Dissolving entropy contribution

This means that the basicity constant measured in solution represents the total basicity of the dissolved aromatic substance. In order therefore to obtain the true basicity constant the measured figure evidently has to be divided by the number of active C-atoms in the molecule. This number, z, which is related to the symmetry of the molecule, is six for benzene, four for p-xylene, one for pentamethylbenzene, six for hexa-methylbenzene, fom for naphthalene, two for anthracene, etc. (see Tables 19 and 22). Thermodynamically, this correction can be justified as an entropy contribution (Mackor et al., 1958a). 

Thermodynamics The equilibrium constant for dissolving an ionic substance is known as the solubility product. It is related to a Gibbs free energy change that depends on a balance of lattice energy and solvation energies, together with an entropy contribution. 

The disorder of a system typically increases when a solid dissolves (Fig. 8.24). Therefore, in most cases, we can expect the entropy of the system to increase when a solution forms. Because TAS is positive, this increase in disorder makes a negative contribution to AG. If AH is negative, we can be confident that AG is negative overall. Therefore, we can expect most substances with negative enthalpies of solution to be soluble. 

In many studies of metal-hydrogen systems, attention has focused on the partial excess entropy of dissolved hydrogen (see Table I) and the evaluation of the various contributions to it. The partial excess entropy of hydrogen can be written... 

The above conclusion is unfortunate for the case of polymeric solutes, because then-entropies of dissolution are unusually small. The repeat units can not become as disordered as can the corresponding monomer molecules since they are constrained to be part of a chain-like structure. Such disordering is particularly difficult if the chain is stiff. Thus, in this situation dissolution is even less likely. Crystalline polymers are also more difficult to dissolve than are their amorphous counterparts since the enthalpy of dissolution also contains a large, positive contribution from the latent heat of fusion. 

Equations (92) and (93) show that the presence of a solvent medium other than a free space much reduces the magnitude of van der Waals interactions. In addition, the interaction between two dissimilar molecules can be attractive or repulsive depending on refractive index values. Repulsive van der Waals interactions occur when n3 is intermediate between nx and n2, in Equation (92). However, the interaction between identical molecules in a solvent is always attractive due to the square factor in Equation (93). Another important result is that the smaller the n - nj) difference, the smaller the attraction will be between two molecules (1) in solvent (3) that is the solute molecules will prefer to separate out in the solvent phase which corresponds to the well-known like dissolves like rule. However there are some important exceptions to the above explanation, such as the immiscibility of alkane hydrocarbons in water. Alkanes have nx = 1.30-1.36 up to 5 carbon atoms, and water has a refractive index of n = 1.33, and very high solubility may be expected from Equation (93) since the van der Waals attraction of two alkane molecules in water is very small. Nevertheless, when two alkane molecules approach each other in water, their entropy increases significantly because of the very high difference in their dielectric constants and also the zero-adsorption frequency contribution consequently alkane molecules associate in water (or vice versa). This behavior is not adequately understood. 

When the ideal gas is dissolved into a solvent at constant pressure, the solvent volume increases by nrksT. This volume increase, which has nothing to do with the molecular interactions, originates from the physical cause that the ideal gas makes the solvent volume increase in order to gain the entropy (at a constant pressure). The original K-B theory naturally includes this contribution in the form nTkB,T. This is true for solutes of monoatomic molecules. However, it is not so obvious if it applies to polyatomic solutes as well. If atoms in the solute molecule could move freely in the solvent, the ideal contribution would be N... 

The dissolved sections will be subjected to BROWNian movements and contribute to an entropy increase. 

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