Chemical characteristics of calcium carbide and its reaction ...

Cal­ci­um car­bide is a chem­i­cal com­pound of cal­ci­um and car­bon, and is a white crys­talline sub­stance when pure. It is ob­tained from the re­ac­tion

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Ca + 2C &#; CaC&#;

Calcium carbide

[Wikimedia]

Cal­ci­um car­bide has great prac­ti­cal sig­nif­i­cance. It is also known as cal­ci­um acetylide.

The chem­i­cal char­ac­ter­is­tics of cal­ci­um car­bide

Cal­ci­um car­bide is not volatile and not sol­u­ble in any known sol­vent, and re­acts with wa­ter to yield acety­lene gas and cal­ci­um hy­drox­ide. Its den­si­ty is 2.22 g/cm³. Its melt­ing point is °C, and its boil­ing point is °C. Since the acety­lene that forms upon con­tact with wa­ter is flammable, the sub­stance is list­ed in haz­ard class 4.3.

Cal­ci­um acetylide was first ob­tained by Ger­man chemist Friedrich Wöh­ler in when he heat­ed an al­loy of zinc and cal­ci­um with coal. The sci­en­tist de­scribed the re­ac­tion of cal­ci­um car­bide with wa­ter. Cal­ci­um car­bide re­acts vig­or­ous­ly with even mere traces of &#;&#;O, re­leas­ing a large amount of heat. If there is an in­suf­fi­cient quan­ti­ty of wa­ter, the re­sult­ing acetylide spon­ta­neous­ly com­busts. Cal­ci­um acetylide re­acts vi­o­lent­ly with aque­ous so­lu­tions of al­ka­lis and di­lut­ed non-or­gan­ic acids. These re­ac­tions re­lease acetylide. With its strong re­duc­tive prop­er­ties, CaC&#; re­duces all met­al ox­ides to pure met­als or turns them into car­bides.

It is eas­i­er to ob­tain cal­ci­um car­bide from its ox­ide than from cal­ci­um it­self, as the ox­ide is re­duced at tem­per­a­tures above °C. The met­al and car­bon com­bine:

CaO + 3C &#; CO&#; + CaC&#;

The re­ac­tion takes place in an elec­tric arc fur­nace, where a mix­ture of un­slaked lime and coke or an­thracite is heat­ed. The tech­ni­cal prod­uct is grey due to the pres­ence of free car­bon, cal­ci­um ox­ide, phos­phide, sul­fide, and oth­er chem­i­cal com­pounds. CaC&#; com­pris­es 80-85% of the prod­uct by mass.

Use of cal­ci­um car­bide

In the past, cal­ci­um car­bide was used in car­bide lamps, where it served as a source of acety­lene flame. Nowa­days these lamps are still used to pow­er light­hous­es and bea­cons, and also in cave ex­plo­ration. CaC&#; also serves as a raw ma­te­ri­al in the de­vel­op­ment of chem­i­cal tech­nolo­gies, most no­tably syn­thet­ic rub­ber. Cal­ci­um car­bide is also used to make vinyl chlo­ride, acety­lene black, acry­loni­trile, acetic acid, ace­tone, eth­yl­ene, styrene, and syn­thet­ic resins.

The old miner carbide lamp

[Deposit Photos]

In met­al­lur­gy, cal­ci­um car­bide is used to de­ox­i­dize met­als and re­duce their oxy­gen and sul­fur con­tent (desul­fu­ra­tion). Cal­ci­um car­bide is used to man­u­fac­ture pow­dered car­bide, a plant growth reg­u­la­tor. 3,000 kW/h of elec­tric­i­ty is re­quired to ob­tain one ton of CaC&#;. For this rea­son, the man­u­fac­ture of the sub­stance is only prof­itable when elec­tric­i­ty costs are low. At the same time, the pro­duc­tion of cal­ci­um car­bide world­wide is con­stant­ly in­creas­ing.

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Cal­ci­um car­bide &#; re­ac­tion with wa­ter

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When cal­ci­um car­bide re­acts with wa­ter, acety­lene is re­leased:

2H&#;O + CaC&#; &#; C&#;H&#;&#; + Ca(OH)&#;

Acety­lene is an in­dus­tri­al sub­stance with an un­pleas­ant smell, which is caused by the im­pu­ri­ties it con­tains (NH&#;, H&#;S, PH&#;, and oth­ers). In its pure form, acety­lene is a col­or­less gas with a char­ac­ter­is­tic faint smell, and it dis­solves in wa­ter.

