Salt-like carbides

The salt-like carbides. Among these are aluminium tricarbide imethanide) AI4C3 (containing essentially C ions) in the crystal lattice and the rather more common dicarbides containing the C ion, for example calcium dicarbide CaCjt these carbides are hydrolysed by water yielding methane and ethyne respectively ... 

Carbides, which are binary compounds containing anionic carbon, occur as covalent and as salt-like compounds. The salt-like carbides are water-reactive and, upon hydrolysis, yield flammable hydrocarbons. Typical hydrolysis reactions include ... 

Salt-like carbides containing individual C anions are sometimes called methanides since they yield predominantly CH4 on hydrolysis. Be2C and AI4C3 are the best-characterized examples, indicating the importance of small... 

Saltlike carbides, 4 647, 648-650 Salt-like nitrides, 77 198-199 Salt mines, 22 799, 800 Salt-out, general separation heuristics for, 22 320... 

High A ij carbides with salt-like, ionic-like properties (carbides of alkali, alkaline-earth metals). 

Carbides. As might he expecled from its position in the periodic table, carbon forms binary compounds with the metals in which it exhibits a negative valence, and binary compounds with the non-metals in which it exhibits a positive valence. A convenient classification of the binary compounds of carbon is into ionic or salt-like carbides, intermediate carbides, interstitial carbides, and cuvalent binary carbon compounds. 

In carbides, carbon is bound to elements with lower or similar EN-values. We distinguish three types of carbides. The salt-like carbides with elements from groups 1, 2 and 3 are decomposed by water A14C3 +12 H20 — 4 Al(OH)3 + 3 CH4. In addition, there are the covalent carbides like SiC and B4C and a intermediate group with most transition metals. In the intermediate group C atoms are located in the octahedral cavities of metal close packings. The melting points vary from 3000 to some extreme values of about 4800 °C and their hardness lies between 7 and 10 on the Mohs scale. Furthermore, the... 

As for hydrides, borides, and carbides, different types of nitrides are possible depending on the type of metallic element. The classifications of nitrides are similarly referred to as ionic (salt-like), covalent, and interstitial. However, it should be noted that there is a transition of bond types. Within the covalent classification, nitrides are known that have a diamond or graphite structure. Principally, these are the boron nitrides that were discussed in Chapter 8. 

The carbides can be divided into four groups (1) salt-like, (2) interstitial, (3) iron-type, and (4) covalent. 

Members of this class can usually be made by heating the metal, its oxide or hydride with carbon, CO or a hydrocarbon. The salt-like carbides are... 

These metals have an atomic radius below 1.3 A. Iron (1.16 A), chromium (1.17 A) and manganese (1.17 A) form carbides with properties intermediate between the salt-like and the interstitial. Structurally FcgC, MugC and NigC have C atoms inside the trigonal prisms formed by the metal atoms. They are easily decomposed by acids and water. In CrgC2 the carbon atoms form chains in the solid. 

The interstitial structures comprise the compounds of certain metallic elements, notably the transition metals and those of the lanthanide and actinide series, with the four non-metallic elements hydrogen, boron, carbon and nitrogen. In chapter 8 we discussed the structures of a number of hydrides, borides, carbides and nitrides of the most electropositive metals, and these we found to be typical salt-like compounds with a definite composition and with physical properties entirely different from those of the constituent elements they are generally transparent to light and poor conductors of electricity. The systems now to be considered are strikingly different. They resemble... 

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. 

The salt-like interstitial compounds, on the other hand, have entirely different properties they are non-conductors of electricity and are easily decomposed by the action of water or dilute acids. Of this group, the salt-like carbides have been subject, to much study and have been classified us follows (i) Those which yield methane wit.h water, e.g. A14C3 (ii) Those which give acetylene with water, e.g. CaC2 and (iii) Those... 

Salt-like carbides. The most electropositive metals form carbides having physical and chemical properties indicating that they are essentially 13 W. A. Frad, Ado. Inorg. Chem. Radiochem., 1968, 11, 153 (a general review). 

Examples of volatile nonmetallic compounds are very familiar methane, ammonia, water. Examples of involatile compounds are silicon carbide (SiC) and silica (Si02). The former is manufactured by heating silica with graphite and is sold under the name carborundum. The commereial produet is black, but when pure it is colourless. It melts at about 2700 C. Pure silica also forms colourless crystals, melting to a colourless liquid at about 1600 °C and boiling at about 2400 C. Both compounds are insulators. Conductivity measurements have also been made on liquid silica, in which state it remains a poor conductor (cf salt-like compounds). 

Salt-like carbides (formed by the elements of Groups I,... 

The term refractory, as stated in Ch. 1, means a material with a high melting point, arbitrarily fixed at >1800°C, and with a high degree of chemical stability. Only the interstitial and covalent carbides fulfill these two qualifications. The intermediate and salt-like carbides do not meet one or both of these conditions and cannot be considered as refractory, yet they are important materials and are briefly reviewed in Secs. 5.3 and 5.4. 

Coordination Number (CN)= 12 C = covalent carbide IM = intermediate carbide S = salt-like carbide... 

The values of Table 2.2 are for the prevaling hybridization in carbide formation (see discussion of hybridization in Ch. 3). One should note that carbon is one of the smallest atoms. Table 2.2 also shows the type of carbide formed, i.e., interstitial (IS), covalent (C), intermediate (IM), or salt-like (S). 

The salt-like (or salinic) carbides are formed with carbon and the most electropositive elements, found in Groups I, II and III to the left of the Periodic Table (Table 2.1.). These elements have an electronegativity difference of about two or more which corresponds to an atomic bond that is at least S0% ionic. 

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