Dissociation of calcium carbonate

In the dissociation of calcium carbonate there will be three phases, viz., two solid phases (CaC03 and CaO) and one gaseous phase (C02). 

Dissociation of calcium carbonate The case of dissociation of calcium carbonate is rather complicated. Its equilibrium can be represented as ... 

Dissociation of calcium carbonate. Tension of dissociation.—Guided by the intuitions of H. Sainte-Claire Deville, H. Debray has proved that the laws of the vaporization of a liquid, laws which we have just recaUed, may be applied almost literally to the chemical decomposition of certain bodies, notably to the dissociation of carbonate of calcium into lime and carbonic acid gas. 

Let us take as a second example a monovariant system formed of calcium carbonate, carbonic acid gas, and lime denote by P the dissociation tension of calcium carbonate for the temperature T if, at the same temperature, the pressure is greater than P, the system will be the seat of a reaction accompanied by a decrease in volume, that is, of a combination of carbonic acid gas with lime if, on the contrary, the pressure is less than P, the system will be the seat of a reaction accompanied by an increase in volume, or of dissociation of calcium carbonate. 

Application to dissociation.—Equation (1) is very easily applied to the case of dissociation of which the dissociation of calcium carbonate is the type. 

EXAMPLE 22-4 The following thermodynamic data are given for the dissociation of calcium carbonate at 25°C AH = 178 kJ AG = 130 kJ AS° = 161 J/mole-deg. Estimate the temperature at which the dissociation pressure is 1 atm, ignoring the correction for heat capacity. 

The dissociation of calcium carbonate has been carefully measured of late by Johnston and Riesenfeld. Johnston s figures are given in the following table ... 

We see that if two phases are in equilibrium y all components capable of passing from one to the other must have the same chemical potential in the two phases, h) Thermal dissociation of calcium carbonate (cf. p. 17). 

Dissociation of Calcium Carbonate A number of studies of this decomposition have concluded that Ea is similar to the dissociation enthalpy for equilibrium conditions, about 173 kJ mol 1. A study of the reaction in vacuum, below I 0-2 Pa (17), however, measured the activation energy as 205 kJ mol-1, or about 30 kJ mol-1 greater than the enthalpy of the dissociation process these values have been supported by subsequent studies (78). This study investigated the reaction between 934 and 1013 K through measurements, by a quartz microbalance, of the constant rate of mass loss during inward interface movement from a single flat reactant crystal face. 

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) ... 

It is, however, only in the case of systems of more than one component that any difficulty will be found for only in this case will a choice of components be possible. Take, for example, the dissociation of calcium carbonate into calcium oxide and carbon dioxide. At each temperature, as we have seen, there is a definite state of equilibrium. When equilibrium has been established, there are three different substances present—calcium carbonate, calcium oxide, and carbon dioxide and these are the constituents of the system between which equilibrium exists. Although these constituents take part in the equilibrium, they are not all to be regarded as components, for they are not mutually independent. On the contrary, the different phases are related to one another, and if two of these are taken, the composition of the third is defined by the equation... 

If calcium oxide is formed in a thermal dissociation of calcium carbonate the texture... 

On the other hand, in a process such as the dissociation of calcium carbonate, the two solid phases are practically inuniscible and their chemical potentials depend only on the temperature (and to a very slight extent, the pressure). These potentials are therefore imable to modify themselves as reaction proceeds. The condition of equilibrium is... 

After the decomposition zone, that is, at the location with axial temperature greater than 900° C, it can be assumed that the dissociation of calcium carbonate, Equation (10.10), an endothermic reaction with AH = -1660kJ/kg CaC03, is essentially complete. 

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