Welder using an acetylene torch

[Deposit Photos]

A sim­ple ex­per­i­ment can be used to demon­strate the re­ac­tion of cal­ci­um car­bide with wa­ter: pour wa­ter into a 1.5 L bot­tle, quick­ly add sev­er­al pieces of cal­ci­um car­bide, and close the bot­tle with a stop­per. As a re­sult of the en­su­ing re­ac­tion be­tween cal­ci­um car­bide and wa­ter, acety­lene col­lects in the bot­tle as pres­sure builds. As soon as the re­ac­tion stops, place a burn­ing piece of pa­per in the bot­tle &#; this should trig­ger an ex­plo­sion ac­com­pa­nied by a fiery cloud. As the walls of the bot­tle can burst as a re­sult of the re­ac­tion, this ex­per­i­ment is dan­ger­ous, and should only be con­duct­ed with strict ob­ser­vance of safe­ty pre­cau­tions.

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To demon­strate the re­ac­tion of cal­ci­um car­bide with wa­ter, the ex­per­i­ment can be re­peat­ed in mod­i­fied form &#; us­ing a six-liter bot­tle. In this case, the com­po­nents must be weighed with pre­ci­sion, be­cause the greater the ra­dius of the bot­tle, the less the con­tain­er can with­stand high pres­sure (as­sum­ing iden­ti­cal ma­te­ri­al and wall thick­ness). A bot­tle with a large ca­pac­i­ty has a large ra­dius, but its walls are ap­prox­i­mate­ly the same &#; ac­cord­ing­ly, it is less re­sis­tant to pres­sure. To pre­vent it from ex­plod­ing, the amount of cal­ci­um car­bide must be cal­cu­lat­ed be­fore­hand. Cal­ci­um has a mo­lar mass of 40 g/mol, while car­bon&#;s is 12 g/mol, so the mo­lar mass of cal­ci­um car­bide is around 64 g/mol. Ac­cord­ing­ly, 64 g of car­bide will yield 22.4 L of acety­lene. The vol­ume of the bot­tle is 6 L, and the pres­sure has risen by ap­prox­i­mate­ly 4 at­mos­pheres.

The bot­tle must with­stand five at­mos­pheres: to con­duct the ex­per­i­ment, we take around 64 g of cal­ci­um car­bide and about 0.5 L of wa­ter. Place a piece of car­bide in­side a small bag. Push the bag into the bot­tle, then quick­ly close the bot­tle with the stop­per. The re­ac­tion of cal­ci­um car­bide with wa­ter con­tin­ues for sev­er­al min­utes, the bot­tle swells up and the process is ac­com­pa­nied by loud bangs, but the bot­tle should with­stand this.

Ball-and-stick model of the undecane molecule

[Wikimedia]

Af­ter the re­lease of acety­lene is com­plete, place a hot rag soaked in hen­de­cane on the bot­tle stop­per, then move away to a max­i­mum safe dis­tance. You will soon see a bright yel­low flash, and a foun­tain of flame up to 4 me­ters high will rise out of the bot­tle. This will burn the stop­per and warp the bot­tle, but the bot­tle should re­main in­tact. This ex­per­i­ment must be con­duct­ed in the open air, far away from flammable and ex­plo­sive ob­jects. Be sure to ob­serve all rel­e­vant safe­ty pre­cau­tions.

Calcium carbide is used as a desulfurizer, dehydrant of steel, fuel in steel making, powerful deoxidizer and as a source of acetylene gas. It is used as a starting material for the preparation of calcium cyanamide, ethylene, chloroprene rubber, acetic acid, dicyandiamide and cyanide acetate. It is used in carbide lamps, toy cannons such as the big-bang cannon and bamboo cannon. It is associated with calcium phosphide and used in floating, self-igniting naval signal flares. Further, it is involved in the reduction of copper sulfide to metallic copper.

This Thermo Scientific Chemicals brand product was originally part of the Alfa Aesar product portfolio. Some documentation and label information may refer to the legacy brand. The original Alfa Aesar product / item code or SKU reference has not changed as a part of the brand transition to Thermo Scientific Chemicals.

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