Solid entropy change with dissolution

To predict the sign of A S, look to see if the process involves a phase change, a change in the number of gaseous molecules, or the dissolution (or precipitation) of a solid. Entropy generally increases for phase transitions that convert a solid to a liquid or a liquid to a gas, reactions that increase the number of gaseous molecules, and dissolution of molecular solids or salts with +1 cations and —1 anions. 

The entropy of ideal dissolution (10.9) can also be easily derived using classical (as opposed to statistical) thermodynamics. This is worth doing here since it provides further insight into the problem. The derivation for ideal gases is very simple, and that for liquids and solids only slightly more complicated. Because we want to look at the effect of volume and pressure changes at constant temperature, we start with the exact differential of S with respect to T and V,... 

With some exceptions, you can predict the change in entropy when a solid or a liquid dissolves to form a solution. The solute particles, which are separate and pure before dissolving, become dispersed throughout the solvent. Therefore, dissolution usually increases the randomness and disorder of the particles, as shown in Figure 16-20, and the entropy of the system increases. For the dissolving of sodium chloride in water. 

The attractive force can be quite large for a polar solvent molecule in direct contact with an ion. This is part of the large exergonic physical change when solid salts dissolve in water. The entropy of mixing also favors dissolution (see Section 3.1.5). Table 3.6 shows several heats of hydration (equivalent to the heat of solution for water as solvent) for various ions, salts, and a few organic structures. 

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