Section 12-2: The Solution Process
Objectives:
List and explain three factors that affect the rate at which a solid solute dissolves in a liquid solvent.
Explain solution equilibrium, and distinguish among saturated, unsaturated, and supersaturated solutions.
Explain the meaning of “like dissolves like” in terms of polar and nonpolar substances.
List the three interactions that contribute to the enthalpy of a solution, and explain how they combine to cause dissolution to be exothermic or endothermic.
Compare the effects of temperature and pressure on solubility.
A. Factors Affecting the Rate of Dissolution
1. increasing the surface area of the solute – dissolution process occurs at the surface of the solute; Fig. 6, p. 407 bottom
2. stirring or shaking – helps to disperse solute particles and increase contact between the solvent and solute surface
3. higher temperatures – collisions among solute particles and solvent are more frequent and of higher energy
B. Solubility
1. solution equilibrium – physical state in which the opposing processes of dissolution and crystallization of a solute occur at the same rates;
Fig. 7, p. 408 bottom
2. types of solutions a. saturated solution – solution that contains the maximum amount of dissolved solute at a given temperature; Fig. 8, p. 409 top b.
unsaturated solution – solution that contains less solute than a saturated solution under the same conditions; Fig. 8, p. 409 top c. supersaturated solution – solution that contains more dissolved solute than a saturated solution contains under the same conditions

3. solubility values a. solubility – amount of that substance required to form a saturated solution with a specific amount of solvent at a specified temperature; Table 4, p. 410 bottom
12.2b b. usually given as grams of solute per 100 g of solvent at a given temperature; solubility of sugar is 204 g per 100 g of water at 20°C c. vary widely, and must be determined experimentally
C. Solute-Solvent Interactions
1.
general concepts a. solubility varies greatly with the type of compounds involved b. “Like dissolves like” is a rough but useful rule for predicting whether one substance will dissolve in another c. similar substances: type of bonding; polarity or nonpolarity; intermolecular forces among the solute and solvent
2.
dissolving ionic compounds in aqueous solution a. polarity of water molecules plays an important role in the formation of solutions of ionic compounds in water b. slightly charged parts of water molecules attract the ions in the ionic compounds and surround them, separating them from the crystal surface and drawing them into the solution c. hydration – solution process with water as the solvent; ions are said to be hydrated ; Fig. 9, p. 411 top; Fig. 10, p. 411 bottom

12.2c
3. nonpolar solvents a. ionic compounds are generally not soluble in nonpolar solvents such as carbon tetrachloride, CCl
4
, and toluene, C
6
H
5
CH
3 b. nonpolar solvent molecules do not attract the ions of the crystal strongly enough to overcome the forces holding the crystal together c. ionic and nonpolar substances differ widely in bonding type, polarity, and intermolecular forces, so their particles cannot intermingle very much
4. liquid solutes and solvents a. immiscible – liquids that are not soluble in each other; oil and water do not mix because oil is nonpolar whereas water is polar
Fig. 11, p. 412, top b. immiscible – liquids that dissolve freely in one another in any proportion; two polar substances, or two nonpolar substances form solutions together easily because their intermolecular forces match
Fig. 12, p. 412, bottom
5. effects of pressure on solubility a. increases in pressure of the solute gas above the solution causes gas particles to collide with the liquid surface more often causing more gas particles to dissolve in the liquid b. decreasing the pressure of the solute gas above the solution allows more dissolved gas particles to escape from solution
6. Henry’s Law a. Henry’s law – the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas on the surface of the liquid;
Fig. 13, p. 413, bottom b. effervescence – rapid escape of a gas from a liquid in which it is dissolved

12.2d
7.
2. effects of temperature on solubility b. increasing the temperature usually increases solubility of solids in liquids; Fig. 15, p. 414 bottom
Table 5, p. 416 top c. effect of temperature on solubility for a given solute is difficult to predict
D. Enthalpies of Solution
1. formation of a solution is accompanied by an energy change; formation of a solid-liquid solution can either absorb energy or release energy as heat;
Fig. 16, p. 415 bottom a. energy changes occur during solution formation because energy is required to separate solute molecules and solvent molecules from their neighbors enthalpy of solution – a. increasing the temperature usually decreases gas solubility; b.
Fig. 14, p. 414 top solvated – solute particle that is surrounded by solvent molecules net amount of energy absorbed as heat by the solution when a specific amount of solute dissolves in a solvent; a. enthalpy of solution is negative when energy is released; dissolve some sodium hydroxide, NaOH, in water, you will find that the outside of the container feels warm to the touch b. enthalpy of solution is positive when energy is absorbed; dissolve some potassium iodide, KI, in water, you will find that the outside of the container feels cold to the touch