A Molecular View of the Solution Process

The intermolecular attractions that hold molecules together in liquids and solids also play a central role in the formation of solutions. When one substance (the solute) dissolves in another (the solvent), particles of the solute disperse throughout the solvent. The solute particles occupy positions that are normally taken by solvent molecules. The ease with which a solute particle replaces a solvent molecule depends on the relative strengths of three types of interactions  

If the solute-solvent attraction is stronger than the solvent-solvent attraction and solute-solute attraction, the solution process is favorable, or exothermic (A/Zsoin 0). If the solute-solvent interaction is weaker than the solvent-solvent and solute-solute interactions, then the solution process is endothermic 0). 

In Section 6.6 we dscussed the solution process from a macroscopic point of view. 

When sodium chloride dissolves in water, the ions are stabilized in solution by hydration, which involves ion-dipole interaction. In general, we predict that ionic compounds should be much more soluble in polar solvents, such as water, liquid ammonia, and liquid hydrogen fluoride, than in nonpolar solvents, such as benzene and carbon tetrachloride. Because the molecules of nonpolar solvents lack a dipole moment, they cannot eflfeclively solvate the Na and Q ions. (Solvation is the process in which an ion or a molecule is surrounded by solvent molecules arranged in a specific manner. The process is called hydration when the solvent is water.) The predominant intermolecular interaction between ions and nonpolar compounds is ion-induced dipole interaction, which is much weaker than ion-dipole interaction. Consequently, ionic compounds usually have extremely low solubihty in nonpolar solvents. 

Example 12.1 illustrates how to predict solubihty based on a knowledge of the intermolecular forces in the solute and the solvent. 

In a supersaturated sodium acetate solution (top), sodium acetate crystals rapidly form when a small seed crystal is added. 

Solute Solvent State of Resulting Solution Examples 

A molecular view of the solution process portrayed as taking place in three steps First the solvent and solute molecules are separated (steps 1 and 2). Then the solvent and solute molecules mix (step 3). 

Dissolution of an Ionic and a Covalent Compound ARIS, Animations 

In Chapter 4, we learned guidelines that helped us predict whether or not an ionic solid is soluble in water. We now take a more general look at the factors that determine solubility. This discussion will enable us to understand why so many ionic substances are soluble in water, which is a polar solvent and it will help us to predict the solubility of ionic and molecular compounds in both polar and nonpolar solvents. 

The saying like dissolves like is useful in predicting the solubility of a substance in a given solvent. What this expression means is that two substances with intermolecuiar forces of similar type and magnitude are likely to be soluble in each other. For example, both carbon tetrachloride (CCI4) and benzene (QHg) are nonpolar liquids. The only intermolecuiar forces present in these substances are dispersion forces [W Section 12.1]. When these two liquids are mixed. 

Like the formation of chemical bonds [W Section 8.9). the formation of intermolecuiar attractions is exothermic If that isn t intuitively obv ious, think of it this way It would require energy to separate molecules that ate attracted to each other. The reverse process, the combination of molecules that attract each other, would give off an equal amount of energy [W Section 5.3). 

Solvation refers in a general way to solute partides being sunounded by solvent molecules. When the solvent is water, we use the more specific temi hydration [ W Section 4.1], 

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A MOLECULAR VIEW OF THE SOLUTION PROCESS Review Questions... 

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