Metal carbonate

Metal carbonate decompositions proceed to completion in one or more stages which are generally both endothermic and reversible. Kinetic behaviour is sensitive to the pressure and composition of the prevailing atmosphere and, in particular, to the availability and ease of removal of C02. The structure and porosity of the solid product and its relationship with the reactant phase controls the rate of escape of volatile product by inter-and/or intragranular diffusion, so that rapid and effectively complete withdrawal of C02 from the interface may be difficult to achieve experimentally. Similar features have been described for the removal of water from crystalline hydrates and attention has been drawn to comparable aspects of reactions of both types in Garner s review [ 64 ]. 

The kinetics of carbonate decompositions have been studied for both their technical importance and their theoretical interest. While both isothermal and non-isothermal techniques have been used, data obtained by rising temperature methods may be unreliable where the influence on the reverse reaction of C02 gas present has not been positively characterized. 

Selected kinetic characteristics for the decomposition of metal carbonates 

Compound Temperature range (K) Atmosphere A dec,298K (kJ mole-1) E (kJ mole-1) Remarks Ref. 

Accordingly, the present survey is mainly concerned with constant temperature methods, though thermal analysis [730] has been shown to be most valuable in identifying the temperature range of stability and the compositions of decomposition intermediates. 

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Carbon dioxide, COj. Sublimes — 78 5 C. A colourless gas at room temperature, occurs naturally and plays an important part in animal and plant respiration. Produced by the complete combustion of carbon-containing materials (industrially from flue gases and from synthesis gas used in ammonia production) and by heating metal carbonates or by... 

As with the hydroxides, we find that whilst the carbonates of most metals are insoluble, those of alkali metals are soluble, so that they provide a good source of the carbonate ion COf in solution the alkali metal carbonates, except that of lithium, are stable to heat. Group II carbonates are generally insoluble in water and less stable to heat, losing carbon dioxide reversibly at high temperatures. 

Most metal carbonates are insoluble and they are precipitated either as the simple carbonate or as the basic carbonate when... 

All Group IV elements form tetrachlorides, MX4, which are predominantly tetrahedral and covalent. Germanium, tin and lead also form dichlorides, these becoming increasingly ionic in character as the atomic weight of the Group IV element increases and the element becomes more metallic. Carbon and silicon form catenated halides which have properties similar to their tetrahalides. 

Organometallic compounds which have main group metal-metal bonds, such as S—B, Si—Mg,- Si—Al, Si—Zn, Si—Sn, Si—Si, Sn—Al, and Sn—Sn bonds, undergo 1,2-dimetallation of alkynes. Pd complexes are good catalysts for the addition of these compounds to alkynes. The 1,2-dimetallation products still have reactive metal-carbon bonds and are used for further transformations. 

Ammonium perchlorate Hot copper tubing, sugar, flnely divided organic or combustible materials, potassium periodate and permanganate, powdered metals, carbon, sulfur... 

Inert Gas Dilution. Inert gas dilution involves the use of additives that produce large volumes of noncombustible gases when the polymer is decomposed. These gases dilute the oxygen supply to the flame or dilute the fuel concentration below the flammability limit. Metal hydroxides, metal carbonates, and some nitrogen-producing compounds function in this way as flame retardants (see Flame retardants, antimony and other inorganic compounds). 

Carbonates. Iron(II) carbonate [563-71-3] FeCO, precipitates as a white soHd when air-free solutions of alkah metal carbonates and iron(II) salts are mixed. The limited tendency of [Fe(H20)g] to hydroly2e is illustrated by the lack of carbon dioxide evolution in this reaction. The soHd rapidly... 

Nickel and other transition metals function as solvent-catalysts for the transformation of carbon species into the diamond aHotrope. At temperatures high enough to melt the metal or metal—carbon mixture and at pressures high enough for diamond to be stable, diamond forms by what is probably an electronic mechanism (see Carbon, diamond-synthetic). 

Alkali Metal Titanates. Alkali metatitanates may be prepared by fusion of titanium oxide with the appropriate alkah metal carbonate or hydroxide. Representative alkah metal titanates ate hsted in Table 14. The alkah metal titanates tend to be more reactive and less stable than the other titanates, eg, they dissolve relatively easily in dilute acids. 

Ditungsten trisiUcide [12138-30-6], W2Si2, gray in color and having an sp gr of 10.9, is insoluble in water, acid, or alkaline solutions. It is readily attacked by HNO —HE and fused alkah-metal carbonates and hydroxides. 

The alkah metal—graphite compounds formed by graphite absorption of the fused metals Na, K, Rb, and Cs, represent a special type of metal—carbon compound (6). These intercalation compounds having formulas MCg are brown MC are gray and MC q are strongly graphitic. 

The bonding between carbon monoxide and transition-metal atoms is particularly important because transition metals, whether deposited on soHd supports or present as discrete complexes, are required as catalysts for the reaction between carbon monoxide and most organic molecules. A metal—carbon ( -bond forms by overlapping of metal orbitals with orbitals on carbon. Multiple-bond character between the metal and carbon occurs through formation of a metal-to-CO TT-bond by overlap of metal-i -TT orbitals with empty antibonding orbitals of carbon monoxide (Fig. 1). 

Pyrolysis. Pyrolysis of 1,2-dichloroethane in the temperature range of 340—515°C gives vinyl chloride, hydrogen chloride, and traces of acetylene (1,18) and 2-chlorobutadiene. Reaction rate is accelerated by chlorine (19), bromine, bromotrichloromethane, carbon tetrachloride (20), and other free-radical generators. Catalytic dehydrochlorination of 1,2-dichloroethane on activated alumina (3), metal carbonate, and sulfate salts (5) has been reported, and lasers have been used to initiate the cracking reaction, although not at a low enough temperature to show economic benefits. 

Many anthraquinone reactive and acid dyes are derived from bromamine acid. The bromine atom is replaced with appropriate amines in the presence of copper catalyst in water or water—alcohol mixtures in the presence of acid binding agents such as alkaU metal carbonate, bicarbonate, hydroxide, or acetate (Ullmaim condensation reaction). 

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