Properties of Solutions. Course Learning Outcomes for Unit III. Reading Assignment. Unit Lesson UNIT III STUDY GUIDE

Transcription

1 UNIT III STUDY GUIDE Properties of Solutions Course Learning Outcomes for Unit III Upon completion of this unit, students should be able to: 1. Describe how enthalpy and entropy changes affect solution formation. 2. Describe the relationship between intermolecular forces and solubility, including use of the like dissolves like rule. 3. Describe the role of equilibrium in the solution process and its relationship to the solubility of a solute. 4. Describe the effect of temperature on the solubility of solids and gases and the relationship between the partial pressure of a gas and its solubility. 5. Calculate the concentration of a solution in terms of molarity, molality, mole fraction, percent composition, and parts per million and interconvert between them. 6. Describe a colligative property and explain the difference between the effects of nonelectrolytes and electrolytes on colligative properties. 7. Calculate the boiling-point elevation, freezing-point depression, and the osmotic pressure of a solution, as well as the vapor pressure of a solvent over a solution. 8. Differentiate between a solution and a colloid. Reading Assignment Chapter 13: Properties of Solutions Unit Lesson The Solution Process Solutions are homogeneous mixtures of two or more substances, which may be solids, liquids, or gases. The interaction between solute and solvent particles is called salvation. When the solvent is water, the interaction is called hydration. The dissolution of ionic substances in water is promoted through hydration of the separated ions by the polar water molecules. For example, the formation of a solution of NaCl in water involves the breaking of attractions among solute particles, the breaking of attractions among solvent particles, and the formation of attractions between solute and solvent particles. Energy and the disorder that results when molecules of the solute and solvent mix to form a solution are the forces driving the solution process. (Graphic below is on page 516 of the textbook.) CHM 1040, General Chemistry II 1

2 UNIT x STUDY GUIDE Graphic depiction of the dissolution of an ionic solid, NaCl, in water (Brown, LeMay, Bursten, Murphy, & Woodward, 2012) Saturated Solutions and Solubility There is a limit to the amount of solute that can be dissolved in a solvent. A saturated solution is a solution in which no more solute can be dissolved. A solution that contains a greater amount of solute than that needed to form a saturated solution is said to be supersaturated. Supersaturated solutions are very unstable solutions, meaning that they do not remain supersaturated for long. Instead, under certain conditions, some of the solute particles will escape from the solution and re-form the pure solute. Whether or not a solute will dissolve in a solvent, and the extent to which it will dissolve, both depend on the solubility of the solute. The solubility is the amount of a solute that will dissolve in a specific solvent under given conditions. Increasing temperature usually increases the solubility of solid and liquid substances and usually decreases the solubility of gases in water. (Graphic below is on page 519 of the textbook.) When rates of dissolution and crystallization become equal, a dynamic equilibrium is established; the above graphic depicts a dynamic equilibrium in a saturated solution with excess ionic solute. (Brown, LeMay, Bursten, Murphy, & Woodward, 2012) Solubility and Expressing Solution Concentration The extent to which one substance dissolves in another depends on the nature of the solute and solvent, the temperature, and the pressure (for gasses). When two substances are similar, they can dissolve in each CHM 1040, General Chemistry II 2

3 other. Polar and ionic solutes tend to dissolve in polar solvents, and nonpolar UNIT solutes x STUDY tend to dissolve GUIDE in nonpolar solvents ( like dissolves like ). Liquids that mix in all proportions are miscible; those that do not dissolve significantly in one another are immiscible. Hydrogen-bonding interactions between solute and solvent often play an important role in determining solubility; for example, ethanol and water, whose molecules form hydrogen bonds with each other, are miscible. Solutions of gasses are greatly affected by changes in temperature. As the temperature increases, the kinetic energy of the solute gas becomes greater. The gas particles acquire more of a tendency to escape from the solvent. Thus, as the temperature increases, the solubility of a gas in a liquid decreases. The solubility of gas in a liquid is also strongly influenced by pressure. When pressure is increased, the rate at which gas molecules strike the surface to enter the solution is increased. The solubility of a gas in any solvent is increased as the pressure of the gas over the solvent increases. A solution s concentration tells how concentrated or dilute a solution is. In other words, the concentration of a solution is the amount of solute in a given amount of solution Concentrations of solutions can be expressed quantitatively through use of several different measures, including mass percentage [(mass solute/mass solution) X 10 2 ], parts per million (ppm), parts per billion (ppb), and mole fraction. The molarity (M) of a solution is defined as the number of moles of solute dissolved in each liter of solution. The molality (m) of a solution is the number of moles of solute dissolved in each kilogram of solvent. (Graphic below is on page 525 of the textbook.) As gases, such as methane, oxygen, carbon monoxide, and helium, are dissolved in water, their solubility decreases with increasing temperature. (Brown, LeMay, Bursten, Murphy, & Woodward, 2012) Colligative Properties A property that depends on the concentration of solute particles, but is independent of their nature, is called a colligative property. Four colligative properties are vapor pressure lowering, freezing-point lowering, boiling point raising, and osmotic pressure. The fact that the magnitude of vapor pressure reduction is proportional to solute concentration in known as Raoult s law, which addresses the lowering of vapor pressure in solutions. Although an ideal solution obeys Raoult s law, differences in solvent solute as compared with solvent solvent and solute solute intermolecular forces cause many solutions to depart from ideal behavior. The amount by which the boiling temperature is raised is the boiling point elevation, another colligative property of solutions. A solution containing a nonvolatile solute possesses a higher boiling point than the pure CHM 1040, General Chemistry II 3

