Calcium carbide structures

HGURE 22.18 Calcium Carbide Structure In the NaCl-type structure for CaC2, the dicarbide ions are in the positions of the chloride ions, making the structure slightly noncubic. 

In addition to the types of compounds discussed so far, the group IVA elements also form several other interesting compounds. Silicon has enough nonmetallic character that it reacts with many metals to form binary silicides. Some of these compounds can be considered as alloys of silicon and the metal that result in formulas such as Mo3Si and TiSi2. The presence of Si22 ions is indicated by a Si-Si distance that is virtually identical to that found in the element, which has the diamond structure. Calcium carbide contains the C22-, so it is an acetylide that is analogous to the silicon compounds. 

Xenon difluoride XeF2 and calcium carbide (form I) CaC2 are isomorphous with Z = 2, the linear triatomic molecule occupies Wyckoff position 2(a) of site symmetry 4/mmm. Hg2Cl2 (calomel) has a very similar crystal structure, although it is tetraatomic. The molecular structure and crystal packing of XeF2 and Hg2Cl2 are compared in Fig. 9.6.15. 

In some cases, if the complex ion is not too far from spherical, the compound may assume a structure similar to the simple ones already described, hut with slight distortion. In calcium carbide, for example, the layout of positive and negative ions is the same as in sodium chloride, but the nonspherical Cl ions are lined up with their axes parallel, distending one dimension of the square units to form rectangles. In potassium nitrate, again similar to the sodium chloride structure, the unit cell has been even further distorted from a perfect cube, all faces becoming rhombuses rather than squares. 

This section presents a brief overview of a few other compounds that have not been described in previous sections. Because it can function as a nonmetal, silicon forms sihcides with several metals. These materials are often considered as alloys in which the metal and silicon atoms surround each other in a pattern that may lead to unusual stoichiometry. Examples of this type are Mo3Si and TiSi2. In some sihcides, the Si-Si distance is about 235 pm, a distance that is quite close to the value of 234 pm found in the diamond-type structure of elemental silicon. This indicates that the structure contains Si22-, and CaSi2 is a compound of this type. This compound is analogous to calcium carbide, CaC2 (actually an acetylide that contains C22- ions (see Chapter 10)). 

Coke is employed for a number of purposes, but the major use is in the manufacture of carbon electrodes for aluminum refining that requires a high-purity carbon— low in ash and sulfur free. In addition, petroleum coke is employed in the manufacture of carbon brushes, silicon carbide abrasives, and structural carbon (pipes, Rashig rings, and so on), as well as calcium carbide manufacture from which acetylene is produced. 

Calcium carbide, CaC2, is the most important of the metal carbides. Its crystal structure resembles that of NaCl, with parallel C2 units, as shown in Figure 8-19. Before compressed gases were readily available, calcium carbide was commonly used as a source of acetylene for lighting and welding carbide headlights were on early automobiles. 

The superoxides have the tetragonal lattice of calcium carbide, in which the anion has an unpaired electron, its structure cf, the oxygen molecule p. 95) being KK (za) (ya) (xa)2(w7c) (v7r). It confers paramagnetism on the compounds, the only ones, among those of Gp. I A, known to be paramagnetic. 

Problem 8.5 What do you suppose the structure of calcium carbide is Can you suggest another name for it What is the nature of its reaction with water ... 

The structure is quite different from that of KCg. Instead of every carbon atom bonded to three other carbon atoms, as in graphite and KC, calcium carbide contains discrete C-i ions with carixm-carbon triple bonds. The structure of CaC2 is similar to the Natl structure, but because Cz is a linear anion (not spherical like Cl") the unit cell of CaCl2 is elongated in one direction making the unit cell tetragonal (see Figure 14.14). This carbide is, hence, ionic. 

The simplest alkyne is ethyne, better known as acetylene (C2H2). The structure and bonding of C2H2 were discnssed in Section 10.5. Acetylene is a colorless gas (b.p. -84°C) prepared by the reaction between calcium carbide and water ... 

Metal-like and Salt-like Interstitial Compounds. A different type of interstitial lattice structure—that of calcium carbide— is shown in Fig. 87, in which the lace-centred cubic calcium atoms are shown as ), and the interstitial carbon atoms are in groups of two. This type of lattice, in view of its slight vertical distortion, is face-centred tetragonal rather than face-centred cubic, but its most interesting feature is tho fact that here the interstitial carbon a,toms are linked together in groups of two. 

Tetraiodoethylene. Tetraiodoethene diiodoform ethylene periodide ethylene tetraiodide. f mol wt 531.70. C 4.52%, I 95.48%, I2C = CI2. Prepd by the action of iodine on diodoacetylene obtained from calcium carbide and iodine. Crystal and molecular structure B. C. Haywood, R. Shirley, Acta Crystallogr. B33, 1765 (1977). 

Silicon accounts for 27.7% of the mass of the Earth s crust and occurs in a wide variety of silicates with other metals, clays, micas, and sand, which is largely Si02. The element is obtained on a small scale by the reduction of silicon(IV) oxide (Si02) by carbon or calcium carbide. For semiconductor applications very pure silicon is produced by direct reaction of silicon with an HCI/CI2 mixture to give silicon tetrachloride (SiCb), which can be purified by distillation. This is then decomposed on a hot wire in an atmosphere of hydrogen. For ultra-pure samples zone refining is used. Unlike carbon, silicon does not form allotropes but has only the diamond type of structure. 

The binary compounds of carbon with metals, metalloids, and certain nonmetals are called carbides. The more active metals form ionic carbides, and the most common of these contain the acetylide ion (C2 ). This ion is isoelectronic with N2, and its Lewis structure, [ C=C ], has a carbon-carbon triple bond. The most important ionic carbide is calcium carbide (CaC2), produced by the reduction of CaO with carbon at high temperature ... 

Boron nitride Calcium carbide alloy refining Boron trichloride alloy, aluminum structural parts Magnesium alloy, bearing Cadmium alloy, brazing Cadmium... 

Therefore, the type of the pyrite, c, and the type of the calcium carbide t, derives from the structure of the NaCl type, that... 

Potassium hexachloroplatinate(IV), K2PtCl6, in antifluorite structure with K occupying all the tetrahedral holes in fee (c) Calcium carbide, CaC2, with occupying octahedral holes in elongated fee Ca ... 

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