Introductory Chemistry PDF

15 Gases, Liquids, and Solids...

15 Gases, Liquids, and Solids CHAPTER CONTENTS Liquid drops adopt a 15.1 Does Liquid Water Exist Beyond Planet Earth? spherical shape because of a macroscopic, measurable 15.2 Dalton’s Law of Partial property known as surface Pressures tension. This property can be 15.3 Properties of Liquids understood at the particulate 15.4 Types of Intermolecular level in terms of the strengths of Forces the attractive forces among the 15.5 Liquid–Vapor particles that make up the liquid. Equilibrium Strong attractive forces lead to 15.6 The Boiling Process high surface tension. 15.7 Water—An “Unusual” Compound 15.8 The Solid State Candyfloss Film/Shutterstock.com 15.9 Types of Crystalline Solids 15.10 Energy and Change of State 15.11 Energy and Change of Temperature: Specific Heat 15.12 Change in Temperature I n Chapter 12, you studied chemical bonds, the attractive forces between atoms or ions that hold them together to form compounds. In this chapter, we focus on covalently bonded molecules and shift our attention to the attractive forces between the molecules. Plus Change of State The strength of these attractive forces is largely responsible for the physical properties of compounds, including melting and boiling points. Thus, attractive forces between mole- cules determine whether a substance exists as a solid, liquid, or gas at a given temperature and pressure. You first considered the particulate character of matter in the kinetic molecular theory in Section 2.3. The particles of a solid were described as holding in fixed position relative to one another. A degree of freedom is reached in the liquid state, in which particles move about among themselves but still remain together at the bottom of the container that holds them. As a gas, the particles gain complete independence from one another and fly about randomly to fill their containers. In this chapter, we discuss the relationship among gases, liquids, and solids and the energy changes that accompany a change of state. 15.1 Does Liquid Water Exist Beyond Planet!Earth? Several sections of this chapter examine the liquid state of matter. As we will dis- cuss in more detail in Section 15.7, liquid water is chemically unusual, with prop- erties that don’t fit the patterns followed by most other substances. But since liquid 551 Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 552 Chapter 15 Gases, Liquids, and Solids water is so prevalent on Earth, we typically don’t spend much time contemplating its uniqueness. One important characteristic of liq- uid water is that it is required for life, as we know it on Earth. Thus, as we investigate the universe to look for other forms of life, it is important to look for the presence of liquid water as well. Our two nearest planetary neighbors, Venus and Mars, each likely had liquid water on their surfaces in the past. In 2016, a team of NASA scientists published the results of the first sophisti- cated, three-dimensional simulation of the history of Venus’s atmo- sphere. They found that not only was it likely that Venus once had surface water but also that the Venusian ocean may have existed for two billion years. In 2018, a team of Italian scientists reported that radar measurements by the Mars Express spacecraft detected a 20-kilometer-wide lake of liquid water that exists under Mars’s south- ern polar plane (Figure 15.1)! Could life exist in this Martian lake? Other bodies in our solar system also appear to have liquid water. One example is Enceladus, Saturn’s sixth-largest moon. In 2005, the unmanned robotic space probe Cassini flew over the south polar region of Enceladus and found geysers of what appeared to be steam and ice! Cassini was directed to fly through the plumes in 2008, discovering that they are composed of water, Wikimedia Commons carbon dioxide, carbon monoxide, and carbon–hydrogen mole- cules. Given that all living organisms have molecules that include carbon atoms, the basic elements needed for life are present on Enceladus. Figure 15.2 is a not-to-scale illustration of the model hypothesized to be the fundamental structure of Enceladus, with Figure 15.1 The southern polar plain on Mars with the a rocky core, a liquid water ocean, and a solid water crust. site of the liquid water lake enclosed in the highlight rect- angle. The colors within the rectangle indicate the nature Is there evidence of liquid water existing beyond the solar sys- of the radar signal. The lake is located at the blue triangle tem? A good answer to this question cannot be given at the current at approximately the center of the rectangle. The white point in time. Certainly, detection of liquid water outside of our solar surface feature above (to the south of) the rectangle is systems is slightly beyond our current capabilities. However, models Mars’s south polar cap, which is made of both solid car- of planets outside of the solar system indicate that planets with a bon dioxide and solid water. radius about 2.5 times that of Earth likely contain water and 35% of the planets larger than Earth are likely to be water-rich. The James Webb Space Tele- scope (Section 8.1) should provide data that can refute or verify these models. In the words of Sara Seager, Professor of Planetary Science and Physics at the Massachusetts Figure 15.2 A cutaway model of the fundamental structure of Enceladus, a moon of Saturn. The solid planetary core is illustrated in black and gray, its liquid ocean is show in blue, and its ice (solid water) crust is shown in white. Geysers emitting water and a mixture of carbon-atom-containing molecules are illustrated in the southern region of the moon. NASA/JPL-Caltech Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  15.2 Dalton’s Law of Partial Pressures 553 Institute of Technology (Figure 15.3), “It’s amazing to think that the enigmatic intermediate-size exoplanets could be water worlds with vast amounts of water.” 