Chemical characteristics of calcium carbide and its reaction with water

Why is it so vigorous?

[Deposit Photos]

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

Ca + 2C → CaC₂

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 is not sol­u­ble in any known sol­vent, and breaks down on con­tact with wa­ter. The den­si­ty of cal­ci­um acetylide is 2.22 g/cm³. Its melt­ing tem­per­a­ture is 2,160 de­grees Cel­sius, and its boil­ing tem­per­a­ture is 2,300 de­grees Cel­sius. For its im­pact on the hu­man body, it is clas­si­fied as a high­ly dan­ger­ous sub­stance (1st class of dan­ger).

Cal­ci­um acetylide was first ob­tained by the Ger­man chemist Friedrich Wöh­ler in 1862 – by heat­ing an al­loy of zinc and cal­ci­um with coal. The sci­en­tist de­scribed the re­ac­tion of cal­ci­um car­bide with wa­ter. Even with traces of Н₂O, cal­ci­um car­bide re­acts vig­or­ous­ly, re­leas­ing a large amount of heat. If there is in­suf­fi­cient wa­ter, the formed acetylide self-com­busts. Cal­ci­um acetylide re­acts vi­o­lent­ly with wa­ter so­lu­tions of al­ka­lis and di­lut­ed non-or­gan­ic acids. Acetylide is re­leased dur­ing the re­ac­tion. With its strong re­duc­tive prop­er­ties, CaC₂ re­duces all met­al ox­ides to pure met­als or car­bides.

It is more con­ve­nient to ob­tain cal­ci­um car­bide from its ox­ide, and not from cal­ci­um it­self. At a high tem­per­a­ture (over 2,000 de­grees Cel­sius), the sub­stance is re­duced. 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 in col­or be­cause of the pres­ence of free car­bon, ox­ide, cal­ci­um phos­phide, sul­fide and oth­er chem­i­cal com­pounds. The mass con­tent of CaC₂ in the prod­uct is from 80% to 85%.

Use of cal­ci­um car­bide

In the past, cal­ci­um car­bide was used in car­bide lamps, where the sub­stance served as a source of acety­lene flame. Nowa­days these lamps are used in cav­ing, and also in light­hous­es and bea­cons. CaC₂ is a raw ma­te­ri­al for the de­vel­op­ment of chem­i­cal tech­nolo­gies. In the man­u­fac­ture of prod­ucts of or­gan­ic syn­the­sis, syn­thet­ic rub­ber is the most im­por­tant of these. Cal­ci­um car­bide is 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 for de­ox­i­da­tion of met­als and to re­duce the con­tent of oxy­gen and sul­fur (desul­fu­ra­tion). Cal­ci­um car­bide is used to man­u­fac­ture pow­dered car­bide reagent, a plant growth reg­u­la­tor. To ob­tain one ton of CaC₂, 3,000 kWt/hours of elec­tric­i­ty is re­quired. For this rea­son, man­u­fac­ture of the sub­stance is only jus­ti­fied with low elec­tric­i­ty prices. At the same time, the world pro­duc­tion of cal­ci­um car­bide is con­stant­ly grow­ing.

Cal­ci­um car­bide – re­ac­tion with wa­ter

When cal­ci­um car­bide re­acts with wa­ter, acety­lene is re­leased:

2H₂O + CaC₂ → C₂H₂ + Ca(OH)₂

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

Welder using an acetylene torch [Deposit Photos]

To un­der­stand how the re­ac­tion of cal­ci­um car­bide with wa­ter takes place, we can con­duct an ex­per­i­ment: pour wa­ter into a 1.5-liter bot­tle, quick­ly add sev­er­al pieces of cal­ci­um car­bide and close the bot­tle with a stop­per. As a re­sult, acety­lene col­lects in the bot­tle un­der ex­ces­sive pres­sure. As soon as the re­ac­tion of cal­ci­um car­bide with wa­ter stops, place a burn­ing piece of pa­per in the bot­tle – an ex­plo­sion takes place, ac­com­pa­nied by a fiery cloud. The walls of the bot­tle ex­plode as a re­sult of the re­ac­tion, so this ex­per­i­ment is dan­ger­ous, and should only be con­duct­ed fol­low­ing safe­ty rules.

Warn­ing! Don’t try to re­peat these ex­per­i­ments with­out a pro­fes­sion­al su­per­vi­sion! Here you’ll find out what ex­per­i­ments with flame you can safe­ly do at home

To demon­strate the re­ac­tion of cal­ci­um car­bide with wa­ter, the ex­per­i­ment can be re­peat­ed in a dif­fer­ent way – us­ing a bot­tle with a ca­pac­i­ty of 6 liters. In this case, we must weigh the com­po­nents we use pre­cise­ly, be­cause the greater ra­dius of the bot­tle, the less the con­tain­er can with­stand high pres­sure (with an iden­ti­cal thick­ness of walls and with iden­ti­cal ma­te­ri­al). A bot­tle of a large ca­pac­i­ty has a large ra­dius, but its walls are ap­prox­i­mate­ly the same – ac­cord­ing­ly, it is less re­sis­tant to pres­sure. To stop it from ex­plod­ing, the amount of cal­ci­um car­bide must be cal­cu­lat­ed be­fore­hand. Cal­ci­um has a mo­lar mass of 40 and car­bon 12, so the mo­lar mass of cal­ci­um car­bide is around 64 m/mole. Ac­cord­ing­ly, 64 g. of car­bide will give 22.4 l of acety­lene. The vol­ume of the bot­tle is 6 l, and so the ex­cess pres­sure is around 4 at­mos­pheres.

The bot­tle must with­stand four 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 with­stands 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. Soon there will be a bright yel­low flash, and a foun­tain of flame up to 4 me­ters high will rise out of the bot­tle. The stop­per burns as a re­sult, and the bot­tle is warped, but it re­mains in­tact. This ex­per­i­ment must be con­duct­ed in the open air, far away from flammable and ex­plo­sive ob­jects. Be sure to fol­low safe­ty rules.