Chemical characteristics of calcium carbide and its reaction with water

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

Ca + 2C → CaC₂

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 2160 °C, and its boil­ing point is 2300 °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 1862 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 Н₂O, 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, CaC₂ 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 2000 °C. The met­al and car­bon com­bine:

CaO + 3C → CO↑ + CaC₂

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. CaC₂ 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. CaC₂ 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.

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 CaC₂. 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.

Cal­ci­um car­bide – re­ac­tion with wa­ter

When cal­ci­um car­bide re­acts with wa­ter, acety­lene is re­leased:

2H₂O + CaC₂ → C₂H₂↑ + Ca(OH)₂

Acety­lene is an in­dus­tri­al sub­stance with an un­pleas­ant smell, which is caused by the im­pu­ri­ties it con­tains (NH₃, H₂S, PH₃, and oth­ers). In its pure form, acety­lene is a col­or­less gas with a char­ac­ter­is­tic faint smell, and it dis­solves in wa­ter.

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.

Warn­ing! Do not at­tempt these ex­per­i­ments with­out pro­fes­sion­al su­per­vi­sion! Look here for ex­per­i­ments with flame you can safe­ly do at home

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.

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.