The Thermodynamics of Solution Formation

A solute is the substance being dissolved. The solvent is the dissolving medium. 

When liquids are mixed, the liquid present in the largest amount is called the solvent. 

Because a mixture, unlike a chemical compound, has a variable composition, the relative amounts of substances in a solution must be specified. The qualitative terms dilute (relatively little solute present) and concentrated (relatively large amount of solute) are often used to describe solution content. However, we need to define solution composition more precisely to perform calculations. For example, in dealing with the stoichiometry of solution reactions in Chapter 4, we found it useful to describe solution composition in terms of molarity, or the number of moles of solute per liter of solution. 

Other ways of describing solution composition are also useful. Mass percent (sometimes called weight percent) is the percent solute by mass in the solution  

In very dilute aqueous solutions the molality and the molarity are nearly the same. 

Example State of Solution State of Solute State of Solvent 

Vodka in watei antifreeze Liquid Liquid Liquid 

Seawater sugar solution Liquid Solid Liquid 

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For the origin of this expression and an excellent discussion of the thermodynamics of solution formation, see Barrow. G. Physical Chemistry, 4th ed. McGraw-Hill New York. 1979. 

Scamehorn et. al. (20) also presented a simple, semi—empirical method based on ideal solution theory and the concept of reduced adsorption isotherms to predict the mixed adsorption isotherm and admicellar composition from the pure component isotherms. In this work, we present a more general theory, based only on ideal solution theory, and present detailed mixed system data for a binary mixed surfactant system (two members of a homologous series) and use it to test this model. The thermodynamics of admicelle formation is also compared to that of micelle formation for this same system. 

A detailed discussion of the thermodynamics of complex formation in aqueous solution lies beyond the scope of this book, but we discuss briefly entropy changes that accompany the formation of coordination compounds in solution, and the so-called chelate ejfect. In Chapter 20, we look further at the thermodynamics of complex formation. 

The thermodynamics of adduct formation by BF3, BCI3 and BBr3 have been much discussed, and reactions with NMe3 (Lewis base L) in the gas phase show that the order of adduct stabilities is L-BF3 < L-BC13 < L-BBr3. Determinations of A H° for reaction 12.34 in nitrobenzene solution reveal the same sequence. 

The enthalpies of formation from the oxides of thirty-two 2-3 and 2-4 spinels, among others Ni2Ge04, have been determined by solution calorimetry in molten oxide solvents at 970 K. Regularities in the thermodynamics of spinel formation and the olivine -spinel transition for orthosilicates of Mg, Fe, Mn, Co, and Ni have been discussed. 

Electrochemical techniques have often been used to study the thermodynamics of solutions. The present study shows that the oxidation of titanium ions in fused alkali chlorides depends dramatically on the composition of the melt. Large cations weakly bonded to chloride ions, such as cesium ions, promote the formation of titanium hexachlorocomplexes, TiCl or TiCl , and results in a large stability range for the high oxidation states. Ti " reduces directly to metallic titanium. This behaviour is similar to that obtained with stronger ligands such as fluoride ions [9, 22]. 

The enthalpy of micellization of many surfactants in aqueous solution has been determined in the past, using mostly cell type and flow microcalorimeters [6-8]. These determinations were based on measurements of the excess heat associated with dilution of a surfactant from a concentration above the cmc to a concentration below the cmc, which results in demicellization of the preexisting micelles. One diffleulty with these determinations relates to the dependence of the heat evolution (AQ) on the initial and final concentrations, probably due to secondary self-aggregations of the surfactants at high concentrations and/or pre-micellar dimer formation at low surfactant concentrations [6,9], These difficulties are at least partially responsible for the lack of consistent data on the thermodynamics of micelle formation [6]. 

A detailed discussion of the thermodynamics of complex formation in aqueous solution lies beyond the scope of... 

In sum, the free-energy curves of the species involved in the reaction provide a good description of the thermodynamics of methyl formate hydrolysis in an aqueous medium. These curves, which are constructed on the basis of the solute-solvent interaction energies with the solvent fluctuation being chosen as reaction coordinate, respond acceptably to the activation barrier of this process. Also, to obtain reasonable results for this reaction in solution one must take into account a mechanism that includes the assistance of various water molecules. It was also observed that the activation barrier depends appreciably on other factors, in particular, the basis set used to describe the systems, the components of the interaction energy used in the fits of the potential functions, and the procedure employed to construct and move the curves. 

Ma.nufa.cture. Nickel carbonyl can be prepared by the direct combination of carbon monoxide and metallic nickel (77). The presence of sulfur, the surface area, and the surface activity of the nickel affect the formation of nickel carbonyl (78). The thermodynamics of formation and reaction are documented (79). Two commercial processes are used for large-scale production (80). An atmospheric method, whereby carbon monoxide is passed over nickel sulfide and freshly reduced nickel metal, is used in the United Kingdom to produce pure nickel carbonyl (81). The second method, used in Canada, involves high pressure CO in the formation of iron and nickel carbonyls the two are separated by distillation (81). Very high pressure CO is required for the formation of cobalt carbonyl and a method has been described where the mixed carbonyls are scmbbed with ammonia or an amine and the cobalt is extracted as the ammine carbonyl (82). A discontinued commercial process in the United States involved the reaction of carbon monoxide with nickel sulfate solution. 

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