Liquids and Intermolecular Forces. Course Learning Outcomes for Unit I. Reading Assignment. Unit Lesson UNIT I STUDY GUIDE

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1 UNIT I STUDY GUIDE Liquids and Intermolecular Forces Course Learning Outcomes for Unit I Upon completion of this unit, students should be able to: 1. Identify the intermolecular attractive interactions that exist between molecules or ions based on their composition and molecular structure and compare the relative strengths of these intermolecular forces. 2. Explain the concept of polarizability and how it relates to dispersion forces. 3. Explain the concepts of viscosity and surface tension in liquids. 4. Recall the names of the various changes of state for a pure substance. 5. Interpret heating curves and calculate quantities related to temperature and enthalpies of phase changes. 6. Define critical pressure, critical temperature, vapor pressure, normal boiling point, normal melting point, critical point, and triple point. 7. Interpret and sketch phase diagrams and explain how and why water s phase diagram differs from most other substances. 8. Explain how the molecular arrangements characteristic of nematic, smectic, and cholesteric liquid crystals differ from ordinary liquids and from each other, and recognize the features of molecules that favor formation of liquid crystalline phases. Reading Assignment Chapter 11: Liquids and Intermolecular Forces Unit Lesson A Molecular Comparison of Gases, Liquids, and Solids Molecular substances can be gases, liquids, or solids. The particles of a gas are far apart from each other, occupy their entire container, and are in rapid motion. By contrast, the particles of a liquid remain close together and do not occupy their entire container. They are, however, in constant motion. Liquid particles also collide with each other and with the walls of the container. Solids show more arrangement of particles, in which the particles are not free to move around. Their movement is limited to vibrations while they remain locked in their fixed positions in the solid. The forces of attraction between neighboring molecules are called intermolecular forces. These forces span a wide range of strengths. The intermolecular forces between the particles of a solid are stronger than those in gases and liquids. The particles cannot overcome these forces and move away from each other. Solids have high densities and are incompressible. (Graphic below is on page 427 of the textbook.) CHM 1040, General Chemistry II 1

2 UNIT x STUDY GUIDE Types of Intermolecular Forces There are four different types of intermolecular forces: dispersion forces, dipole dipole forces, and hydrogen bonding. Dispersion forces are present in all molecular substances (and atoms, for atomic substances such as He, Ne, Ar, and so forth). As molecular weight increases, the polarizability of a molecule increases, which results in stronger dispersion forces. Dipole dipole forces are attractions between opposite charges of neighboring permanent dipoles. Dipole dipole forces increase in strength as the polarity of the molecule increases. The strong dipole-dipole force between the hydrogen atom of one molecule and the F, O, or N atom of another molecule is called a hydrogen bond. Hydrogen bonds are generally stronger than dipole dipole or dispersion forces. Ion dipole forces exist between an ion and a polar molecule. Ion dipole forces are important in solutions in which ionic compounds are dissolved in polar liquids. (Graphic below is on page 434 of the textbook.) CHM 1040, General Chemistry II 2

3 UNIT x STUDY GUIDE Properties of Liquids Viscosity is the friction, or resistance to motion, that exists between the molecules of a liquid when they move past each other. The viscosity of a liquid depends on its intermolecular forces. The stronger the attraction between the molecules of a liquid, the greater its resistance to flow and thus the greater its viscosity. Viscosity increases as temperature decreases. At lower temperatures, the speed of liquid molecules decreases, so they stay closer together. It becomes more difficult for molecules to overcome the intermolecular forces between them. They do not flow as easily and the viscosity of the liquid increases. Surface tension is imbalance of forces at the surface of a liquid. The uneven forces make the surface behave as if it had a tight film stretched across it. Surface tension explains the beading of raindrops of the shiny surface of a car. Small quantities of a liquid take on spherical shapes in order to minimize surface area. Surface tension is greater in liquids with strong intermolecular forces of attraction. The adhesion of a liquid to the walls of a narrow tube and the cohesion of the liquid account for capillary action and the formation of a meniscus at the surface of a liquid. (Graphic is on page 437 of the textbook.) Phase Changes A phase change is the conversion of a substance from one of the three physical states of matter to another. A phase change always involves a change in energy. The change of state from liquid to gas is called vaporization. The opposite change, from gas to liquid, is called condensation. The change of state from solid to liquid is called melting. The reverse transformation of liquid into solid is called freezing. The conversion of a solid directly into a gas is called sublimation. The reverse transformation of a gas directly into a solid is called deposition. The amount of heat necessary to vaporize a given amount of a liquid at its boiling point it is called its heat of vaporization. The heat that is necessary to convert a given amount of a solid into a liquid is called heat of fusion. The amount of heat necessary to convert a given amount of a solid into a gas is called heat of sublimation. Critical temperature is the temperature above which the substance cannot exist in the liquid state. Critical pressure is the lowest pressure at which the substance can exist as a liquid at the critical temperature. When the temperature exceeds the critical temperature and the pressure exceeds the critical pressure, the liquid CHM 1040, General Chemistry II 3