15.2 Dalton’s Law of Partial Pressures Goal 1 Given the partial pressure of each component in a mixture of gases, find the total pressure. 2 Given the total pressure of a gaseous mixture and the partial pressures of all components except one, or information from which those partial pressures can Sam Ogden/Science Source be obtained, find the partial pressure of the remaining component. In Section 4.2, five macroscopic properties of gases are identified: 1. Gases may be compressed. 2. Gases may be expanded. Figure 15.3 Sara Seager, 3. Gases have low densities. Professor of Planetary Science 4. Gases may be mixed in a fixed volume. and Physics at the Massachusetts Institute of Technology. Her science 5. Gases exert constant pressure on the walls of their container uniformly in all research focuses on theory, directions. computation, and data analysis of These properties are explained by the kinetic molecular theory and the ideal exoplanets, those that are outside of gas model described in Chapters 4 and 14. A gas is made up of tiny molecules that our solar system. are widely separated from one another so that they occupy the whole volume of the container that holds them. It is the vast, open space between molecules that makes it possible for gases to mix. If Gas A is added to Gas B in a rigid container (con- stant volume), the A molecules distribute themselves throughout the open space between the B molecules. The particle volume of the A and B molecules is negligible compared with the macroscopic volume occupied by the gas as a whole. One thing changes, though: pressure. It goes up. Figure 15.4 illustrates an experiment. First, note that the volume of all three containers is the same. The gas pressure in the nitrogen vessel is 186 mm Hg, and the pressure in the oxygen vessel is 93 mm Hg, as shown. When the two samples are 3 The N2 and O2 samples are mixed in the same 1.00-L flask at 258C. 0.0100 mol N2 0.0100 mol N2 0.0050 mol O2 0.0050 mol O2 258C 258C 258C 1-L flask 1-L flask 1-L flask P = 186 mm Hg P = 93 mm Hg P = 279 mm Hg 0.0100 mol of N2 0.0050 mol of O2 The total pressure, 279 in a 1.00-L flask at in a 1.00-L flask at mm Hg, is the sum of the 258C exerts a pres- 258C exerts a pres- pressures of the individual sure of 186 mm Hg. sure of 93 mm Hg. gases (186 + 93) mm Hg. 1 2 4 Figure 15.4 Experimental apparatus to demonstrate Dalton’s Law of Partial Pressures. Nitrogen gas exerts a pressure of 186 mm Hg. Oxygen gas exerts a pressure of 93 mm Hg. If the gases are combined in a container of the same volume at the same temperature, the total pressure is the sum of the individual pressures: 186 mm Hg 1 93 mm Hg 5 279 mm Hg. Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 554 Chapter 15 Gases, Liquids, and Solids mixed into a single vessel, the pressure exerted by the combined gases is the sum of the pressures of the individual gases, 186 mm Hg 1 93 mm Hg 5 279 mm Hg. In effect, each gas continues to exert the same pressure it exerted before the gases were mixed. After mixing, the individual gases are exerting their pressures in the same container. The total pressure is the sum of the two individual pressures. John Dalton, of atomic theory fame, summed up these observations in what is now known as Dalton’s Law of Partial Pressures: The total pressure exerted by a Dalton’s atomic theory proposes mixture of gases is the sum of the partial pressures of the gases in the mixture. that all matter is made up of tiny The partial pressure of one gas in a mixture is the pressure that gas would exert if particles called atoms. His theory it alone occupied the same volume at the same temperature. Mathematically, this is is described in Section 5.2. P 5 p1 1 p2 1 p3 1... where P is the total pressure and p1, p2, p3,... are the partial pressures of gases 1, 2, 3,.... Notice that we use an uppercase P for total pressure and a lowercase p for partial pressure. Active Example 15.1 Dalton’s Law of Partial Pressures I In a gas mixture the partial pressure of methane is 154 torr; of ethane, 178 torr; and of propane, 449 torr. Find the total pressure exerted by the mixture. Think Before You Write The problem statement gives you the partial pressures of the components of a gas mixture, and it asks for total pressure. This indicates that you apply Dalton’s Law of Partial Pressures. Answers Cover the left column with your cut-out shield. Reveal each answer only after you have written your own answer in the right column. P 5 p1 1 p2 1 p3 5 Apply the relationship P 5 p1 1 p2 1 p3 1.... Set up 154 torr 1 178 torr 1 449 torr 5 781 torr and solve. You improved your understanding of Dalton’s Law of Partial What did you learn by solving this Active Example? Pressures. Practice Exercise 15.1 A scientist sets up an artificial atmosphere with four gases. Their partial pressures are 0.78 bar nitrogen, 0.21 bar oxygen, 0.009 bar argon, and 0.025 bar water. (See the discussion of air in a SCUBA tank in Figure 15.5.) What is the total pressure? Figure 15.5 SCUBA (self- contained underwater breathing apparatus) diving. The solubility of gases in the blood is proportional to their partial pressures. If air is compressed, the partial pressures of nitrogen and oxygen are increased, leading to problems such as nitrogen narcosis and oxygen toxicity. Helium is added to the breathing mixture for NOAA deep-sea diving. Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  15.2 Dalton’s Law of Partial Pressures 555 Figure 15.6 Laboratory prepara- KClO3 tion of oxygen. The heat applied to (and a trace of MnO2 as a catalyst) Oxygen the test tube decomposes the solid + KClO3 into gaseous O2 and solid water vapor KCl. The oxygen is directed into the bottom of an inverted bottle that is initially filled with water. As the oxygen accumulates in the bottle, it displaces the water and is saturated with water vapor until the bottle is filled with a mixture of oxygen and water vapor. Water Gases generated in the laboratory may be collected by bubbling them through water, as shown in Figure 15.6. As the oxygen bubbles rise through the water, they become saturated with water vapor. The gas collected is therefore actually a mixture of oxygen and water vapor. The pressure exerted by the mixture is the sum of the partial pressure of the oxygen and the partial pressure of the water vapor (Figure 15.7). As you will see in Section 15.5, water vapor pressure depends only on temperature. Its values at different temperatures may be found by doing a search for “water vapor pressure table” on the Internet or by consulting a refer- ence book. Active Example 15.2 Dalton’s Law of Partial Pressures II Oxygen is generated for a laboratory experiment by bubbling the gas through water, as illustrated in Figures 15.6 and 15.7. The total pressure of the oxygen saturated with water vapor is 755 torr. The temperature of the gas mixture is 22°C, and water vapor pressure at that temperature is 19.8 torr. What is the partial pressure of the oxygen? Think Before You Write You are given the total pressure of a mixture of gases, and you are also given the partial pressure of one of the components. You are asked to determine the partial pressure of the other component of the mixture. This should lead you to recognize that this is an application of Dalton’s Law of Partial Pressures. Answers Cover the left column with your cut-out shield. Reveal each answer only after you have written your own answer in the right column. P 5 pH2O 1 pO2 Rewrite P = p1 1 p2 1 p3 1... in terms of the total Rearranging, pO2 5 P ! pH2O 5 pressure of the mixture, the partial pressure of oxygen, and the partial pressure of water so that it specifically applies 755 torr ! 19.8 torr 5 735 torr to this problem. Then rearrange the equation so that the unknown, pO2, is isolated on one side of the equals sign. Finish by substituting the measured quantities and solving. You improved your understanding of Dalton’s Law of Partial What did you learn by solving this Active Example? Pressures. Practice Exercise 15.2 Calculate the partial pressure of oxygen (in atm) in a sample collected over water, given that the total pressure of the mix- ture is 0.98 atm and the partial pressure of water vapor is 16.5 mm Hg. Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 556 Chapter 15 Gases, Liquids, and Solids Figure 15.7 Applying Dalton’s Gas + H2O Law of Partial Pressures to a gas 2 Ptotal = Pgas + PH2O vapor collected over water. When the water levels inside and outside of 1 Water levels inside the tube are equalized, the pressure and outside the test tube are the same to inside the tube is equal to atmo- equalize pressure. spheric pressure, and thus Ptotal may be easily measured. The water vapor Gaseous product pressure depends on temperature, of reaction Don Mason/Bridge/Corbis so its value can be looked up in a reference source. The pressure of H2O the gaseous product is, therefore, Water level of the total pressure minus the water tube equal to vapor pressure. that of pan 15.3 Properties of Liquids Goal 3 Explain the differences between the physical behavior of liquids and gases in terms of the relative distances among particles and the effect of those distances on intermolecular attractions. 