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Acetylene
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Acetylene

   
The chemical compound acetylene, also called ethyne, was discovered in 1836 by Edmund Davy, in England; its chemical formula is C2H2 and its structure is:

Acetylene is a colorless and extremely flammable gas at standard temperature and pressure with a garlic-like odor. Acetylene can violently decompose if the pressure of the gas exceeds 100 kPa in its free state, so it is shipped and stored dissolved in acetone. The majority of acetylene's chemical energy is contained in the carbon-carbon triple bond.

Above 400 C (which is quite low for a hydrocarbon), the pyrolysis of Acetylene will start. The main products are the dimer vinylacetylene (C4H4) and benzene. At temperatures above 900 C, the main product will be soot.

The principal raw materials for acetylene manufacture are calcium carbonate (limestone) and coal. The calcium carbonate is first converted into calcium oxide and the coal into coke, and then the two are reacted with each other to form calcium carbide. Calcium carbide and water are then reacted by any of several methods to produce acetylene. Acetylene can also be manufactured by the partial combustion of methane with oxygen, or by the cracking of hydrocarbons.

Approximately 80 percent of the acetylene produced annually in the United States in used in chemical synthesis. The remaining 20 percent is used primarily for oxyacetylene welding and cutting. Combustion with oxygen produces a flame of over 3300 C, releasing 11,800 J/g.

Acetylene is also used in the acetylene lamp or carbide lamp, formerly found in mines (not to be confused with the (Humphry)Davy lamp), and on cars; it is still sometimes used by cavers. In this context, it is generated by adding calcium carbide (CaC2) pellets to water. The lamp was first patented in Duluth, Minnesota on October 21, 1902 (U.S. Patent No. 711 871).

Nowadays acetylene is used for carburization (i.e. hardening) of steel. Research in the last ten years has concluded that acetylene is the best hydrocarbon available for this purpose.


Burning Ice Cubes: a magic trick / experiment in acetylene synthesis

Required: Calcium carbide pellets, matches, ice cubes, borosilicate oven glass bowl (or preferably beaker), pencil and paper (optional)

Preparation: Do this experiment outside or in a fume hood. The ice need not be pure.

Experimentation:

1. Place 3-5 calcium carbide pellets in the bottom of the beaker or bowl.

2. Explain that the "rocks" are there to hold the beaker or bowl down.

3. Place ice cubes in beaker or bowl and light with a match.

4. Explain if you wish. The reaction is the following: CaC2+2H2O → Ca(OH-)2 + C2H2,
    followed by: 2C2H2+5O2 → 4CO2+2H2O. A pencil and paper may come in handy.

Clean Up:

The remaining calcium hydroxide can be saved for use in chemistry or cleaning purposes, or generously diluted and poured down the drain. Diluting can be accomplished by running water from the tap down the drain along with the calcium hydroxide - pour slowly. Wash out bowl or beaker with soap and water.

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