~~Nonideal Solutions
Nonideal Solutions - раздел Образование, Part One THE PREDICATE Consider A Solid Substancc Dissolving In A Liquid Solvent. The Solid Is Such ...
Consider a solid substancc dissolving in a liquid solvent. The solid is such that when melted, it is converted to a liquid that, in turn, can form an ideal solution with the solvent. The dissolution of the solid can be pictured as occurring in two hypothetical stages:
solid solute —» liquid solute —» solute in solution.
The second of these steps docs not involve any energy change, for the solution formed is ideal. In contrast, the first step does involve the absorption of energy in the amount per mole of solute. Consequently, while the tendency toward maximum entropy favours the dissolution of the solid, the tendency toward minimum energy favours the solid remaining undissolved. Therefore, the solubility of the solid is limited, and a saturated solution which represents the best compromise between maximizing entropy and minimizing energy is formed. Sincc AHfuJ is related to the strength of attractive forces between solute molecules, wc can deduce that the magnitudes of these same forces determine the solubility of the solid in ideal solutions.
By using some carc, wc can extend our aigumcnts to nonideal solutions. Two liquids which mix with the evolution of heat will be infinitely soluble in each other, for both energy and entropy cffccts favour their mixing. Two liquids which mix with the absorption of heat may have limited solubility in each other, for if the mixing proccss is energetically unfavourable, the tendency toward maximum molccular chaos may or may not be sufficient to allow the liquids to mix in all proportions. Likewise, the solubility of a solid is likely to be small if it enters the solution only with considerable absorption of heat. On the other hand, if the dissolution of the solid is accompanied by evolution of heat, the solubility of the solid may be quite high. Even with these generalizations it is difficult to predict or even rationalize qualitatively the solubilities of substanccs that form markedly nonideal solutions, for the energy and entropy changes that accompany the mixing of strongly irreacting molecules are subtle and difficult to anticipate.
