Spontaneous Combustion of Acetylene

Materials

1 Liter Graduated Cylinder

Large Watch Glass

Plastic 1 mL Dropper

100 mL 3M Hydrochloric Acid

Small bottle of Calcium Carbide Chunks (about 1 gram)

Bottle of Fresh Bleach

Gloves and Goggles

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 pipete 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.

Waste Management

The leftover HCl solution should be neutrailzed and then poured down the sink.