ISSN print edition: 0366-6352 ISSN electronic edition: 1336-9075 Registr. No.: MK SR 9/7 Published monthly

Determination of equilibrium quantities of the systems formed by thermal-decomposition according to the reaction A(cond) reversibleB(cond) + C(g). 2. Divariant equilibrium states at thermal dehydration of CaC₂O₄ . H₂O

I. Nerád, I. Proks, and S. Šaušová

institute of inorganic Chemistry, Slovak Academy of Sciences, CS-84236 Bratislava

Abstract: Temperature dependence of composition of solid phase coexisting in equilibrium with water vapour in the system formed at thermal dehydration of CaC₂O₄ . H₂O was determined by isobaric-isothermal method. Temperature was changed in the range 80°C-360°C, the partial pressure of water vapour was kept at the values 7.6 kPa, 22.9 kPa, and 85.2 kPa, respectively. It was found that in the region of dehydration or rehydration of substantial amount of water the equilibrium composition of solid phase changes fluently with temperature while the partial pressure of water vapour remains constant. Thus the equilibrium is divariant and the coexisting phases are solid solution of CaC₂O₄ with CaC₂O₄ . H₂O and water vapour.When the composition of the solid solution was close to CaC₂O₄.H₂O we observed at dehydration or rehydration formation of the sequence of divariant equilibrium states with relatively constant composition of solid phase. Narrow temperature intervals corresponding to constant composition of solid phase moved with increasing pressure of water vapour to higher temperatures. It is assumed that this corresponds to gradual successive liberation or fixation of water molecules in different nonequivalent positions in the structure of solid solution. Relative distribution of water molecules bound in different positions in the structure of solid solution depends on temperature and pressure of water vapour.The enthalpy of dehydration of solid solution determined from the slope of the dependence of equilibrium pressure of water vapour on reciprocal temperature does not depend on composition of the solid solution in the, region of dehydration of main part of water and it equals  ∆H_dch = (69.8 +/- 3) kJ mol^-1.

Full paper in Portable Document Format: 456a721.pdf

Chemical Papers 45 (6) 721–730 (1991)