Magnesium carbonate, dissociation

The lime-burning process requires sufficient heat to be transferred to the limestone to preheat it to the dissociation temperature and to decompose the calcium and magnesium carbonates. 

Calcination is the heating of a substance so that a physical, or chemical change occurs. In the case of limestone, it refers to the dissociation of calcium and magnesium carbonates. 

If produced by a thermal dissociation of magnesium carbonate the reactivity of the... 

Magnesium carbonate, which is found in some quantities with limestone deposits, also dissociates as according to the reaction... 

The dissociation of both calcium and magnesium carbonate is endothermic and needs elevated temperature to drive the reaction (see Figures 10.1 and 10.2). The reaction rate increases with increasing temperature. For magnesium carbonate, the reaction commences at 250°C (480°F) and requires 410°C (770°F) to go to completion at atmospheric pressure (Figure 10.2) compared with calcium carbonate... 

Bases also constitute the active ingredient in antacids. There are hundreds of different brands of antacids, most of which contain one or more of the following bases sodium bicarbonate (NaHCOj), calcium carbonate (CaCOj), magnesium carbonate (MgC03), magnesium hydroxide (MglOHlj), and aluminum hydroxide (A1(0H)3). These antacids dissociate in water to produce a metal ion and a base. Sodium bicarbonate, for example, produces sodium ions and basic bicarbonate ions (HCOj ) in solution ... 

Tantalum. Numerous methods developed to extract tantalum metal from compounds included the reduction of the oxide with carbon or calcium the reduction of the pentachloride with magnesium, sodium, or hydrogen and the thermal dissociation of the pentachloride (30). The only processes that ever achieved commercial significance are the electrochemical reduction of tantalum pentoxide in molten K TaF /KF/KCl mixtures and the reduction of K TaF with sodium. 

Sihcon carbide is comparatively stable. The only violent reaction occurs when SiC is heated with a mixture of potassium dichromate and lead chromate. Chemical reactions do, however, take place between sihcon carbide and a variety of compounds at relatively high temperatures. Sodium sihcate attacks SiC above 1300°C, and SiC reacts with calcium and magnesium oxides above 1000°C and with copper oxide at 800°C to form the metal sihcide. Sihcon carbide decomposes in fused alkahes such as potassium chromate or sodium chromate and in fused borax or cryohte, and reacts with carbon dioxide, hydrogen, ak, and steam. Sihcon carbide, resistant to chlorine below 700°C, reacts to form carbon and sihcon tetrachloride at high temperature. SiC dissociates in molten kon and the sihcon reacts with oxides present in the melt, a reaction of use in the metallurgy of kon and steel (qv). The dense, self-bonded type of SiC has good resistance to aluminum up to about 800°C, to bismuth and zinc at 600°C, and to tin up to 400°C a new sihcon nitride-bonded type exhibits improved resistance to cryohte. 

Thermal stability. The degree to which a compound resists dissociation or other chemical alteration at elevated temperatures. Magnesium oxide is stable up to its melting point (2800° C.) and beyond, and hence is considered to have high thermal stability calcium bicarbonate decomposes at 100° to carbon dioxide, water, and calcium carbonate, and hence is thermally unstable. As used in the text, the term indicates chemical integrity up to a designated temperature. 

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