4 For two liquids, given comparative values of physical properties that depend on intermolecular attractions, predict the relative strengths of those attractions; or, given a comparison of the strengths of the intermolecular attractions, predict the relative values of physical properties that the attractions cause. The properties of liquids are easy to observe and describe—more so than the prop- erties of gases. To understand liquid properties, however, it is helpful to compare the structure of a liquid with the structure of a gas. In Chapter 4, you learned that gas particles are so far apart that attractive and repulsive forces between the par- ticles are negligible. These forces are electrostatic in character. They are inversely related to the distance between the particles; the smaller the distance is, the stronger the forces are. In a liquid, particles are very close to one another. Consequently, the intermolecular attractions in a liquid are strong enough to affect its physical properties. We can now compare the properties of liquids with five properties of gases that were listed in Section 4.2 (Figures 4.2 through 4.5): 1. Gases may be compressed; liquids cannot. Liquid particles are “touchingly close” to one another. There is no space between them, so they cannot be pushed closer, as in the compression of a gas. 2. Gases may be expanded; liquids cannot. The strong attractions between liquid particles hold them together; the volume of a liquid is constant, no matter the container size. 3. Gases have low densities; liquids have relatively high densities. Density is mass per unit volume—mass divided by volume. If the particles of a liquid are close together compared with the particles of a gas, a given number of liquid parti- cles will occupy a much smaller volume than the same number of particles will occupy as a gas. A small denominator in the density ratio for a liquid means a higher value for the ratio. 4. Gases may be mixed in a fixed volume; liquids cannot. When one gas is added to another, the particles of the second gas occupy some of the space between the particles of the first gas. There is no space between particles of a liquid, so com- bining liquids must increase volume. Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  15.3 Properties of Liquids 557 5. Gases exert constant pressure on the walls of their container uniformly in all directions; the pressure in a liquid container increases with increasing depth. For small samples of enclosed gases, gas pressure is independent of external fac- tors such as gravitational forces. Liquid pressures depend on the depth of the liquid due to variation in weight at varying depth. A liquid has several measurable properties with values that depend on inter- molecular attractions, the tendency of the particles to stick together. In fact, if you think in terms of the “stick-togetherness” as equated with the strength of the inter- molecular attractions, you can usually predict relative values of these properties for two liquids. The greater the stick-togetherness is, the stronger the intermolecular attractions are. We will now identify five of these properties. Vapor Pressure The open space above any liquid contains some particles in the gas- eous, or vapor, state. This is due to evaporation (Figure 15.8). The partial pressure exerted by these gaseous particles is called vapor pressure. If the gas space above the liquid is closed, the vapor pressure increases to a definite value called the equilibrium vapor pressure. In Section 15.5, you will study the mechanics by which that pres- sure is reached. Vapor pressure is inversely related to intermolecular attractions. If stick-togetherness is high between liquid particles, very few liquid particles “escape” into the gaseous state, so the vapor pressure is low. Heat of Vaporization Energy must be transferred to a liquid to first overcome intermolecular attractions, then separate liquid particles from one another, and continue to keep them apart (Figure 15.9 ). The quantity of energy required to change one mole of a liquid to its vapor while at constant temperature and pres- sure is called heat of vaporization. The energy released in the opposite process, as vapor condenses to the liquid phase, is called heat of condensation. The greater the stick-togetherness is, the greater is the amount of energy transferred into or out of the liquid. Some of the liquid phase molecules are moving with a kinetic energy large enough to overcome the intermolecular attractions in the At the same time, some liquid and escape to the gas phase. molecules in the gas re-enter the liquid. Vapor Charles D. Winters Liquid Figure 15.8 Evaporation. Some molecules at the surface of the liquid phase have sufficient energy to escape and enter the vapor phase. Some molecules in the vapor phase will re-enter the liquid phase when they make contact with the surface. The partial pressure exerted by the mole- cules in the vapor phase is called vapor pressure. Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 558 Chapter 15 Gases, Liquids, and Solids Vapor DH vaporization DH condensation (endothermic) (exothermic) Charles D. Winters Liquid Figure 15.9 Heats of vaporization and condensation. Energy must be transferred to a system to overcome the attractive forces that are exerted among liquid molecules. When an equal quantity of vapor condenses to a liquid, an equal amount of energy is transferred out of the vapor. Boiling Point Liquids can be changed to gases by boiling. The boiling process is discussed in more detail in Section 15.6. At the boiling point, the average kinetic energy of the liquid particles is high enough to overcome the forces of attraction that hold the particles in the liquid state. When stick-togetherness is high, it takes more motion (a higher temperature) to separate the particles within the liquid, where boiling occurs. The trends in vapor pressure, molar heat of vaporization, and boiling point are shown for several substances in Table 