A kinetic and mechanistic study of the isothermal decomposition of calcium maleate dihydrate and calcium fumarate trihydrate

The kinetics of the isothermal decomposition of calcium maleate dihydrate (between 460 and 490 degrees C) and calcium fumarate trihydrate (between 460 and 530 degrees C) were studied. Kinetic measurements were carried out using the accumulatory method for the evolution of permanent product gases in a calibrated pre-evacuated glass apparatus. The product gases, from both reactants, were shown by mass spectrometry to be dominated by CO2 with relatively smaller amounts of CO, C2H2 and C2H4. The solid decomposition products, from both reactants, were identified using X-ray powder diffraction and IR spectroscopy as a mixture of CaCO3 and Ca(OH)(2) together with carbonaceous residue. Due to the uncertainity of the composition of the solid residue, approximate reaction stoichiometry for the decomposition of both reactants is given. Analysis of kinetic data was somewhat complicated by the intervention of a secondary reaction possibly involving gaseous species. Kinetic data of both reactants were found to fit the zero-order equation in the range 0.05 < alpha < 0.47. Afterwards, data were best fitted to the second-order rate equation in the range 0.47 < alpha < 0.78. NMR analysis of partially decomposed calcium maleate showed no isomerization to the more stable fumarate anion. Scanning electron microscopy (SEM) revealed that neither of these two reactants underwent comprehensive melting during the decomposition reaction.