Thermal dissociation of calcium carbonate
Theoretical aspects.Thermal dissociation of CaCO3 is a solid-gas heterogeneous reaction and the equilibrium constant will depend on CO2 because it is the only gaseous substance. The dissociation occurs following the equation:
In a hard fusible tube (fig1), made by quartz or enameled porcelain, connected with one of the extremity to a vacuum pump an a manometer, a sample of CaCO3 is introduced. The tube may be heated to a constant temperature. Before the oven heating, the pump is connected a few minutes until the vacuum is obtained inside. Turn off the pump tap and then heat gradually.CO2 will be released inside of the tube and will 'press on the manometric liquid'.
If vacuum is made in tube again, one will see that the pressure will rise gradually till the corresponding characteristic value is reached. This operation can be repeated many other times till the entire amount of CaCO3 is decomposed. CaO remains in the tube.If the tube gets cool, the pressure gradually decreases passing through the same characteristic values of the temperature(as during heating).CaO2 will partially react with CaO recovering CaCO3 .When the tube gets to the ordinary temperature the pressure is practically null.
For a certain pressure, the thermal dissociation is limited. The dissociation occurs till the CaO2 resulted from the reaction reaches a maximum pressure called tension of dissociation similar with vapor tension of a liquid. This tension depends on temperature; the tension of dissociation increases with the temperature and is dependant on tube volume and also on amount of CaCO3.
A hard fusible tube containing a sample of CaCO3 is situated inside the electric oven. The tub is obturated at one extremity and connected through a three ways tap to a manometer and a vacuum pump at the other extremity. A thermoelectric couple touching the porcelain tube is fixed in the middle of the oven; the electric wires are connected to a milivoltmeter with a scale in centidegrees. The oven can be gradually heated till 780ºC by varying the current intensity with a rheostat.
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