4 and gas phases cannot be distinguished, and the substance is in a state called UNIT a supercritical x STUDY GUIDE fluid. (Graphic below is on page 439 of your textbook.) Vapor Pressure The vapor pressure of a liquid indicates the tendency of the liquid to evaporate. The pressure exerted by a vapor that is in equilibrium with its corresponding liquid at a given temperature is called the equilibrium vapor pressure of the liquid. The higher the vapor pressure of a liquid, the more readily it evaporates and the more volatile it is. Volatile liquids are liquids that evaporate readily and have relatively weak forces of attraction between their particles. The boiling point is the temperature at which the equilibrium vapor pressure of the liquids equals the atmospheric pressure. The normal boiling point is the temperature at which the vapor pressure equals 1 atm. (Graphic below is on page 444 of the textbook.) CHM 1040, General Chemistry II 4

5 UNIT x STUDY GUIDE Phase Diagrams A phase diagram is a graph of pressure versus temperature. It shows the conditions under which the phases of a substance exist. It also reveals how the states of a system change with changing temperature and pressure. The three regions of the phase diagram are labeled solid, liquid, and gas. This means that only the labeled state can exist under the combinations of temperature and pressure represented by that region. Two phases exist in equilibrium along the three lines of the diagram. The point of intersection of these lines near the center of the diagram is the only combination of pressure and temperature at which all three states can exist at equilibrium. The point on the diagram at which all three phases coexist in equilibrium is called the triple point. The melting point at 1 atm is the normal melting point. (Graphic below is on page 445 of the textbook.) Liquid Crystal A liquid crystal is a substance that exhibits one or more ordered phases at a temperature above the melting point of the solid. Liquid crystals are nematic and smectic. In smectic liquid crystals, the long axes of the CHM 1040, General Chemistry II 5

6 molecules are perpendicular to the plan of the layers. Nematic liquid crystals are UNIT less x STUDY ordered. GUIDE Although the molecules in nematic liquid crystals are aligned with their long axis parallel to one another, they are not separated into layers. A cholesteric liquid crystal is composed of molecules that align parallel to each other within a layer, as they do in nematic liquid crystalline phases, but the direction along which the long axes of the molecules align rotates from one layer to the next to form a helical structure. The color of cholesteric liquid crystals changes with temperature and therefore they are suitable for use as sensitive thermometers. Molecules that exhibit liquid crystallinity are usually long and rodlike. (Graphic below is on page 450 of the textbook.) 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) Test Yourself To learn more about our topics in Unit I, please refer to the Chapter 11 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 11 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 I material. Liquids and Intermolecular Forces Chapter 11 To learn more about Liquids and Intermolecular Forces, please watch the Chapter 11 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 11 from the dropdown box at the top of the screen, then click GO. CHM 1040, General Chemistry II 6

7 When the Chapter 11 Study Area bar shows on the left side of the screen, select UNIT Videos x STUDY and GUIDE 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 11, these exercises are found on pages 432, , 441, 445, , , and 452. 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. Non-graded learning activities are provided to aid students in their course of study. These are non-graded activities, so you do not have to submit them. Key Terms 1. Capillary action 2. Critical temperature 3. Dipole dipole forces 4. Dispersion forces 5. Dynamic equilibrium 6. Heat of fusion, sublimation, and vaporization 7. Hydrogen bonding 8. Intermolecular forces 9. Normal boiling point and melting point 10. Phase changes in liquids 11. Phase diagram 12. Polarizability 13. Supercritical fluid 14. Surface tension 15. Vapor pressure CHM 1040, General Chemistry II 7