15.1. Viscosity Particles in a liquid are free to move about relative to one another; they “flow.” Some liquids flow more easily than others. Water, for example, can be poured much more freely than syrup, and syrup pours more readily than honey (Figure 15.10). The ability of a liquid to flow is measured by its viscosity. Viscosity is an internal resis- tance to flow, and it is partially based on intermolecular attractions. When comparing particles of about the same size, more stick-togetherness means higher viscosity. Surface Tension When a liquid is broken into “small pieces,” it forms spherical drops (see the photograph on the opening page of this chapter). A sphere has the smallest surface area possible for a drop of any given volume. This tendency toward a minimum surface is the result of surface tension. Within a liquid, each particle is attracted in all directions by the particles around it. At the surface, however, the Charles D. Winters Table 15.1 Physical Properties of Liquids Figure 15.10 Viscosity. Honey pours relatively slowly, and thus it Heat of Normal has a relatively high viscosity. This Vapor Pressure at Vaporization Boiling Point Intermolecular macroscopic characteristic indicates Substance 20°C (torr) (kJ/mol) (°C) Attractions that there are relatively strong inter- Mercury 0.0012 59 357 Strongest molecular attractions among the molecules that make up the mixture Water 17.5 41 100 commonly known as honey. Benzene 75 31 80 Ether 442 26 35 Weakest Ethane 27,000 15 289 Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  15.3 Properties of Liquids 559 Surface molecules have unbalanced forces acting on them, and they are pulled inward. Charles D. Winters Interior molecules are pulled in all directions, so no net force acts on them. Figure 15.11 Surface tension. Unbalanced attractive forces at the surface of a liquid pull sur- face molecules downward and sideways but not up. Molecules within the water are attracted in all directions. Surface tension is a result of the difference in these forces. attraction is almost entirely downward, pulling the surface molecules into a sort Marek Mierzejewski/Shutterstock.com of tight skin over a standing liquid. Similarly, in a spherical drop, the attraction at the surface is almost entirely inward. This is surface tension (Figure 15.11). Its effect in water may be seen when a needle floats if placed gently on a still surface, or when small bugs run across the surface of a quiet pond (Figure 15.12). High stick- togetherness at the surface means more resistance to anything that would break through or stretch that surface. Figure 15.12 Insect walking on a summary of... Properties of Liquids and Intermolecular Attractions water. The surface tension of water Vapor pressure: Liquids with relatively strong intermolecular attractions evaporate less readily, creates a difficult-to-penetrate skin yielding lower vapor concentrations and therefore lower vapor pressures than liquids with weak that will support small bugs or thin intermolecular attractions. pieces of dense metals, such as a needle or razor blade. A bug literally Heat of vaporization: The heat of vaporization of a liquid with strong intermolecular attractions is runs on the water; it does not float higher than the heat of vaporization of a liquid with weak intermolecular attractions. in it. Boiling point: Liquids with strong intermolecular attractions require higher temperatures for boiling than liquids with weak intermolecular attractions. Viscosity: Liquids with strong intermolecular attractions are generally more viscous than liquids with weak intermolecular attractions. Surface tension: Liquids with strong intermolecular attractions have higher surface tension than liquids with weak intermolecular attractions. Target Check 15.1 a) What main difference between gases and liquids at the particulate level accounts for the large differences in their macroscopic properties? b) Intermolecular attractions are stronger in A than in B, with all other potentially influencing factors being about equal. Which do you expect will have the higher surface tension, molar heat of vaporization, vapor pressure, boiling point, and viscosity? c) X has a higher molar heat of vaporization than Y. Which do you expect will have a higher vapor pressure? Why? Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 560 Chapter 15 Gases, Liquids, and Solids 15.4 Types of Intermolecular Forces Goal 5 Identify and describe or explain induced dipole forces, dipole forces, and hydro- gen bonds. 6 Given the structure of a molecule, or information from which it may be deter- mined, identify the significant intermolecular forces present. 