Hydrolysis of calcium carbide and characteristics of the substance

How calcium acetylide reacts with water

Calcium carbide lights [Flickr]

Car­bides are formed in the in­ter­ac­tion of car­bon with met­als, at high tem­per­a­tures. Cal­ci­um car­bide is the most im­por­tant of all car­bides, and pure CaC₂ is a sol­id sub­stance that crys­tal­lizes well. Its col­or­less crys­tals are formed by the ions Ca²⁺ and C₂²⁻. This sub­stance is also known as cal­ci­um acetylide.

The his­to­ry of ob­tain­ing the com­pound and its phys­i­cal prop­er­ties

Cal­ci­um car­bide has be­come wide­ly used in mod­ern in­dus­try. The mol­e­cule of the sub­stance was first syn­the­sized in 1862 by the Ger­man chemist Friedrich Wöh­ler.

Friedrich Wöhler [Wikimedia]

The sci­en­tist ob­tained cal­ci­um car­bide by the fol­low­ing method: he pre­pared a com­pound of cal­ci­um and zinc, then heat­ed it with coal. As a re­sult of the re­ac­tion, car­bide formed. The chem­i­cal for­mu­la of the com­pound is CaC₂. The in­dus­tri­al method of ob­tain­ing car­bide was dis­cov­ered in 1892 by the sci­en­tist Moissan.

By its phys­i­cal prop­er­ties, cal­ci­um car­bide is a crys­talline sub­stance with a melt­ing tem­per­a­ture of 2300 de­grees Cel­sius. This fig­ure is ap­pli­ca­ble ex­clu­sive­ly for the pure com­pound. For car­bide con­tain­ing im­pu­ri­ties, oth­er melt­ing tem­per­a­tures ap­ply. The main ag­gre­gate state of the sub­stance is sol­id, and the col­or of car­bide varies from grey to brown.

Chem­i­cal prop­er­ties and meth­ods of ob­tain­ing the sub­stance

Cal­ci­um car­bide ab­sorbs wa­ter well — this process is ac­com­pa­nied by a break­down re­ac­tion. Car­bide dust has an ir­ri­tant ef­fect on the skin, the mu­cous mem­branes and res­pi­ra­to­ry or­gans. So when work­ing with the com­pound, you must use dust or gas masks. Cal­ci­um car­bide in­ter­acts with oxy­gen at a high tem­per­a­ture, with cal­ci­um car­bon­ate form­ing in this re­ac­tion. In a re­ac­tion with ni­tro­gen, a syn­the­sis of cal­ci­um cyanamide forms. At high tem­per­a­tures, cal­ci­um car­bide en­ters into a re­ac­tion with phos­pho­rous, chlo­rine and ar­senic. One of the most im­por­tant prop­er­ties of the sub­stance is the break­down by wa­ter.

Cal­ci­um car­bide is man­u­fac­tured as fol­lows: pul­ver­ized coke and un­slaked lime are mixed. The re­sult­ing mix­ture is placed in elec­tric fur­naces and melt­ed. Equal mass­es of cal­ci­um ox­ide and coke are tak­en. The process takes place at a tem­per­a­ture of 1900 de­grees Cel­sius. The com­pound comes out of the fur­nace and is poured into spe­cial molds. The hard­ened cal­ci­um car­bide is bro­ken up and sort­ed by the size of the pieces. Gran­ules of the sub­stance are di­vid­ed into four groups ac­cord­ing to their sizes: 25×80, 15×25, 8×15, 2×8.

Granules of calcium carbide [Flickr]

By its com­po­si­tion, tech­ni­cal cal­ci­um car­bide con­tains 75-80% of the orig­i­nal sub­stance. Im­pu­ri­ties such as lime, car­bon and oth­ers ac­count for up to 25% of the to­tal mass. Cal­ci­um sul­fide and phos­phide con­tained in tech­ni­cal car­bide cre­ate a very un­pleas­ant smell. The re­ac­tion for ob­tain­ing СаС₂:

СаО + 3С → СаС₂ + СО↑

The for­ma­tion of cal­ci­um acetylide is ac­com­pa­nied by an ab­sorp­tion of heat, and so the re­ac­tion of its break­down takes place with the re­lease of en­er­gy.

Hy­drol­y­sis re­ac­tion of cal­ci­um car­bide

Car­bide can be re­gard­ed as cal­ci­um acetylide, i.e. as a cal­ci­um de­riv­a­tive of acetylide, or an acetylide salt. Like ev­ery salt formed by a strong base and a weak acid, cal­ci­um car­bide is hy­drolyzed by wa­ter. The re­ac­tion of cal­ci­um car­bide with wa­ter takes place vi­o­lent­ly, with re­lease of heat – slaked lime and acety­lene form. We also rec­om­mend you to read a more de­tailed de­scrip­tion of a re­ac­tion be­tween car­bide and wa­ter.

Space-filling model of solid acetylene [Wikimedia]

For lab­o­ra­to­ry pur­pos­es, acety­lene can be ob­tained from the hy­drol­y­sis of cal­ci­um car­bide. To avoid lo­cal over­heat­ing, it is rec­om­mend­ed to use gen­er­a­tors in which cal­ci­um car­bide is im­mersed in a rel­a­tive­ly large amount of wa­ter. The acety­lene that is re­leased in the re­ac­tion is con­tam­i­nat­ed by im­pu­ri­ties of am­mo­nia, oxy­gen and hy­dro­gen sul­fide (its main mass is ab­sorbed by the wa­ter al­ka­line medi­um), hy­dro­gen phos­phide, sil­i­con hy­dride and hy­dro­gen ar­senide. This re­ac­tion is in­ter­est­ing from a his­tor­i­cal stand­point, for in the era of gas lamps, pri­vate hous­es and pub­lic build­ings were lit with acety­lene lamps. They were even in­stalled in the first cars. Por­ta­ble acety­lene lamps are used in mines to this day. Nowa­days, in the re­ac­tion of the hy­drol­y­sis of cal­ci­um car­bide, much more acety­lene is ob­tained than in the past, but acety­lene is no longer the fi­nal prod­uct of syn­the­sis. The sub­stance is used as an in­ter­me­di­ate prod­uct for the syn­the­sis of or­gan­ic com­pounds. On the ba­sis of the hy­drol­y­sis re­ac­tion of cal­ci­um car­bide, sev­er­al meth­ods for de­ter­min­ing wa­ter con­tent have been de­vel­oped. In the ma­jor­i­ty of them, the amount of acety­lene is mea­sured by mano­met­ric or vol­u­met­ric meth­ods. Oth­er meth­ods have found a lim­it­ed use based on the com­bus­tion of acety­lene, in which oxy­gen con­sump­tion or flame in­ten­si­ty is mea­sured.

The use of cal­ci­um car­bide

Cal­ci­um car­bide is used in var­i­ous spheres of life, above all in in­dus­tri­al syn­the­sis. Cal­ci­um car­bide is re­quired for the man­u­fac­ture of acetic acid, syn­thet­ic rub­ber, eth­yl­ene, ace­tone, styrene and vinyl chlo­ride. Cal­ci­um car­bide is valu­able for its use in the syn­the­sis of cyanide sub­stances and fer­til­iz­ers. The sub­stance is used in agri­cul­ture – a car­bide-car­bamide reg­u­la­tor is used for reg­u­lat­ing plant growth. This com­pound is also used in the process of man­u­fac­tur­ing cal­ci­um cyanamide – the re­ac­tion is based on heat­ing cal­ci­um car­bide with ni­tro­gen. Cal­ci­um car­bide is also used in the re­duc­tion of al­ka­line met­als, and in gas weld­ing.