Chemistry: A Molecular Approach Chapter 12: Liquids, Solids, and Intermolecular Forces PDF

11/1/2024 Chemistry: A Molecular Approach Sixth Edition Chapter 12 Liquids, Solids, and...

11/1/2024 Chemistry: A Molecular Approach Sixth Edition Chapter 12 Liquids, Solids, and Intermolecular Forces Copyright © 2023 Pearson Education, Inc. All Rights Reserved 1 States of Matter The fundamental difference between states of matter is the distance between particles (previous dry ice video). Because in the solid and liquid states particles are closer together, we refer to them as condensed phases. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 2 1  11/1/2024 The States of Matter The state a substance is in at a particular temperature and pressure depends on two antagonistic entities: –The kinetic energy of the particles. –The strength of the attractions between the particles. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 3 Intermolecular Forces The attractions between molecules are not nearly as strong as the intramolecular attractions (bond) that hold compounds together. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 4 2  11/1/2024 Intermolecular Forces These intermolecular attractions are, however, strong enough to control physical properties, such as boiling and melting points, vapor pressures, and viscosities. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 5 Intermolecular Forces These intermolecular forces as a group are referred to as van der Waals forces. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 6 3  11/1/2024 van der Waals Forces London dispersion forces Dipole–dipole interactions Hydrogen bonding Copyright © 2023 Pearson Education, Inc. All Rights Reserved 7 London Dispersion Forces While the electrons in the 1s orbital of helium would repel each other (and, therefore, tend to stay far away from each other), it does happen that they occasionally wind up on the same side of the atom. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 8 4  11/1/2024 London Dispersion Forces At that instant, then, the helium atom is polar, with an excess of electrons on the left side and a shortage on the right side. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 9 London Dispersion Forces Another helium atom nearby, then, would have a dipole induced in it, as the electrons on the left side of helium atom 2(B) repel the electrons in the cloud on helium atom 1(A). Copyright © 2023 Pearson Education, Inc. All Rights Reserved 10 5  11/1/2024 London Dispersion Forces London dispersion forces, or dispersion forces, are attractions between an instantaneous dipole and an induced dipole. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 11 London Dispersion Forces These forces are present in ALL molecules, whether they are polar or nonpolar. The tendency of an electron cloud to distort in this way is called polarizability. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 12 6  11/1/2024 Factors Affecting London Forces enhance The shape of the molecule affects the strength of dispersion forces: long, skinny molecules (like n- pentane) tend to have stronger diminish dispersion forces than short, fat ones (like neopentane). This is due to the increased surface area in n-pentane. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 13 Factors Affecting London Forces The strength of dispersion forces tends to increase with increased molecular weight. Larger atoms have larger electron clouds that are easier to polarize. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 14 7  11/1/2024 Boiling Points of n-Alkanes Copyright © 2023 Pearson Education, Inc. All Rights Reserved 15 Alkane Boiling Points Branched chains have lower boiling points than straight chains. The straight-chain isomers have more surface-to-surface (surface area) contact. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 16 8  11/1/2024 Dipole–Dipole Interactions Molecules that have permanent dipoles are attracted to each other. –The positive end of one is attracted to the negative end of the other, and vice versa. –These forces are only important when the molecules are close to each other. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 17 Dipole–Dipole Interactions comparable mass The more polar the molecule, the higher its boiling point. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 18 9  11/1/2024 Which Have a Greater Effect? Dipole–Dipole Interactions or Dispersion Forces If two molecules are of comparable size and shape, dipole– dipole interactions will likely be the dominating force. If one molecule is much larger than another, dispersion forces will likely determine its physical properties. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 19 How Do We Explain This? The nonpolar series (SnH4 to CH4) follow the expected trend. The polar series follow the trend until you get to the smallest molecules in each group (H2O, HF, NH3). Copyright © 2023 Pearson Education, Inc. All Rights Reserved 20 10  11/1/2024 Hydrogen Bonding The especially dipole–dipole interactions experienced when H is bonded to N, O, or F are unusually strong. We call these interactions hydrogen bonds. N, O, F have very high electronegativity Copyright © 2023 Pearson Education, Inc. All Rights Reserved 21 Hydrogen Bonding Hydrogen bonding arises in part from the high electronegativity of nitrogen, oxygen, and fluorine. Also, when hydrogen is bonded to one of those very electronegative elements, the hydrogen nucleus is exposed (more partial positive). Water expands when it freezes at a pressure of 1 atm about 9% Making ice less dense than liquid water Copyright © 2023 Pearson Education, Inc. All Rights Reserved 22 11  11/1/2024 Practice problem Which force below increases in strength as the molecular weight of the compound increases? a. Hydrogen bonding b. Ion-dipole forces c. Dipole-dipole forces d. London dispersion forces Copyright © 2023 Pearson Education, Inc. All Rights Reserved 23 Practice problem Which substance below has a greater density in its liquid state than in its solid state? a. Iron b. Glass c. Water d. Carbon dioxide Copyright © 2023 Pearson Education, Inc. All Rights Reserved 24 12  11/1/2024 Practice problem Which compound below is not capable of forming hydrogen bonds? a. CH4 b. NH3 c. H2O d. HF Copyright © 2023 Pearson Education, Inc. All Rights Reserved 25 Practice problem Which compound below has the highest boiling point? a. H2O b. H2S c. H2Se d. H2Te Copyright © 2023 Pearson Education, Inc. All Rights Reserved 26 13  11/1/2024 Practice problem Which element below has the highest boiling point? a. Kr b. F2 c. Cl2 d. Br2 Copyright © 2023 Pearson Education, Inc. All Rights Reserved 27 Practice problem Why is the boiling point of SnH4 higher than that of CH4? A. SnH4 is more polar than CH4. B. SnH4 is smaller in size than CH4. C. SnH4 has greater internal dispersion forces than in CH4. D. SnH4 is ionic in structure and CH4 is molecular. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 28 14  11/1/2024 Ion–Dipole Interactions Ion–dipole interactions (a fourth type of force) are important in solutions of ions. The strength of these forces is what makes it possible for ionic substances to dissolve in polar solvents. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 29 Summarizing Intermolecular Forces Copyright © 2023 Pearson Education, Inc. All Rights Reserved 30 15  11/1/2024 Type and Relative Strength of Intermolecular Forces Table 12.4 Types of Intermolecular Forces Generally weak moderate strong Very strong *The dispersion force can become very strong (as strong and even stronger than the others) for molecules of high molar mass. Methane, butane, octane, oil, wax, plastic bottle. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 31 Practice problem Which force below is the strongest intermolecular attractive force? a. Hydrogen bonding b. Ion-dipole forces c. Dipole-dipole forces d. London dispersion forces Copyright © 2023 Pearson Education, Inc. All Rights Reserved 32 16  11/1/2024 Intermolecular Forces Affect Many Physical Properties dimer The strength of the attractions between particles can greatly affect the properties of a substance or solution. comparable mass Copyright © 2023 Pearson Education, Inc. All Rights Reserved 33 Viscosity Resistance of a liquid to flow is called viscosity. It is related to the ease with which molecules can move past each other. Viscosity increases with stronger intermolecular forces and decreases with higher temperature. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 34 17  11/1/2024 Surface Tension Density= 4.68 g/mL Surface tension results from the net inward force experienced by the molecules on the surface of a liquid. It is a property of liquids that results from the tendency of liquids to higher potential E minimize their surface area. Because they have fewer neighbors to attract them, the surface molecules are less stable than those in the interior. – Have a higher potential energy To minimize their surface area, liquids form drops that are spherical. Video here Copyright © 2023 Pearson Education, Inc. All Rights Reserved 35 Surface Tension The (tight) surface tension of a liquid is the energy required to break the surface and increase the surface area a given amount. – Surface tension of H2O  72.8 mJ/m2 ▪ At room temperature – Surface tension of C6H6  28 mJ/m2 Copyright © 2023 Pearson Education, Inc. All Rights Reserved 36 18  11/1/2024 Factors Affecting Surface Tension The stronger the intermolecular attractive forces, the higher the surface tension will be. Raising the temperature of a liquid reduces its surface tension. – Raising the temperature of the liquid increases the average kinetic energy of the molecules. – The increased molecular motion makes it easier to stretch the surface. Video here Copyright © 2023 Pearson Education, Inc. All Rights Reserved 37 Capillary Action Capillary action is the ability of a liquid to flow up a thin tube against the influence of gravity. – The narrower the tube, the higher the liquid rises. Capillary action is the result of two forces working in conjunction, the cohesive and adhesive forces. – Cohesive forces hold the liquid molecules together. – Adhesive forces attract the outer liquid molecules to the tube’s surface. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 38 19  11/1/2024 Capillary Action The adhesive forces pull the surface liquid up the side of the tube, and the cohesive forces pull the interior liquid with it. The liquid rises up the tube until the force of gravity counteracts the capillary action forces. The narrower (more relatively surface area inside the tube) the tube diameter, the higher the liquid will rise up the tube. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 39 Practice problem If the inside surface of each tube were coated with wax, would the general shape of the water meniscus change? Would the general shape of the mercury meniscus change? Shape of Water Meniscus Shape of Hg Meniscus A. Yes, Inverted U Yes, downward U B. No change Yes, downward U C. No change No change D. Inverted U No change Copyright © 2023 Pearson Education, Inc. All Rights Reserved 40 20  11/1/2024 Phase Changes Copyright © 2023 Pearson Education, Inc. All Rights Reserved 41 Phase Changes Copyright © 2023 Pearson Education, Inc. All Rights Reserved 42 21  11/1/2024 Energy Changes Associated with Changes of State The heat of fusion is the energy required to change a solid at its melting point to a liquid. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 43 Energy Changes Associated with Changes of State The heat of vaporization is defined as the energy required to change a liquid at its boiling point to a gas. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 44 22  11/1/2024 Energy Changes Associated with Changes of State The heat of sublimation is defined as the energy required to change a solid directly to a gas. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 45 Heat of Fusion The amount of heat energy required to melt one mole of the solid is called the heat of fusion, Hfus. – Sometimes called the enthalpy of fusion It is always endothermic; therefore, Hfus is . It is somewhat temperature dependent. Hcrystallization  H fusion Generally  H fusion is much less than H vap. Hsublimation  H fusion  Hvaporization Copyright © 2023 Pearson Education, Inc. All Rights Reserved 46 23  11/1/2024 Energy Changes Associated with Changes of State The heat added to the system at the melting and boiling points goes into pulling the molecules farther apart from each other. The temperature of the substance does not rise during a phase change (plateau). Copyright © 2023 Pearson Education, Inc. All Rights Reserved 47 Practice problem Isopropyl alcohol feels cool to the touch because it has an (X) heat of (Y). a. X = exothermic, Y = vaporization b. X = endothermic, Y = vaporization c. X = exothermic, Y = fusion d. X = endothermic, Y = fusion Copyright © 2023 Pearson Education, Inc. All Rights Reserved 48 24  11/1/2024 Practice problem When heat is added to ice at zero degrees Celsius, what will happen? a. The temperature will increase. b. The temperature will decrease. c. The temperature will not change. d. A supercritical fluid will form. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 49 Practice problem Substance X has a boiling point of 150 degrees F and a freezing point of 15 degrees F. The condensation point of X a. is 150 degrees F. b. is 15 degrees F. c. is 165 degrees F. d. is 135 degrees F. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 50 25  11/1/2024 Practice problem Is it possible to calculate the heat of sublimation for a substance given its heats of vaporization and fusion? If so, what is the relationship? A. No, because we are not dealing with state functions. B. No, because we need heat of melting. C. Yes, ΔHsub = ΔHfus + ΔHvap D. Yes, ΔHsub = ΔHfus – ΔHvap Copyright © 2023 Pearson Education, Inc. All Rights Reserved 51 Vapor Pressure At any temperature some molecules in a liquid have enough energy to break free. As the temperature rises, the fraction of molecules that have enough energy to break free increases. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 52 26  11/1/2024 Vapor Pressure As more molecules escape the liquid, the pressure they exert increases. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 53 Vapor Pressure Vapor pressure: the liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 54 27  11/1/2024 Vapor Pressure The boiling point of a liquid is the temperature at which its vapor pressure equals atmospheric pressure. The normal boiling point is the temperature at which its vapor pressure is 760 torr. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 55 Vapor Pressure The natural log of the vapor pressure of a liquid is inversely proportional to its temperature. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 56 28  11/1/2024 Vapor Pressure This relationship is quantified in the Clausius–Clapeyron equation: ln P = −Hvap/RT + C, where C is a constant. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 57 Practice problem As the temperature increases, does the rate of molecules escaping into the gas phase increase or decrease? A. Increases B. Decreases Copyright © 2023 Pearson Education, Inc. All Rights Reserved 58 29  11/1/2024 Practice problem At high altitudes, the boiling point of water is a. 100 degrees Celsius. b. greater than 100 degrees Celsius. c. less than 100 degrees Celsius. d. equal to its freezing point. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 59 Practice problem What is the vapor pressure of ethylene glycol at its normal boiling point? A. 260 torr B. 460 torr C. 660 torr D. 760 torr Copyright © 2023 Pearson Education, Inc. All Rights Reserved 60 30  11/1/2024 Phase Diagrams The phase diagram plots pressure in atmospheres versus temperature in degrees Celsius. Phase diagrams display the state of a substance at various pressures and temperatures, and the places where equilibria exist between phases. Fusion curve Vaporization curve Sublimation curve Copyright © 2023 Pearson Education, Inc. All Rights Reserved 61 Supercritical Fluid As a liquid is heated in a sealed container, more vapor collects, causing the pressure inside the container to rise, the density of the vapor to increase, and the density of the liquid to decrease. At some temperature, the meniscus between the liquid and vapor disappears, and the states commingle to form a supercritical fluid. Supercritical fluids have properties of both gas and liquid states. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 62 31  11/1/2024 The Critical Point The temperature required to produce a supercritical fluid is called the critical temperature. The pressure at the critical temperature is called the critical pressure. At the critical temperature or higher temperatures, the gas cannot be condensed to a liquid, no matter how high the pressure gets. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 63 Phase Diagrams The liquid–vapor interface starts at the triple point (T), at which all three states are in equilibrium, and ends at the critical point (C), above which the liquid and vapor are indistinguishable from each other. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 64 32  11/1/2024 Phase Diagrams Each point along this red line is the boiling point of the substance at that pressure. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 65 Phase Diagrams The interface between liquid and solid (blue) marks the melting point of a substance at each pressure. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 66 33  11/1/2024 Phase Diagrams Below the triple point the substance cannot exist in the liquid state. Along the solid–gas line (green) those two phases are in equilibrium; the sublimation point at each pressure is along this line. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 67 Phase Diagram of Water Note the high critical temperature and critical pressure. –These are due to the strong van der Waals forces between water molecules (H bond). Copyright © 2023 Pearson Education, Inc. All Rights Reserved 68 34  11/1/2024 Phase Diagram of Water The slope of the solid–liquid line (fusion curve) is negative. –This means that as the pressure is increased at a temperature just below the melting point, water goes from a solid to a liquid at a lower temperature. The water fusion curve has a negative slope because, unlike most substances, ice is less dense than liquid water, meaning applying pressure to ice actually causes it to melt at a lower temperature. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 69 Phase Diagram of Carbon Dioxide Carbon dioxide cannot exist in the liquid state at pressures below 5.11 atm (triple point); CO2 sublimes at normal pressures. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 70 35  11/1/2024 Features of phase diagrams Triple point - Temperature and pressure at which all three phases are in equilibrium. Critical point – Point above which the liquid and gas phases are indistinguishable. Critical temperature - The minimum temperature for liquefaction of a gas using pressure (T at critical point) Critical pressure - Pressure required for liquefaction (P at critical point) 71 Liquid Crystals Some substances do not go directly from the solid state to the liquid state. In this intermediate state, liquid crystals have some traits of solids and some of liquids. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 72 36  11/1/2024 Practice problem The temperature and pressure at which all three phases exist simultaneously is called the _______ point of a substance. a. boiling b. freezing c. triple d. critical Copyright © 2023 Pearson Education, Inc. All Rights Reserved 73 Practice problem The highest temperature at which a substance can exist in its liquid state is called its _______ point. a. boiling b. freezing c. triple d. critical Copyright © 2023 Pearson Education, Inc. All Rights Reserved 74 37  11/1/2024 Practice problem If the pressure exerted on a liquid is increased, while the temperature is held constant, what type of phase transition will eventually occur? A. Freezing B. Melting C. Vaporization D. Condensation Copyright © 2023 Pearson Education, Inc. All Rights Reserved 75 Copyright This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials. Copyright © 2023 Pearson Education, Inc. All Rights Reserved 76 38