Topic: 3. Properties of Solutions
Concept: B. Types of solutions and general properties
Concept Overview:
The solubility of a compound is defined by how much of it will dissolve in a given quantity of solvent. In order for a compound to dissolve in solution, two forces must be overcome: the intermolecular forces in the solute and the intermolecular forces in the solvent. For this situation to occur, there must exist some new intermolecular forces between the solute and solvent molecules that can overwhelm the original intermolecular forces.
Liquid-liquid solubility
When one liquid can dissolve another liquid, the the two liquids are called miscible. If two liquids are not miscible, all attempts at mixing them will result in two layers in the solution, with the more dense material at the bottom of the container. Those who like Italian salad dressing are familiar with immiscible liquids--the dressing separates if left undisturbed for any period of time, and must be mixed just before serving. The best way to predict miscibility of two liquids is to use the empirical (experimentally-determined) rule like dissolves like. The feature of the liquids to compare is polarity. Two polar liquids are likely to be miscible, as breaking of dipole-dipole interactions among each separate liquid is overcome by new dipole-dipole interactions between solute and solvent. Polar liquids will not mix with non-polar liquids, because the dipole-dipole attractions within the pure polar fluid are strong enough that the nonpolar fluid will be excluded.
Solids as solutes
Simple molecular solids work in a similar way as liquids. If they are held together solely by intermolecular forces, the "like dissolves like" rule applies. The text has a good picture of this as Figure 14.4 on page 661. Other solids, held together by covalent bonds, such as diamonds, sand, etc., are not likely to be soluble in anything; the energy to break the covalent bond is too high.
Ionic solids are the most interesting solid solutes. Some of them dissolve quite readily, and some don't. Back in Chem 11, you were introduced to a set of solubility rules. Here, we will present a way to predict the solubility of a compound.
In Chem 11, you also should have been introduced to Hess's Law, which states that you can determine the DH of a reaction from the DH°f of both the products and the reactants. For example, you can use the following reaction to predict the solubility of KF:
KF(s) ---> KF(aq)
DH°soln = DH°f [KF(aq)] - DH°f [KF(s)]
A negative DH°solution would predict that the solid is soluble, because the solvation process is exothermic. A positive DH°solution may at first suggest that a compound is not soluble. Be careful, though; a positive DH°solution only states that the dissolution process is endothermic. Salts with endothermic heats of solution may be soluble. The most common example of a soluble salt with a positive DH°solution is ammonium nitrate, the ingredient in chemical cold packs. Breaking the inner bag allows dry ammonium nitrate to contact water, making the pack instantly cold.
