Carbonates heterogeneous equilibria

In heterogeneous equilibria, not all the species present are in the same state. An example of a heterogeneous equilibrium reaction is heating calcium carbonate in a closed system so that the carbon dioxide gas produced cannot escape and equilibrium is established. The equation for the reaction is ... 

However, not all the CO2 is present in the melt in the form of carbonate ion molecular CO2 is also present, based on the heterogeneous equilibrium... 

Heterogeneous equilibrium. C02 dissolves in water to give carbonic acid (which is mostly dissolved C02, as described in Box 6-4). 

The mass-action equations have been written in the same form as those given by Marynowski et al. (6) so that the equilibrium constants can be used directly. (Should more accurate data become available, the equilibrium yields calculated here will require revision.) The fourth equation, which applies to the heterogeneous equilibrium between carbon and nitrogen, is included for completeness but is unnecessary for the general solution. It can be shown that when the total pressure of the system is F, the partial pressure of cyanogen radicals is given by the equation ... 

Heterogeneous equilibrium involves reactants and products in dilTcrent phases. For example, when calcium carbonate is heated in a closed vessel, the following equilibrium reaction occurs ... 

In applying Le Chatelier s principle to a heterogeneous equilibrium, the effect of pressure changes on solids and liquids can be ignored because the volume (and concentration) of a solid or a liquid is nearly independent of pressure. Consider, for example, the high-temperature reaction of carbon with steam, the first step in converting coal to gaseous fuels ... 

In determining the concentration of the I2 species present at equilibrium in the aqueous solution, use is made of a heterogeneous equilibrium the distribution of molecular iodine I2 between two immiscible solvents, water and carbon tetrachloride. 

Schindler, P. W. (1967) Heterogeneous Equilibria Involving Oxides, Hydroxides, Carbonates and Hydroxide Carbonates. In Equilibrium Concepts in Natural Water Systems, Adv. Chem. Ser., No. 67, American Chemical Society, Washington DC, p. 196. 

ScHiNDi.KR, P. W. 1967, Heterogeneous equilibria involving oxides, hydroxides, carbonates, and hydroxide carbonates, In Equilibrium concepts in natural water sy.stems. Am. Chem. Soc. Adv. Chem. Ser. 67, pp. 196 221. Washington, DC Am. Chem. Soc. 

However, experimentai resuits show that the position of a heterogeneous equilibrium does not depend on the amounts of pure solids or liquids present (see Fig. 13.6). The fundamental reason for this behavior is that the concentrations of pure solids and liquids cannot change. Thus the equilibrium expression for the decomposition of solid calcium carbonate might be represented as... 

Akad. Nauk BSSR, 10(10), 778-782 (1966) (Phase Relations, Phase Diagram, Review) [1966Schl] Schuermann, E., Hirsch, J., Contributions to the Heterogeneous Equilibrium of Iron-Silicon-Carbon System (in German), Giesserei Tech.-Wiss. Beih., 18(1), 1-16 (1966) (Experimental, Phase Diagram, 29)... 

FIGURE 14.6 Heterogeneous Equilibrium The concentration of solid carbon (the number of atoms per unit volume) is constant as long as some sohd carbon is present. The same is true for pure bquids. For this reason, the concentrations of solids and pure liquids are not included in equihbrium constant expressions. 

The system, therefore, is at equilibrium at a given temperature when the partial pressure of carbon dioxide present has the required fixed value. This result is confirmed by experiment which shows that there is a certain fixed dissociation pressure of carbon dioxide for each temperature. The same conclusion can be deduced from the application of phase rule. In this case, there are two components occurring in three phases hence F=2-3 + 2 = l, or the system has one degree of freedom. It may thus legitimately be concluded that the assumption made in applying the law of mass action to a heterogeneous system is justified, and hence that in such systems the active mass of a solid is constant. 

In heterogeneous catalytic hydrogenations suprafacial (as) addition of hydrogen would be expected, as the transfer of hydrogen atoms from the catalyst surface to the reactant is usually assumed. However, in some Pt catalyzed reactions antarafacial (trans) addition of hydrogen is also observed. The ratio of diastereomeric products formed is determined by the chemisorption equilibrium of the surface intermediates and by the relative rates of hydrogen entrance to the different unsaturated carbon sites. Both effects are influenced by steric factors. 

In general, lack of equilibrium during the pyro-processing stage and heterogeneity of composition make it impractical to calculate the phase content of fly ash from its chemical analysis. Two other factors, common to all types of fly ash, further complicate evaluation of the potential to use fly ash. One is the presence of unburnt carbon, which should preferably not exceed a few wt% the other is the presence of alkali-sulphates (see Groppo et al., 2004). When used in cementitious formulations, free... 

The equilibrium between carbon and hydrogen at high temperatures has been treated thermodynamically by several authors (I, 2, 5, 7). The approach is to formulate the various reactions which could occur, to apply to each the appropriate mass action equation, and to solve the set of simultaneous equations so obtained. A distinction is made between heterogeneous and homogeneous systems since for the latter it is necessary to specify the ratio of carbon to hydrogen in the system. 

When the system is heterogeneous—i.e., the temperature and pressure are such that solid carbon exists in equilibrium with its vapor, the value of Pci is uniquely determined by the temperature and can be calculated directly from the equilibrium constant Ki. Hence in a heterogeneous system, the partial pressure of cyanogen radicals and of cyanogen depend only on the temperature... 

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