|Chemistry 8th Edition / Chang|
|Student Study Guide
THE SOLUTION PROCESS (12.1 – 12.2)
Some Definitions. The main subject of this chapter concerns the formation and properties of liquid solutions. Recall from Chapter 4 that a solution is a homogeneous mixture of two or more substances. The component in greater quantity is called the solvent, and the component in lesser amount is called the solute. In aqueous solutions water is the solvent. Solutes can be liquids, solids, or gases. Several terms are used to describe the degree to which a solute will dissolve in a solvent:
The Solution Process. Dissolving is a process that takes place at the molecular level and can be discussed in molecular terms. When one substance dissolves in another, the particles of the solute disperse uniformly throughout the solvent. The solute particles occupy positions that are normally taken by solvent molecules. The ease with which a solute particle may replace a solvent molecule depends on the relative strengths of three types of interactions:
Imagine the solution process as taking place in three steps as shown in Figure 12.1. Step 1 is the separation of solvent molecules. Step 2 is the separation of solute molecules. These steps require inputs of energy to overcome attractive intermolecular forces. Step 3 is the mixing of solvent and solute molecules; it may be exothermic or endothermic. According to Hess's law (see Section 6.6 of the text) the heat of solution is given by the sum of the enthalpies of the three steps:
Hsoln = H1 + H2 + H3
The solute will be soluble in the solvent if the solute-solvent attraction is stronger than the solvent-solvent attraction and solute-solute attraction. Such a solution process is exothermic. Only a relatively small amount of the solute will be dissolved if the solute-solvent interaction is weaker than the solvent-solvent and solute-solute interaction; then the solution process will be endothermic. Solvation is the process in which a solute particle (an ion or molecule) is surrounded by solvent molecules due to strong solute-solvent attractive forces. When water is the solvent the process is called hydration.
Figure 12.1. A molecular view of the solution process. Think of the solute molecules and solvent molecules first being spread apart, and then being mixed together. The relative strength of forces holding solvent molecules together H1, solute particles together H2, and the forces between solvent and solute molecules H3 in the solution are important in determining the solubility.
Here we see that the solution process is assisted by an exothermic heat of solution. Yet there are a number of soluble compounds with endothermic heats of solution. In addition every substance is somewhat soluble no matter what the value of Hsoln. It turns out that all chemical processes are governed by two factors. The first of these is the energy factor. In other words, does the solution process absorb energy or release energy?
Disorder or randomness is the other factor that must be considered. Processes that increase randomness or disorder are also favored. In the pure state, the solvent and solute possess a fair degree of order. Here we mean the ordered arrangement of atoms, molecules, or ions in a three-dimensional crystal. Where order is high, randomness is low.
The order in a crystal is lost when the solute dissolves and its molecules are dispersed in the solvent. The solution process is accompanied by an increase in disorder or randomness. It is the increase in disorder of the system that favors the solubility of any substance. The tendency of a system to become more disordered is discussed in more detail in Chapter 18
A General Solubility Rule. The most general solubility rule is that "like dissolves like." In this rule, the term "like" refers to molecular polarity. "Like dissolves like" means that substances of like polarity will mix to form solutions. And substances of different polarity will be immiscible, or will tend only slightly to form solutions. The rule predicts that two polar substances will form a solution, and two nonpolar substances will form a solution, but a polar substance and a nonpolar substance will tend not to mix. For example, water and oil, being polar and nonpolar substances, respectively, are immiscible. Water and ethanol, both being polar, are miscible. Oil will dissolve in carbon tetrachloride because both substances are nonpolar. Ionic compounds tend to be quite soluble in water (a polar solvent).
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