7 Given the molecular structures of two substances, or information from which they may be obtained, compare or predict relative values of physical properties that are related to them. It was stated in the preceding section that attractive forces between particles are elec- trostatic in character; the attractions are between positive and negative charges. But atoms and molecules are electrically neutral. How can there be electrostatic attrac- tions? The answer is that the distribution of electrical charge within the molecule is P/Review If you have drawn not always uniform. Some molecules are polar and some are nonpolar. In addi- a Lewis diagram of a molecule tion, some molecules are large and some are small. Molecular polarity and size both (Section 13.2), you can predict that it contribute to intermolecular attraction and therefore to physical properties. is polar if either of these conditions Three kinds of intermolecular forces can be traced to electrostatic attractions: exists: a central atom has a lone dipole forces, induced dipole forces, and hydrogen bonds. pair of electrons, or a central atom 1. Dipole forces. A polar molecule is sometimes described as a dipole. The attraction is bonded to atoms of different between dipoles is between the positive pole of one molecule and the negative elements (see Section 13.6). pole of another. Figure 15.13 shows the alignment of dipoles, one of several ways polar molecules attract one another. Table 15.2 compares the boiling points of four pairs of substances that have about the same molecular size, indicated approximately by their molar masses. In each pair, the boiling point of the substance with polar molecules is higher than the boiling point of the nonpolar substance. This is because polar molecules have stronger intermolecular attractions than nonpolar molecules. The BrCl …because the 2. Induced dipole forces. Attractions between nonpolar molecules are called molecule is polar. more electro- induced dipole forces. These are also called dispersion forces, London forces, or The Br atom negative Cl acquires a partial atom attracts London dispersion forces. They are believed to be the result of shifting electron positive charge… electrons away clouds within the molecules. If the electron movement in a molecule results in a from Br. temporary concentration of electrons at one side of the molecule, the molecule d+ d– d+ d– becomes a “temporary dipole.” This is shown in the rightmost of the center pair of molecules in Figure 15.14. The electrons repel the electrons in the molecule Br Cl Br Cl next to it, pushing them to the far side of that molecule. The second molecule is thus “induced” to form a second temporary dipole (see the right pair in Figure!15.14). As long as these dipoles exist—a very small fraction of a second in each case—there is a weak attraction between them. The strength of induced dipole forces depends on the ease with which elec- Cl Br Cl Br tron distributions can be induced to be distorted or “polarized.” Large mole- cules, with many electrons and with electrons far removed from atomic nuclei, are more easily polarized than small molecules. Larger molecules also generally d– d+ d– d+ The partial negative region of one molecule is attracted to the partial positive region of the neighboring molecule. Table 15.2 Boiling Points of Polar Versus Nonpolar Substances This results in dipole-dipole forces among the molecules. Polar or Molecular Boiling Polar or Molecular Boiling Formula Nonpolar Mass (u) Point (°C) Formula Nonpolar Mass (u) Point (°C) Figure 15.13 Dipole forces. Mole- N2 Nonpolar 28 2196 GeH4 Nonpolar 77 288 cules tend to arrange themselves by CO Polar 28 2192 AsH3 Polar 78 263 bringing oppositely charged regions close to one another and forcing SiH4 Nonpolar 32 2112 Br2 Nonpolar 160 59 similarly charged regions away from PH3 Polar 34 288 ICl Polar 162 97 one another. Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  15.4 Types of Intermolecular Forces 561 2 2 1 2 1 1 Two nonpolar atoms or molecules Momentary attractions and Correlation of the electron (depicted as having an electron repulsions between nuclei and motions between the two cloud that has a time-averaged electrons in neighboring atoms or molecules (which are spherical shape). molecules lead to induced dipoles. now dipoles) leads to a lower energy and stabilizes the system. Figure 15.14 Induced dipole forces. Electron clouds in nonpolar molecules are constantly shifting. The temporary dipole in the right molecule in the center “induces” the molecule next to it to become another temporary dipole (right pair). The “instantaneous dipoles” are attracted to one another briefly. A small fraction of a second later, the clouds shift again and continue to interact with one another or with nearby molecules. Figure 15.15 Induced dipole forces and molecular size. Larger molecules, with a greater num- ber of electrons, polarize more easily than smaller molecules. Bromine (left) exists as a liquid at room conditions, and iodine (right) is a solid. Both molecules are nonpolar and have induced dipole forces as the dominant intermolecular force. However, bromine molecules have 70 electrons, Charles D. Winters whereas iodine molecules have 106 electrons, so iodine molecules polarize more easily. Thus, the intermolecular attractive forces among iodine molecules are greater than among bromine mole- cules. This particulate-level difference is responsible for the macroscopic observations that iodine Br2 I2 is a solid and bromine is a liquid at room temperature. have greater mass. Consequently, intermolecular forces tend to increase with increasing molecular mass among otherwise similar substances (Figure 15.15). Notice in Table 15.2 the increase in boiling points for both polar and nonpolar molecules as molecular mass increases. 