4 solvent. The constant Kb is called the molal boiling point elevation constant. The UNIT value x STUDY of Kb depends GUIDE on the solvent. Freezing point depression is the colligative property that describes the ability of a dissolved solute to lower the freezing point of its solution. The molal freezing point depression constant, Kf, measures the lowering of the freezing point of a solution. The temperature changes are given by the equations Tb = Kbm. The boiling point elevation ( Tb) is the difference between the boiling point of the solution and the boiling point of the pure solvent. It is directly proportional to the number of solute particles per mole of solvent particles. Osmosis is the movement of solvent molecules through a semipermeable membrane from a less concentrated to a more concentrated solution. The pressure required to prevent osmosis is known as the osmotic pressure ( ) of the solution. The osmotic pressure can be measured in units of gas pressure, such as atm. The osmotic pressure of a solution is proportional to the solution s molarity: P = MRT. (Graphic below is on page 538 of the textbook.) Graphic depiction of the crenation and hemolysis that occur during osmosis through red blood cell walls (Brown, LeMay, Bursten, Murphy, & Woodward, 2012) Colloids Particles that are intermediate in size between those in solutions and suspensions form mixtures known as colloidal dispersions, or simply colloids. Particles between 1 nm and 1000 nm in diameter may form colloids. A useful physical property of colloids, which is the scattering of visible light, is referred to as the Tyndall effect. The Tyndall effect can be used to distinguish between a solution and a colloid. Colloids in water are classified as hydrophilic colloids and hydrophobic colloids. Hydrophilic colloids are solutions containing extremely large molecules, such as proteins. Hydrophilic colloids are common in living organisms, in which large molecular aggregates remain suspended because they have many polar, or charged, atomic groups on their surfaces that interact with water. A hydrophobic colloid normally would not be stable in water, and the particles would clamp together. Hydrophobic colloids, such as small droplets of oil, may remain in suspension through adsorption of charged particles on their surfaces. (Graphic below is on page 543 of the textbook.) CHM 1040, General Chemistry II 4

5 UNIT x STUDY GUIDE Hydrophilic colloidal particle showing the groups that help keep a macromolecule suspended in water (Brown, LeMay, Bursten, Murphy, & Woodward, 2012) Reference Brown, T. L., LeMay, H. E., Jr., Bursten, B. E., Murphy, C. J., & Woodward, P. M. (2012). Chemistry: The central science (12th custom ed.). Upper Saddle River, NJ: Prentice Hall. Learning Activities (Non-Graded) Video Resources YouTube (youtube.com) offers a variety of video tutorials on chemistry topics. Viewing a video that explains a difficult concept can help you obtain a better grasp of the material being discussed. It is suggested that you view more than one video on a topic in order to see different approaches. Viewing multiple videos on a topic will also help you determine the validity of the videos. For Unit III, topics you might search for on YouTube include: Solvation Hydration Crystallization Saturated, unsaturated Supersaturated solution Solubility Miscible and immiscible Henry s law Mass percentage Parts per million (ppm), and parts per billion (ppb) Molality Colligative property Raoult s law Osmosis and osmotic pressure A word of caution: Like any unmonitored Web site, some of the information at YouTube may be faulty; remain aware of this as you select videos to watch. For example, if a video contradicts or disagrees with something in the textbook, find a different video. CHM 1040, General Chemistry II 5

6 Test Yourself UNIT x STUDY GUIDE To learn more about our topics in Unit III, please refer to the Chapter 13 Test Yourself in Mastering Chemistry. After accessing MasteringChemistry, click on the Study Area tab in the navigation bar. Next click the Access the Study Area link. When the Study Area screen appears, select Chapter 13 from the dropdown box at the top of the screen, then click GO. Select Question Sets in section two of the Chapter Guide, under Test Yourself. Complete the Practice Quiz and the Additional Practice Questions. As you complete each activity, you can submit it for grading and find out if you have mastered that particular area. Working these problems will provide you with practice that will help you master the unit II material. Properties of Solutions Chapter 13 To learn more about Properties of Solutions, please watch the Chapter 13 Videos and Activities that are available in Mastering Chemistry. After accessing MasteringChemistry, click on the Study Area tab in the navigation bar. Next click the Access the Study Area link. When the Study Area screen appears, select Chapter 13 from the dropdown box at the top of the screen, then click GO. When the Chapter 13 Study Area bar shows on the left side of the screen, select Videos and Activities from the red menu bar. Click on a link in the list to view an activity or movie/video. When the selected presentation ends, return to the list by clicking the return arrow at the top left of the page or by clicking on the Videos and Activities link. Watching these videos and activities will help you master the concepts and actions that will help you succeed in this course. Apply What You Have Learned As you read the sections of the chapter, stop and work the sample exercises to make sure you understood all the concepts and actions that were covered in that section. In Chapter 13, these exercises are found on pages , 524, , , , , 535, 536, 538, 539, , and Additional practice that will help you master the chapter s educational content is provided in the exercises on pages Answers to selected exercises are provided in the back of the textbook. If there is anything you do not understand or an exercise you are unable to successfully complete, contact your instructor for additional explanation or information. These are non-graded activities, so you do not have to submit them. Key Terms 1. Colligative properties of solution 2. Crystallization 3. Henry s law 4. Hydration 5. Ideal solution 6. Mass percentage 7. Miscible and immiscible 8. Molal boiling-point-elevation constant 9. Molal freezing-point-depression constant 10. Molality 11. Osmosis and osmotic pressure 12. Parts per million (ppm) and parts per billion (ppb) 13. Raoult s law 14. Saturated, unsaturated, and supersaturated solution 15. Solubility 16. Solvation CHM 1040, General Chemistry II 6