Liquids and Solutions

1. Liquids and Solutions Lesson 1

2. Liquids • A liquid can take the shape of its container, but has a fixed volume. • Density and compression: • At 25oC and 1 atm, liquids are much denser than gases. • Intermolecular forces hold the liquid particles together. • Compression occurs in liquids; however, there is less compression than in gases.

3. Fluidity and Viscosity • Fluidity is the ability to flow. • Both gases and liquids are classified as fluids because they can flow. • Liquids can also diffuse through another liquid, although much slower than when a gas diffuses through another gas. • Viscosity is a measure of the resistance of a liquid to flow. • Viscosity is determined by the type of intermolecular forces, shape of the particles, and temperature. • Viscosity decreases with temperature. • An increase in temperature causes an increase in the average kinetic energy.

4. Surface Tension and Capillary Action • Surface tension: the energy required to increase the surface area of a liquid by a given amount. It is a measure of the inward pull by particles in the interior of a liquid. • Compounds that lower the surface tension of water are sometimes called surface active agents or surfactants. • Capillary action: the movement of water in to small spaces due to the adhesion of water molecules to each other and the adhesion of water molecules to other substances.

5. Characteristics of Solutions • Attractive forces exist among the particles of all substances. • Attractive forces exist between the pure solute particles, pure solvent particles, and between the solute and solvent particles. • If the attractive forces between the solvent and the solute particles are greater than the attractive forces holding the solute particles together, the solvent particles will pull the solute particles apart and surround them. • The process of surrounding solute particles with solvent particles to form a solution is called solvation.

6. Aqueous Solutions of Ionic Compounds • Water molecules are dipoles with partially positive and partially negative ends. • When an ionic compound is placed in water, the water molecules collide with the compound. • The charged ends of the water molecules attract the positive ions and the negative ions. • The attraction between the ions and the water is greater than the attraction among the ions in the compound, so the ions break away from the surface of the compound and are surrounded by water molecules. • Solvation continues until the entire compound has dissolved and all ions are distributed through the water.

7. Aqueous Solutions of Molecular Compounds Polar molecular compounds will dissolve in water due to the attractive forces of the polar water molecules. The attractive forces between two different polar molecules is greater than the attractive forces between two molecules of the same substance. Nonpolar molecular compounds will not dissolve in water because there is little attraction between the polar water molecules and the nonpolar molecules.

8. Factors that Affect the Rate of Solvation • To increase the rate of solvation • Agitating the mixture by stirring and shaking moves the dissolved particles away from the contact surfaces more quickly and thereby allows new collisions between the solute and solvent particles to occur. • Raising the temperature of the solvent increases the kinetic energy of its particles, resulting in more frequent collisions and collisions with greater energy than those that occur at lower temperatures.

9. Heat of Solution Energy is required to overcome the attractive forces within the solute and within the solvent, so both steps are endothermic. When solute and solvent particles mix, the particles attract each other and energy is released. This step is exothermic. The overall energy change that occurs during the solution formation process is called the heat of solution.

10. Solubility • Solubility refers to the maximum amount of solute that will dissolve in a given amount of solvent at a specified temperature and pressure. • Solubility is expressed in grams of solute per 100 g of solvent. See Table 15-2 on page 457. • As long as the solvation rate of a solution is greater than the crystallization rate, the net effect is continuing solvation. • You can keep adding solute to the solution and the solvent will dissolve it.

11. However, once the crystallization rate and the solvation rate equalize, no more solute can be dissolved in the solution and the solution becomes saturated. An unsaturated solution can have more solute dissolved in it, but a saturated solution cannot.

12. Factors that Affect Solubility • Temperature • Many substances are more soluble at high temperatures than at low temperatures. • Supersaturated solutions hold more solute in the solution at higher temperatures than saturated solutions. • These solutions are unstable and a slight jarring of the solution can cause the solute to precipitate out of solution. • However, a few substances have lower solubility at high temperatures. See Fig. 15-7 on page 458.

13. Pressure • Pressure affects the solubility of gaseous solutes. • The solubility of a gas in any solvent increases as its external pressure increases. • Once the external pressure is released, the gas comes out of solution to form bubbles. • Henry’s Law: At a given temperature, the solubility (S) of a gas in a liquid is directly proportional to the pressure (P) of the gas above the liquid. • So S1/P1 = S2/P2 • You can manipulate the equation to solve for any of the variables.

14. Example Problem If 0.85 g of a gas at 4.0 atm dissolves in 1.0 L of water at 25oC, how much will dissolve in 1.0 L of water at 1.0 atm and the same temperature? S1 = 0.85 g/L S2 = ? g/L P1 = 4.0 atm P2 = 1.0 atm Do Practice Problems 1 and 2 on page 461.