Delving into the Kinetics of Reversible Thermal Decomposition of Solids Measured on Heating and Cooling

This work explores the differences in the kinetics of reversible thermal decomposition measured respectively during heating and cooling. The kinetics of the process is measured by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The thermal dehydrations of calcium oxalate monohydrate (CaC 2 O 4 ·H 2 O) and calcium sulfate dihydrate (CaSO 4 ·2H 2 O) are studied as examples of reversible decomposition. The kinetics is analyzed by an advanced isoconversional method that demonstrates that on cooling the activation energy decreases with decreasing temperature, whereas on heating it decreases with increasing temperature. This qualitative difference can be understood by modifying an earlier proposed kinetic model to account for a dependence of the equilibrium pressure on conversion. The model is applied to the thermal dehydration of CaC 2 O 4 ·H 2 O, CaSO 4 ·2H 2 O, and lithium sulfate monohydrate (Li 2 SO 4 ·H 2 O) studied previously. The results indicate that in the reversible decomposition on cooling the equilibrium pressure has much stronger dependence on conversion than for the same process on heating. The dramatic difference in the evolution of the equilibrium pressure explains the qualitative difference in the temperature dependencies of the activation energy evaluated respectively from the cooling and heating data.