Solution, enthalpy change

We can consider the solution process as having three components, each with an associated enthalpy change Solute particles separate from one another (AHsoiute)> solvent particles separate from one another ( Hgolventh solute and solvent particles mix (AHj ). The overall enthalpy change, AHsoln> is... 

The chemistry of Lewis acid-base adducts (electron-pair donor-acceptor complexes) has stimulated the development of measures of the Lewis basicity of solvents. Jensen and Persson have reviewed these. Gutmann defined the donor number (DN) as the negative of the enthalpy change (in kcal moL ) for the interaction of an electron-pair donor with SbCls in a dilute solution in dichloroethane. DN has been widely used to correlate complexing data, but side reactions can lead to inaccurate DN values for some solvents. Maria and Gal measured the enthalpy change of this reaction... 

Enthalpy changes for reactions in solution can be determined using standard enthalpies of formation of aqueous ions, applying the general relation... 

We can generalize this result by stating that extrapolating enthalpy changes in solution to infinite dilution gives the enthalpy change AHa for the process5... 

The enthalpy change for this process, in which we mix pure liquids, is known as an integral enthalpy of solution. 

In this process, the original solution is diluted by the addition of pure solvent, and hence, the enthalpy change is called an integral enthalpy of dilution. 

Solution In this example, it is assumed that we add a solute to a large enough volume of solution so that the composition of the mixture does not change. The enthalpy change for this process is referred to as a differential enthalpy of solution. We can represent this process by... 

From the nature of apparent properties, we note that the apparent molar enthalpy assigns all of the enthalpy change in forming a mixture to the solute. The result, as shown in equation (7.79), is that all we need to do to calculate AH for a solution process is find the difference in oL between the products and reactants. Thus, to solve Example 7.2 using apparent molar enthalpies, we would write... 

To show how we can calculate relative apparent molar enthalpies from enthalpies of dilution, consider as an example, a process in which we start with a HC1 solution of molality m = 18.50 mol-kg-1 and dilute it to a concentration of m = 11.10 mol-kg-1. The initial solution contains 3 moles of H20 per mole of HC1 (A = 3) while the final solution has A = 5. The enthalpy change for that process is measured. Then the m = 11.10 mol-kg-1 solution is diluted to one with m = 4.63 mol-kg-1 and its enthalpy of dilution measured. The series continues as illustrated below,... 

Arrecognizes that in the infinitely dilute solution HC1 is already completely separated into ions so that no enthalpy change is involved in the ionization process. 

Because reactions in the body take place in aqueous solution, this value is not the same as the enthalpy change for the reaction in the body. However, the two values are fairly close. Therefore, the oxidation of glycine, which we have found to be exothermic, is a potential source of energy in the body. 

Potassium nitrate dissolves readily in water, and its enthalpy of solution is +34.9 kj-niol. (a) Does the enthalpy of solution favor the dissolving process (b) Is the entropy change of the system likely to be positive or negative when the salt dissolves (c) Is the entropy change of the system primarily a result of changes in positional disorder or thermal disorder ... 

The change in molar enthalpy when a substance dissolves is called the enthalpy of solution, AF/so. The change can be measured calorimetrically from the heat released or absorbed when the substance dissolves at constant pressure. However,... 

A/ /so, is the sum of the enthalpy change required to separate the molecules or ions of the solute, the lattice enthalpy,... 

In the second hypothetical step, we imagine the gaseous ions plunging into water and forming the final solution. The molar enthalpy of this step is called the enthalpy of hydration, AHhvd, of the compound (Table 8.7). Enthalpies of hydration are negative and comparable in value to the lattice enthalpies of the compounds. For sodium chloride, for instance, the enthalpy of hydration, the molar enthalpy change for the process... 

When we include the data, the limiting enthalpy of solution of sodium chloride, the enthalpy change for the process... 

The entry where the row labeled Na+ intersects the column labeled Cl, for instance, is the enthalpy change, -784 kj-mol, for the process Na+(g) + Cl (g) —> Na (aq) + Cl(aq) the values here apply only when the resulting solution is very dilute. 

Because the enthalpy of solution is positive, there is a net inflow of energy as heat when the solid dissolves (recall Fig. 8.23b). Sodium chloride therefore dissolves endothermically, but only to the extent of 3 kj-mol-1. As this example shows, the overall change in enthalpy depends on a very delicate balance between the lattice enthalpy and the enthalpy of hydration. 

Polymerization of 4-bromo-6,8-dioxabicyclo[3.2.1 ]octane 2 7 in dichloromethane solution at —78 °C with phosphorus pentafluoride as initiator gave a 60% yield of polymer having an inherent viscosity of 0.10 dl/g1. Although it is not described explicitly, the monomer used seems to be a mixture of the stereoisomers, 7 7a and 17b, in which the bromine atom is oriented trans and cis, respectively, to the five-membered ring of the bicyclic structure. Recently, the present authors found that pure 17b was very reluctant to polymerize under similar conditions. This is understandable in terms of a smaller enthalpy change from 17b to its polymer compared with that for 17a. In the monomeric states, 17b is less strained than 17a on account of the equatorial orientation of the bromine atom in the former, whereas in the polymeric states, the polymer from 17b is energetically less stable than that from 17a, because the former takes a conformation in which the bromine atom occupies the axial positioa Its flipped conformation would be even more unstable, because the stabilization by the anomeric effect is lost, in addition to the axial orientation of the methylene group. 

In addition to chemical reactions, the isokinetic relationship can be applied to various physical processes accompanied by enthalpy change. Correlations of this kind were found between enthalpies and entropies of solution (20, 83-92), vaporization (86, 91), sublimation (93, 94), desorption (95), and diffusion (96, 97) and between the two parameters characterizing the temperature dependence of thermochromic transitions (98). A kind of isokinetic relationship was claimed even for enthalpy and entropy of pure substances when relative values referred to those at 298° K are used (99). Enthalpies and entropies of intermolecular interaction were correlated for solutions, pure liquids, and crystals (6). Quite generally, for any temperature-dependent physical quantity, the activation parameters can be computed in a formal way, and correlations between them have been observed for dielectric absorption (100) and resistance of semiconductors (101-105) or fluidity (40, 106). On the other hand, the isokinetic relationship seems to hold in reactions of widely different kinds, starting from elementary processes in the gas phase (107) and including recombination reactions in the solid phase (108), polymerization reactions (109), and inorganic complex formation (110-112), up to such biochemical reactions as denaturation of proteins (113) and even such biological processes as hemolysis of erythrocytes (114). 

Fig. 1.2 Standard enthalpy changes of (a) the complexing of lanthanide ions in aqueous solution by EDTA" ( left-hand axis) (b) the standard enthalpy change of reaction 2, the dichloride being a di-f...

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