Energy changes and the solution process

Energy Changes and the Solution Process Freezing-Point Depression of Solutions... 

A solution is a single-phase mixture of more than one compound, and the driving force for its spontaneous formation from the pure compounds at constant T and p is the negative Gibbs free energy change of the mixing process, —AG, as... 

Here, AGM is the free energy change for the above process and is called the free energy of mixing. GAB is the free energy of the solution, while GA and GB are the free energies of the pure components A and B respectively. A solution forms if GAB < (GA + GB) i.e., if AGM is negative. 

Therefore, all chlorine and bromine present in the compound under study should be converted to Cl- (aq) and Br- (aq) during the bomb process. The contribution of reactions 7.68-7.70 to the internal energy change of the bomb process can be taken into account, after their extent has been determined by chemical analysis of the final bomb solution. 

Consider, for example, the saturated solution of a sparingly soluble crystal. Let AII>a, and AS, t denote the heat of solution and the entropy of solution when a few additional pairs are taken into the saturated solution. The condition for equilibrium between the solid and the solution. is, of course, that there shall be no change in the free energy in this process a saturated solution is one for which AF is zero. Hence we may write at once... 

It is clear that nonconfigurational factors are of great importance in the formation of solid and liquid metal solutions. Leaving aside the problem of magnetic contributions, the vibrational contributions are not understood in such a way that they may be embodied in a statistical treatment of metallic solutions. It would be helpful to have measurements both of ACP and A a. (where a is the thermal expansion coefficient) for the solution process as a function of temperature in order to have an idea of the relative importance of changes in the harmonic and the anharmonic terms in the potential energy of the lattice. 

When two pure substances are mixed, they form a solution if the free energy change for the process is negative. The process of formation of a solution between two substances, A and B, can be represented as... 

The gas-phase lifetime of N20- is 10-3 s in alkaline solutions, it is still >10-8 s. Under suitable conditions, N20- may react with solutes, including N20. The hydrated electron reacts very quickly with NO (see Table 6.6). The rate is about three times that of diffusion control, suggesting some faster process such as tunneling. NO has an electron affinity in the gas phase enhanced upon solvation. The free energy change of the reaction NO + eh (NO-)aq is estimated to be --50 Kcal/mole. Both N02- and N03- react with eh at a nearly diffusion-controlled rate. The intermediate product in the first reaction, N02-, generates NO and... 

Table III gives values of the changes in Gibbs energy, enthalpy, entropy, and heat capacity of the solution process as calculated from the equations of Table I. Figure 1 shows the recommended noble gas mole fraction solubilities at unit gas partial pressure (atm) as a function of temperature. The temperature of minimum solubility is marked.

In order for mixing and solution to occur, it is essential that the change in free energy, AG, which is the driving force in the solution process, decrease to below zero. A/f and AG are equal to the change in enthalpy and change in entropy, and for constant temperature the relationship is the classical Gibbs equation ... 

The calculation of the energy for the interaction between the solvent and solute is more complicated. Formally, the free energy for the process, AC/ini, is composed of two chemical parts and an essentially entropic term. The chemical terms are associated with van der Waals interactions AGvdw> electrostatic effects, AG, between solute and solvent. The entropic term measures the free volume i.e., the volume a molecule explores before encountering another, which is assumed to be proportional to the molar volume of the solvent. Thus, the free energy change associated with solute-solvent interactions at temperature T is given by... 

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