Ethyne (Acetylene) is used with $\ce{O2}$ for welding. My question is that why is ethene not used? What properties of ethyne make it suitable for welding?
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An acetylene molecule is composed of two carbon atoms and two hydrogen atoms. The two carbon atoms are held together by what is known as a triple carbon bond. This bond is useful in that it stores substantial energy that can be released as heat during combustion.The triple bond which makes the oxy-acetylene flame the hottest of all gas flames is also responsible for two rather exceptional properties of acetylene gas.
The first is: If free gaseous acetylene is subjected to severe shock, or a source of ignition, some of the triple bonds may break, releasing enough energy to cause all the other molecules in the enclosed volume to decompose into carbon and hydrogen with explosive force. The force of such an explosion is not so great as that released by the explosion of most mixtures of acetylene and oxygen, or acetylene and air, but it is substantial, and can be withstood only by extra-heavy-wall steel tubing.
The other property of acetylene is that the flammability range of mixtures of air and acetylene is broader than that of any other fuel gas/air mixture. Acetylene/air mixtures can be ignited when they contain anywhere from 2.5 percent acetylene to 80 percent acetylene. Mixtures of methane (the principal component of natural gas) and air are flammable when they contain as little as 5 percent methane and not more than 15 percent methane.
To be suitable for welding operations, a fuel gas, when burned with oxygen, must have the following:
- High flame temperature.
- High rate of flame propagation.
- Adequate heat content.
- Minimum chemical reaction of the flame with base and filler metals.
Among the commercially available Oxy Fuel welding fuel gases, acetylene (combination of hydrogen and carbon) most closely meets all these requirements. Other gases, fuel such as MAPP gas, propylene, propane, natural gas, and proprietary gases based on these, have sufficiently high flame temperatures but exhibit low flame propagation rates.
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6$\begingroup$ One additional factor is that the burning of acetylene actually occurs in two steps, and this creates the inner and outer flame envelopes. It's useful for welding to have a hot center and a cooler outer area, because preheating, post-heating, and gas shielding can be products of the outer flame while welding occurs mostly in the inner flame. $\endgroup$– piojoCommented Mar 1, 2015 at 14:15
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1$\begingroup$ It sounds from the first paragraph that it's because the C-C triple bond is so strong that you get a lot of energy from burning acetlyne. $\endgroup$– richardbCommented Mar 1, 2015 at 21:33
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1$\begingroup$ dicyanoacetylene/oxygen flame is hottest. $\endgroup$– MithoronCommented Mar 2, 2015 at 1:13
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1$\begingroup$ @CCR technicality ... Bond breaking is not exothermic. If disturbing a container of acetylene gas broke some bonds and that was it, then there would be no explosion. I think that you mean is that the result of breaking the bonds - the formation of new bonds - is what causes the explosion $\endgroup$ Commented Mar 4, 2015 at 20:38
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1$\begingroup$ Another interesting fact about acetylene. The molecule is so unstable, storing the solitary gas under pressure presents a high risk of spontaneous explosion. Therefore commercial welding tanks dissolve the acetylene gas in acetone and furthermore use a spongy stone within the tank to limit energy dissipation from sloshing. Welding regulators typically limit the pressure to under 15 psig to keep the acetylene somewhat stable outside the tank. $\endgroup$ Commented May 8, 2015 at 14:18