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Spontaneous Combustion of Acetylene
Procedure
- Add a few small calcium carbide chunks to 100ml 3M HCl in a 1 liter graduated
cylinder to generate acetylene gas.
- Cover the grad cylinder with a watch glass to contain the gas.
- When the carbide is dissolved remove the watch glass and place it on the table.
- Quickly add a pipeete full of bleach to the cylinder. Chlorine gas is
generated which will immediately ignite the acetylene.
Note: The bleach must be from a freshly opened bottle for this demo to
work well.
(Must use ultra
bleach, because ~5% sodium hypochlorite is needed).
Reactions
Calcium carbide reacts with water to yield acetylene as shown.
CaC2(s) + 2 ~ H2O(l) --> C2H2(g)
+ Ca2+(aq) + 2OH-(aq)
Bleach is about 5% NaOCl. When added to HCl, chlorine gas is made.
ClO-(aq) + Cl-(aq) --> Cl2(g)
+ H2O (l)
The tendency of Cl2 to remove hydrogen is so great that it reacts
spontaneously with acetylene to produce an exothermic reaction.
C2H2(g) + Cl2(g) --> 2HCl + heat
C2H2(g) + O2(g) --> 4CO2(g)
+ 2H2O (g) + heat
In a limited supply of oxygen there is a smokey flame because of the formation
of carbon.
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For questions or comments contact webmaster@chem.ucsd.edu
Last Updated:
10/30/02
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