Negative entropy of dissolution

When a nonpolar molecule is surrounded by water, stronger than normal water-water interactions are formed around the solute molecule to compensate for the weaker interactions between solute and water. This results in an increasingly ordered arrangement of water molecules around the solute and thus a negative entropy of dissolution. The decrease in entropy is roughly proportional to the nonpolar surface area of the molecule. The association of two such nonpolar molecules in water reduces the total nonpolar surface area exposed to the solvent, thus reducing the amount of structured water, and therefore providing a favourable entropy of association. 

Both entropy and enthalpy change have to be considered when dissolving a salt in any solvent Dissolution can lead to either a positive or negative overall entropy change. In polymer electrolytes, a negative entropy of dissolution is common and can be an important consideration at higher temperatures. This effect arises because the dielectric constant of the solvent polymer (solid or liquid) is usually... 

The unfavorable Gibbs energy (AG° > 0) for the dissolution of methane in water is the result of a strongly negative entropy of solution (AA° 0), which prevails over... 

The value of —19.5 e.u. for the apparent entropy of activation obtained at the beginning of the extraction includes a term for the limited number of sites where dissolution could occur (Table IV and Figure 11). It is considered probable that the dissolution of material from the pores (process Rr) occurring simultaneously can account for part of the large negative value. Chariot (I)... 

The dissolution of a solute in a solvent has associated with it a free-energy change, AG = AH — TAS. The enthalpy change is the heat of solution (AHsoin), and the entropy change is the entropy of solution (ASsoin). Heats of solution can be either positive or negative, depending on the relative strengths of solvent-solvent, solute-solute, and solvent-solute intermolecular forces. Entropies of solution are usually positive because disorder increases when a pure solute dissolves in a pure solvent. 

FIGURE 17.5 When NaCl dissolves in water, the crystal breaks up, and the Na+ and Cl- ions are surrounded by hydrating water molecules. The polar H20 molecules are oriented such that the partially negative O atoms are near the cations and the partially positive H atoms are near the anions. Disruption of the crystal increases the entropy but the hydration process decreases the entropy. For dissolution of NaCl, the net effect is an entropy increase. 

The dissolution process is favored by a negative enthalpy change but opposed by a decrease in entropy. Decrease in entropy upon dissolution is characteristic of uncharged solutes. The standard free energy change is... 

Water is associated in a dynamic manner with nonpolar groups, but only in rare cases (where crystalline clathrates can be formed) is this water able to be isolated along with the hydrophobic groups. The phrase hydro-phobic hydration is used to describe this layer of water. The motion of water molecules is slowed down in the vicinity of nonpolar groups. Hydrophobic groups induce stmcture formation in water, hence the negative entropy (-A5) of their dissolution in water and... 

Wood and DeLaney have measured the solubility of He, Ng, Ar, and ethane in pure A-methylacetamide from 35 to 70°C and used the data to calculate the free energy, heat and entropy of solution, as well as the change in heat capacity for the dissolution process. (See Appendix 2.3.2.) Non-polar solutes appear to be much more soluble in iV-methyl-acetamide than in water. Entropies of solution are not as negative as for aqueous solutions, but the change in heat capacity on dissolution is much more negative than in water. These results lead to some interesting conclusions as to the structural effects of non-polar solutes on this solvent. 

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