Properties of Aqueous Solutions as Demonstrated by Calcium Chloride. The Effect of Temperature on the Extent of Dissociation and on the Number of Bound Waters

Water solutions of CaCl2 are important in many fields beside chemistry, such as biology, botany, enhanced petroleum recovery, construction, CO2 capture, etc. The hydrodesmic (water-binding) number of CaCl2 decreases from 12 waters strongly bound by 1 mol of CaCl2 at 0 °C to only 6 waters at 250 °C. The extent of dissociation decreases from 2.55 to 1.78. Bound waters are not available as solvents for any additional solute. This changes the effective molality, which is linearly related to decreases in vapor pressure up to the point of 50% bound water. The diminution of the dielectric constant of water with increasing temperature causes increases in ion pairing and diminishes the number of unpaired ions present rather than their water-binding affinity. From 0 to 250 °C, a plot of hydrodesmic numbers versus extents of dissociation is linear. Understanding what causes these changes enables the calculation of solution properties at any temperature from 0 to 250 °C.