Cloner
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mechanisms of calcium carbide reactivity
What is the mechanism of calcium carbide attacking a protic liquid? While sodium acetylide would be strongly alkaline, I am not so sure about calcium
acetylide.
Would calcium carbide react with compounds like ethylsulfuric acid?
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DJF90
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Carbide rips the protons off of water to form acetylene and OH(-) ions. Ethylsulfuric acid is a stronger acid than water and as such I expect the
reaction to be faster (read: more violent, possibly explosive).
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ammonium isocyanate
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I have personally tried this with the addition of calcium carbide to water, vinegar (4% acetic acid), and sulfuric acid.
Obviously, the reactions with the acids were faster, although the difference with the vinegar wasn't huge. The small-scale reaction with sulfuric
acid resulted in an acetylene explosion. However, this could have been triggered by phosphine gas released by calcium phosphide impurities (which,
given the smell of diphosphine I easily detected, were very much present, and this poses an additional poisoning hazard).
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Cloner
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A watery solution of an acid will obviously react with calcium carbide because a base is formed (calcium hydroxide). However, my question purely
concerns anhydrous conditions.
Sodium acetylides react as base, but mercury acetylide does not react with water. This is why I don't understand the mechanism of carbides reacting
with other compounds.
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DJF90
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Mecury acetylide is very likely to be covalent (and as such I expect polymer chains of -[Hg(2+)-C2(2-)]-, whereas these other examples (Calcium
carbide, Sodium carbide) are ionic- or at least have a much greater amount of ionic character. I believe this is where the claimed difference in
activity is. Although you are mistaken regarding the acid reacting with calcium carbide because a base (Ca(OH)2) is formed - even an anhydrous acid
(or near anhydrous - 98% H2SO4 for example) - will react with calcium carbide (It would go against all logic if it didnt) and no Ca(OH)2 is produced -
its just an acid base interaction - a proton transfer between the acid and the acetylide (base), eg:
H2SO4 [acid]+ CaC2 [base] => C2H2 (acetylene)[conjugate acid] + CaSO4 [conjugate base]
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sonogashira
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I spent a (thankfully!) small time working in a foundry when I was young.
I recall they used calcium carbide in a test for water. It was added to the sand (used for molding) and shaken in a small pressure vessel- the
pressure of the gas gave a good idea of the amount of water present.
I don't know if it answers any question- but that is my knowledge of calcium carbide, hehe 
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kmno4
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CaC2 does not react with organic acids in anhydrous solutions (from some old article).
CaC2 has very high melting point, lattice energy must be also high, not so easy to destroy.
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Cloner
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That is true. I tried it out and CaC2 barely reacts with concentrated sulfuric acid (a few bubbles were observed), only upon addition of a significant
(ca 50% of the volume) amount of water, the reaction starts to become vigourous.
How about ethanol? Does it react with alcohols directly or is it an equilibrium between alcoholate and hydroxide that makes a reaction possible?
[Edited on 9-9-2009 by Cloner]
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kmno4
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CaC2 is inert to aliphatic alcohols starting from ethanol (with methanol I did not try) and is excellent for dehydrating ethanol (many articles, also
topics in this forum).
Unfortunately thermodynamic data are not available to study such reactions
Alcohols are strong acids comparing to acetylene, but it is not the only factor playing role in reaction.
[Edited on 9-9-2009 by kmno4]
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chloric1
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Quote: Originally posted by kmno4  | CaC2 is inert to aliphatic alcohols starting from ethanol (with methanol I did not try) and is excellent for dehydrating ethanol (many articles, also
topics in this forum).
Unfortunately thermodynamic data are not available to study such reactions
Alcohols are strong acids comparing to acetylene, but it is not the only factor playing role in reaction.
[Edited on 9-9-2009 by kmno4] |
That is surprising but sometimes chemistry is that way. Obviously being ionic is not enough to convert this carbide to alcoholate. Now what about
sodium carbide and alcohol? I have to admit that this would prove different. I never catch wind of calcium ethoxide. I might go as far to say that
only metallic calcium could replace the hydrogen of ethanol. Barium Oxide on the other hand forms alcholates easier because of the larger Ba ion
increases alkalinity.
I guess someone could build a carbon arc setup, mix barium oxide with carbon and make barium carbide. Is Sodium carbode made the same way? Sodium
hydroxide and oxide are considerably more volatile than alkaline earths. I fear that the intense heat of the carbon arc will simple send sodium up in
flames.
Fellow molecular manipulator
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Cloner
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Does anyone have a clou how the mechanism would be going?
As for other carbides. n-butyllithium will deprotonate acetylene. Supposedly so will sodium hydride. The resulting solution of acetylide can be used
in organic chemistry. However, this is the alkalimetal ion of acetylene.
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