Relation between the Kinetics and Thermodynamics in the Endothermic Decomposition of Solids
Decomposition measurements with Mg(OH)2 in water vapour, performed by a technique based on recording the temperature inside the specimen, on its surface and in the furnace metal block, were evaluated by calculating the heat-flow into the specimen, maximum reaction rate and first-order rate constants« The temperature dependence of the latter was correlated using the Planck radiation law and respecting the influence of the equilibrium proximity. The results were: A=6.6×-1, E=135.7 kJ/mole. Since E=ΔH≠+ RT, the activation enthalpy ΔH≠ = 130.3 kJ/mole at 650 K is greater than the enthalpy change from tabulated data (math)(kJ/mole) by 12 kJ/mole. It corresponds to the enthalpy difference between a low-temperature (active) and high-temperature (stabile) MgO from Mg(0H)2. Therefore, the activation enthalpy here is equivalent to the enthalpy change of the reaction Mg(OH)2(s)sMgO(active)+H2O(g). This type of mechanism may be common for the endothermic decomposition of solids.
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