Kinetics and thermodynamics of decomposition of barium carbonate

The equilibrium vapour pressure of barium carbonate and the vacuum decomposition rates of the (001) face of its single crystal were measured by torsion–effusion and torsion–Langmuir techniques, respectively. The rate of the free surface decomposition reaction was constant for product layers up to about 1 mm thickness. The apparent activation enthalpy of decomposition for the reaction BaCO3(s)= BaO(s)+ CO2(g) is 225.9 kJ mol–1, which is less than the enthalpy of the equilibrium reaction, 252.1 kJ mol–1. The apparent activation entropy for the reaction is 53.5 J mol–1 K–1. This entropy is also less than the entropy of the equilibrium reaction, 146.6 J mol–1 K–1 in the temperature range studied. These results are shown to be consistent with theoretical predictions if condensed phase diffusion or a surface step prior to desorption is rate limiting.