3. Hydrogen bonds. Some polar molecules have intermolecular attractions that are much stronger than ordinary dipole forces. These molecules always have a hydrogen atom bonded to an atom that is small and highly electronegative and that has at least one unshared pair of electrons. Nitrogen, oxygen, and flu- P/Review Electronegativity orine are generally the only elements whose atoms satisfy these requirements. estimates the strength with which an Study Figure 15.16 to learn how to recognize hydrogen bonds. atom attracts the pair of electrons that forms a bond between it and another atom. Covalent Electronegative Lewis bonds are polar when there is an Element Diagram Examples electronegativity difference between H the bonded atoms. See Section 12.5. Nitrogen H N H N H H N C H H H H Ammonia Methylamine H Figure 15.16 Recognizing hydro- gen bonding. Hydrogen bonds occur Oxygen O O H O C H when a hydrogen atom is covalently H H H H bonded to a small atom that is highly electronegative and has one or more Water Methanol unshared electron pairs. Fluorine, oxygen, and nitrogen atoms fit this description. The hydrogen bond is Fluorine H F H F F H F between the atom of one of these H H elements in one molecule and the hydrogen atom of a nearby molecule. F Hydrogen bonds between molecules Hydrogen fluoride are illustrated with dashed lines in the hydrogen fluoride example. Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 562 Chapter 15 Gases, Liquids, and Solids The covalent bond formed between the hydrogen atom and the atom of nitrogen, oxygen, or fluorine is strongly polar. The electron pair is shifted away from the hydrogen atom toward the more electronegative atom. This leaves the hydrogen nucleus—nothing more than a proton—as a small, highly concen- trated region of positive charge at the edge of a molecule. The negative pole of another molecule, which is the region near an unshared electron pair on a nitro- gen, oxygen, or fluorine atom, can get quite close to the hydrogen atom of the first molecule. This results in an extra strong attraction between the molecules. This kind of intermolecular attraction is a hydrogen bond. Be sure to keep in mind that a hydrogen bond is an intermolecular force, an attraction between different molecules. It is not a covalent bond between atoms in the same molecule. The dotted lines in Figure 15.17 represent hydrogen bonds between water molecules. Although a hydrogen bond is much stronger than an ordinary dipole–dipole force, it is roughly one-tenth as strong as a cova- lent bond between atoms of the same two elements. Of the three kinds of intermolecular forces, hydrogen bonds are the strongest. When present between small molecules, hydrogen bonds are primarily responsible for the physical properties of a liquid. Dipole forces are the next strongest and induced dipole forces are the weakest of the three. Induced dipole forces are pres- ent between all molecules. In small molecules, induced dipole forces are import- ant only when the other forces are absent. But between large molecules—molecules that contain many atoms or even few atoms that have many electrons—induced dipole forces are quite strong and often play the main role in determining physical properties. Figure 15.18 summarizes the kinds of intermolecular forces and their effects on boiling points of similar compounds in three chemical families. We recommend that you study it carefully. Target Check 15.2 Identify the true statements and rewrite false statements to make them true. a) Induced dipole forces are present only with nonpolar molecules. b) All other things being equal, hydrogen bonds are stronger than dipole–dipole forces. c) Polar molecules have a net electrical charge. d) Intermolecular forces are magnetic in character. e) H2O displays hydrogen bonding, but H2S does not. Water molecules are arranged in tetrahedra connected by hydrogen bonds. Figure 15.17 Hydrogen bonding in solid water (ice). Intermolecular hydrogen bonds are pres- ent between the electronegative oxygen region of one molecule and the electropositive hydrogen region of a second molecule. Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it.  15.4 Types of Intermolecular Forces 563 100 H 2O Temperature (8C) 0 H2Te SbH3 NH3 H2Se SnH4 H2S AsH3 PH3 GeH4 2100 SiH4 CH4 0 2 3 4 5 Period Figure 15.18 Intermolecular attractions illustrated by boiling Dipole forces: The molecules in the rectangle in the Period points of hydrogen-containing compounds. The plot shows boiling 4 hydrogen-containing compounds—H2Se, AsH3, and GeH4—are point temperature (°C) versus period in the periodic table. Liquids about the same size (nearly equal molar mass), and none of them with strong intermolecular attractions usually boil at higher tem- has hydrogen bonding. They differ most in polarity. GeH4 has tet- peratures than liquids with weak intermolecular attractions. These rahedral molecules; it is nonpolar. The trigonal pyramidal attractions are caused by induced dipole forces, dipole forces, and molecules of AsH3 are polar but less so than bent H2Se molecules. hydrogen bonding. Holding two of these variables essentially con- The least polar compound, GeH4, has the lowest boiling point, stant and changing the third, we can see how each variable affects and the most polar compound, H2Se, has the highest boiling the attractions by comparing the boiling points. point. The