Calcium carbonate, decomposition reversibility

Consideration thus far has been on only balanced reactions which occur in one phase, that is, homogeneous reactions. There are, of course, a great many reactions which occur between substances in different phases, and these are known as heterogeneous reactions. Numerous reversible, heterogeneous reactions are known, and it is pertinent now to bestow consideration on how far the law of mass action can be applied to such cases. The familiar reaction of the decomposition of calcium carbonate thermally - a well-known example of a reversible reaction represented by the equation... 

Decomposition. A decomposition reaction can be considered to be the reverse of a combination reaction. In a decomposition reaction, one substance (the reactant) decomposes to form two or more products. For example, calcium carbonate (limestone) decomposes at high temperatures to calcium oxide (lime) and carbon dioxide. This reaction is used industrially to produce large quantities of lime. 

The concept of a reversible chemical reaction may be illustrated by the decomposition of calcium carbonate, which when heated forms calcium oxide and carbon dioxide gas. At equilibrium, this system exerts a definite decomposition pressure of C02 for a given temperature. When the pressure falls below this value, CaCOj decomposes. Assume now that a cylinder is fitted with a frictionless piston and contains CaC03, CaO, and C02 in equilibrium. It is immersed in a constant-temperature bath, as shown in Fig. 2.5, with the temperature adjusted to a value such that the decomposition pressure is just sufficient to balance the weight on the piston. The system is in mechanical equilibrium, the temperature of the system is equal to that of the bath, and the chemical reaction is held in balance by the pressure of the COj. Any change of conditions, however slight,... 

Reversible reactions. Many solid-gas reactions are reversible, e.g., dehydration of crystal hydrates, so that rate equations for such processes should include terms for the rate of the reverse reaction. If the rates of contributing forward and reverse reactions are comparable, the general set of kinetic models (Table 3.3.) will not be applicable. The decomposition step in a reversible reaction thus needs to be studied [94] under conditions as far removed from equilibrium as possible (e.g. low pressures or high flow rates of carrier gas) and sensitive tests are required for determining whether the kinetics vary with the prevailing conditions. Sinev [95] has calculated that, for the decomposition of calcium carbonate, the rate of the reverse reaction is comparable with that of the forward reaction even when small sample masses (10 mg) and high flow rates (200 cm s ) of inert gas are used. Interpretation of observations becomes more difficult and the reliabihty of conclusions decreases if local inhomogeneities of kinetic behaviour develop within the reactant mass. 

Kinetic data measured for the decomposition of calcium carbonate under isothermal and under programmed-temperature conditions [11] and varied reaction environments influencing the ease of removal of the CO2 product, show that the apparent values of the kinetic parameters k, A and may be influenced by sample heating rate, reactant self-cooling, sample mass, geometry and particle size, which determine the rate because of the reversible nature of the decomposition [12]. These effects can lead to compensation behaviour [13]. 

For a reversible process, the dissociation enthalpy of a solid reactant may be determined from the variation of equilibrium decomposition pressure, pe, with temperature, for example, the dissociation of calcium carbonate (17) ... 

When higher-temperature heat is available, other systems are possible. A school in Munich, Germany, dehydrated 7 metric tons of zeolites using 130°C steam in the district heating system during off-peak hours, then passed moist air over them in the day to recover the heat.201 Pellets of calcium hydroxide containing zinc, aluminum, and copper additives were dehydrated to calcium oxide using solar heat from a solar concentrator then the reaction was reversed to recover the heat.202 Zeolite 13X has been used to store carbon dioxide obtained from the decomposition of calcium carbonate at 825°C (15.2).203 Such temperatures are available with solar furnaces (where a whole field of mirrors focus on the reaction vessel). 

Problem 8.19. Calcium carbonate is heated in a crucible open to the atmosphere. The reaction is a decomposition and the equation for the reaction is CaCOjfs) CaO(s) + C02(g). Is the reaction reversible as performed ... 

When investigating the aqueous-phase bicarbonate hydrogenation with ruthenium and rhodium complexes, Benyei and J06 observed certain activity for the reverse reaction, that is, formate decomposition. [RuCl2(mTPPMS)2]2 (mTPPMS = meta-monosulfonated triphenylphosphine) decomposed sodium formate and formic acid (41), while RhCl(mTPPMS)3 slowly decomposed calcium formate and promoted calcium carbonate precipitation (42). 

Bcdinm and caustic litne are formed, Na 0, Ca 8, = Ka S, Ca 0 but if the resulting mass bo treated with water, caustic soda and sulphide of calcium are reproduced. It cannot be reasonably supposed, that when in the case of the pure salts the above-mentioned reaction is well known to take place, in the production of black ash, exactly the reverse action should ensue with the same salts and by a similar mode of treatment, Tet it is well known that in the preparation of black ash, oaustio lime may be, and in the early days of the manufacture in this country was, used instead of carbonate of lime. But in such a case a predisposing cause, if the term may be allowed, determines the reaction. When zinc and water are placed in contact, the water is not decomposed bat if an acid be added, decomposition of the water rapidly takes place. So when sulphate of sodium and lime ore fused together no change occurs but whon charucal is also present, and hot air is blown upon the mass as in the reverberatory black-ash furnace, decomposition immediately follows. 

Discussion The preparation of ammonia from ammonium chloride and calcium hydroxide resembles the preparation of carbon dioxide from an acid and a carbonate in that a double decomposition is immediately followed by the spontaneous breaking up of one of the products into water and a gas. In both cases also the secondary reaction is reversible and with ammonia this reversal, together with the great solubility of ammonia gas itself, is so marked as to require the use of solid material instead of solutions, in order to reduce as much as possible the amount of water present. 

The first reaction predominates if the product contains a large amount of water (-18%). This reaction is analogous to the disproportionation of aqueous hypochlorite. However, disproportionation is much slower in solid calcium hypochlorite than in solution. Under dry conditions, the second reaction predominates. It is catalyzed by transition metals including iron and manganese. It may occur explosively 150°C. Thus, calcium hypochlorite products usually contain some water or an additive such as magnesium sulfate heptahydrate. The third reaction is the reverse of chlorination. The fourth reaction is due to the adsorption of carbon dioxide from air or the release of carbon dioxide from carbonate salt impurities. It is accelerated by water and temperature. The first reaction accounts for -70%, and the second reaction -30%, of the decomposition of solid calcium hypochlorite made in the United States and stored in sealed containers at 25°C. ... 

The development of calcium phosphate-based ceramics at high temperature requires taking into account the thermal stability of these compounds. We can distinguish two schemes of decomposition according to the temperature irreversible decompositions (condensation of hydrogenophosphate ions, decomposition of carbonate ions, of hydroxide ions, etc.) at low temperature (150-1,000°C) and reversible decomposition (decomposition of the apatite into TCP, TTCP and lime) at high temperatures (T > 1,000°C). 

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