.php
(Last accessed December 2009)
Health & Safety checked, March 2009
Updated 22 Apr 2009
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