Liquid entropy change with dissolution

At this temperature, the entropy change for dissolution of liquid hydrocarbons in water is zero. However, the entropy of protein denaturation is far from zero at this temperature but amounts to 17.6 J - K l per mole of amino acid residues (Privalov, 1979), a value that corresponds to an 8-fold increase of the number of possible configurations and is close to the value expected for the helix-coil transition of polypeptides (Schellman, 1955). This difference shows that an oil drop is an inadequate model for a globular protein. A more suitable model resembles that of a small crystal with a quite definite positive melting entropy (see also Bellow, 1977, 1978). 

The simplified hole model was shown to describe the data on C02 solubility in the alkali metal halide melts with good accuracy. The entropy changes in the process of dissolution are close to — 1 J mol-1 K-1, which agrees with the data of Novozhilov [311], and the solubility data obtained for the molten chlorides are in good agreement with Ref. [311]. An interesting fact was revealed— the solubility of C02 increased by four times upon the addition of a small concentration of Ni2+ ( 10-3 mol kg-1), introduced into molten NaCl as an admixture. A study of the kinetics of the dissolution process showed that the rate of C02 dissolution in alkali metal halide melts was defined by the rate of transfer of C02 from the gaseous phase into the liquid, but not by the diffusion and convection of the dissolved molecules in the melt. 

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,... 

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. 

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. 

More complex changes in the ternary phase diagram with temperature will be encountered in reality as a result of more pronounced differences in the entropy of the crystalline species and numerous non-idealities such as the temperature dependence of their enthalpy and entropy or finite interaction energies in the liquid phase. As a result of this, combinations of congruent melting and incongruent dissolution or vice versa may occur (Figure 12.6). 

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