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Spontaneous Combustion of Acetylene
Procedure
Note: The bleach must be from a freshly opened bottle for this demo to work well.
- 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.
(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 + heatC2H2(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.
