Thermodynamic investigation of disorder in the hydrates of sodium carbonate

Measurements have been made of the molar heat capacity of crystalline anhydrous sodium carbonate and of the monohydrate and decahydrate from ∼15 to ∼300°K, of the integral heat of solution in water at 25°C of these three salts, and of the water vapour dissociation pressure at 25°C for the salt pairs, anhydrous salt + monohydrate and monohydrate + decahydrate. The calorimetric entropies at 25°C have been compared with the values obtained from the experimental entropy changes for the two equilibria and the calorimetric entropy of the anhydrous salt. For the monohydrate, the calorimetric entropy is 40.19±0.2 cal deg.–1 mole–1 and the entropy derived from the equilibrium study is 40.16±0.1 cal deg–1, implying that the monohydrate achieves a state of perfect order at 0°K. For the decahydrate, however, the calorimetric entropy is less than the value derived from the equilibrium study by 1.5±0.5 cal deg.–1 mole–1, this entropy difference being the same as that found for sodium sulphate decahydrate. The disorder persisting in both salts at 0°K probably derives from the same cause, randomness in some of the hydrogen bonding. Some dissociation pressure measurements have been made using anhydrous sodium carbonate in a finely divided form. The values differ appreciably from those obtained using the macrocrystalline salt. Reasons are given for believing that previous dissociation pressure measurements made on systems involving the metastable heptahydrate are not reliable.