Intermolecular Forces and the Solution Process

By using the solution concentration definitions, we were able to convert from one concentration unit to another. This skill is used frequently by chemists. 

PRACTICE EXAMPLE A A 16.00% aqueous solution of glycerol, H0CH2CH(0H)CH20H, by mass, has a density of 1.037 g/mL. What is the mole fraction of glycerol in this solution  

Which of the several concentration units described in Section 14-2 are temperature-dependent and which are not Explain. 

We can often understand a process if we analyze its energy requirements this approach can help us to explain why some substances mix to form solutions and others do not. In this section, we focus on the behavior of molecules in solution, specifically on intermolecular forces and their contribution to the energy required for the dissolution process. 

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Solution Concentration 14-3 Intermolecular Forces and the Solution Process... 

Intermolecular Forces and the Solution Process—Predictions about whether two substances will mix to form a solution involve knowledge of intermolecular forces between like and unlike molecules (Figs. 14-2 and 14-3). This approach makes it possible to identify an ideal solution, one whose properties can be predicted from properties of the individual solution components. Most solutions are nonideal. 

For a complete quantitative description of the solvent effects on the properties of the distinct diastereoisomers of dendrimers 5 (G = 1) and 6 (G = 1), a multiparameter treatment was used. The reason for using such a treatment is the observation that solute/solvent interactions, responsible for the solvent influence on a given process—such as equilibria, interconversion rates, spectroscopic absorptions, etc.—are caused by a multitude of nonspecific (ion/dipole, dipole/dipole, dipole/induced dipole, instantaneous dipole/induced dipole) and specific (hydrogen bonding, electron pair donor/acceptor, and chaige transfer interactions) intermolecular forces between the solute and solvent molecules. It is then possible to develop individual empirical parameters for each of these distinct and independent interaction mechanisms and combine them into a multiparameter equation such as Eq. 2, "... 

The ease of dissolving a solute in a solvent is governed by intermolecular forces. Energy and the disorder that results when molecules of the solute and solvent mix to form a solution are the forces driving the solution process. 

A useful property of liquids is their ability to dissolve gases, other liquids and solids. The solutions produced may be end-products, e.g. carbonated drinks, paints, disinfectants or the process itself may serve a useful function, e.g. pickling of metals, removal of pollutant gas from air by absorption (Chapter 17), leaching of a constituent from bulk solid. Clearly a solution s properties can differ significantly from the individual constituents. Solvents are covalent compounds in which molecules are much closer together than in a gas and the intermolecular forces are therefore relatively strong. When the molecules of a covalent solute are physically and chemically similar to those of a liquid solvent the intermolecular forces of each are the same and the solute and solvent will usually mix readily with each other. The quantity of solute in solvent is often expressed as a concentration, e.g. in grams/litre. 

The first step in the solution process of a polymeric material by a good solvent is a swelling. Providing the solvent is a good solvent, the intermolecular forces in linear and branched polymers are broken and the polymer dissolves. 

At best, this approach provides a quantitative index to solvent polarity, from which absolute or relative values of rate or equilibrium constants for many reactions, as well as absorption maxima in various solvents, can be derived. Since they reflect the complete picture of all the intermolecular forces acting in solution, these empirical parameters constitute a more comprehensive measure of the polarity of a solvent than any other single physical constant. In applying these solvent polarity parameters, however, it is tacitly assumed that the contribution of intermolecular forces in the interaction between the solvent and the standard substrate is the same as in the interaction between the solvent and the substrate of interest. This is obviously true only for closely related solvent-sensitive processes. Therefore, an empirical solvent scale based on a particular reference process is not expected to be universal and useful for all kinds of reactions and absorptions. Any comparison of the effect of solvent on a process of interest with a solvent polarity parameter is, in fact, a comparison with a reference process. 

Successful chromatography requires a proper balance of intermolecular forces between the three active parts in the separation process, the solute, the mobile phase, and the stationary phase the polarities for these three parts should be carefully blended for a good separation to be realized in a reasonable time. 

A plot of vapor pressure data for a hypothetical system, assuming c hjRT is unity, is shown in fig. 1.9. The positive deviations from ideality indicate that the mixing process is endothermic. As the parameter A/z is increased, the deviations increase in the positive direction. Obviously, the theory also predicts negative deviations from Raoult s law when A/z is negative, that is, when the mixing process is exothermic. Under these conditions the intermolecular forces between the two species A and B are attractive, and the escaping tendency of each is less than it would be if the solution were ideal (A/z = 0). 

For simplicity, we can imagine the solution process taking place in three distinct steps (Figure 12.2). Step 1 is the separation of solvent molecules, and step 2 entails the separation of solute molecules. These steps require energy input to break attractive intermolecular forces therefore, they are endothermic. In step 3 the solvent and solute molecules mix. This process can be exothermic or endothermic. The heat of solution is given by... 

The solution process is dependent upon intermolecular forces Solutions form when the intermolecular attractive forces between solute and solvent molecules are about as strong as those that exist in the solute alone or in the solvent alone. For example, NaCI dissolves in water because ... 

We begin by considering what happens at the molecular level when a substance dissolves, paying particular attention to the role of intermolecular forces. TWO important aspects of the solution process are the natural tendency of particles to mix and changes in energy. 

Next we study the formation of solutions at the molecular level and see how intermolecular forces affect the energetics of the solution process and solubility. (12.2)... 

We have studied basic definitions in chemistry, and we have examined the properties of gases, liquids, solids, and solutions. We have discussed chemical bonding and intermolecular forces and seen how chemical kinetics and chemical equilibrium concepts help us understand the nature of chemical reactions. It is appropriate at this stage to apply our knowledge to the study of one extremely important system the atmosphere. Although Earth s atmosphere is fairly simple in composition, its chemistry is very complex and not fully understood. The chemical processes that take place in our atmosphere are induced by solar radiation, but they are intimately connected to natural events and human activities on Earth s surface. 

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