same trend appears with the Period 3 and Period 5 Induced dipole forces: The Group 4A/14 hydrogen-containing hydrogen-containing compounds. This indicates that, other things compounds (blue line) all have tetrahedral molecular geometries. They being equal, intermolecular attractions increase as molecular are nonpolar, and they have no hydrogen bonding. The only intermo- polarity increases. lecular forces are induced dipole forces. The molecules differ only in Hydrogen bonding: The high boiling points of H2O and molecular size (mass), ranging from CH4 (the smallest) to SnH4 (the NH3 violate the trends in which small molecules boil at lower tem- largest). The boiling points of the four compounds increase as their peratures than large molecules that are otherwise similar. H2O and molecular sizes increase. Except for H2O and NH3, the same trend NH3 are the only two substances shown that have hydrogen bond- appears for Group 5A/15 (black line) and Group 6A/16 (red line) hydro- ing. This indicates that, for small molecules in particular, hydrogen gen-containing compounds. This suggests that, other things being bonding causes exceptionally strong intermolecular attractions. equal, intermolecular attractions increase as molecular size increases. Target Check 15.3 Determine the molecular geometry and polarity of each of the following and, from that, identify the strongest intermolecular forces present. a) CH4 b) CO2 c) OF2 d) HOCl (the oxygen atom is central) Target Check 15.4 Identify the molecule in each pair that you would expect to have the stronger intermolecular forces and state why. a) CCl4 or CBr4 b) NH3 or PH3 Your Thinking Thinking About Mental Models Particulate-level mental models of what causes each of the intermolecular forces are needed for a complete understanding of this concept. First, be sure that you grasp the distinction between intramolecular forces, which are chemical bonds between atoms within a single molecule, and intermolecular forces, the Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 564 Chapter 15 Gases, Liquids, and Solids attractive forces that operate between whole molecules and other particles. Your “mental movie” needs to be based on an electron cloud model of a molecule or atom, such as the quantum model shown in Figure 11.18. No matter how many atoms are used to make up the particle, consider only the outer electron cloud of the particle for this mental model. Consider Figure 15.14. The electron clouds in the left pair show the most common state for nonpolar molecules—one in which the outer electrons are evenly distributed throughout the molecule. Induced dipole forces are a result of a short-term, uneven shifting of this electron cloud so that one area of the particle has a slight excess of electrons and the opposite side is slightly deficient in electrons, relative to the normal balanced distribution. This makes the molecule just a little polar, with very weak negative and positive regions. The charged region of this molecule then induces (hence the name of this force) its neighbor to adjust its electron distribution. If the electron density of a molecule is shifted to the left (the right particle of the center pair in Figure 15.14), the negative region repels the electrons in the right side of its neighbor, inducing it to shift its electron density to the left (the right pair in Figure 15.14). Now, a slightly negative region is near a slightly positive region, and there is a weak attraction. If you can imagine this process, you have a good start at your mental model of induced dipole forces. Now imagine a particle in which the uneven distribution of electrons is permanent. This is the case in polar molecules, as illustrated in Figure 15.13. The chlorine end of each molecule is the portion with a greater electron density and the accompanying partial negative charge. The region of a polar molecule with a partial negative charge is attracted to the positively charged end of a neighboring polar molecule. The third type of mental model for intermolecular forces that you need to develop is to imag- ine how hydrogen bonds form. Before you think about the intermolecular forces, consider an individual molecule that has a hydrogen atom covalently bonded to oxygen, such as water. The bonding electrons in the O—H bond are displaced toward the oxygen atom, which is relatively small and highly electronegative. This leaves the hydrogen atom nucleus—a positively charged proton—at the end of the molecule with very little surrounding electron density. That positively charged region will be strongly attracted to the oxygen-atom region of a neighboring molecule, given that the oxygen atom not only has two unshared pairs of electrons but also has its bond- ing electrons shifted toward it and away from the hydrogen atom. The result is an intermolecular attractive force between the highly electronegative atom of one molecule and the highly elec- tropositive hydrogen atom of a neighboring molecule. The ability to imagine the particulate-level reasons for intermolecular attractive forces is a pow- erful tool in your “thinking as a chemist” knowledge stockpile. The more you understand this type of particulate-level behavior, the better you will understand the macroscopic properties that result from it. 15.5 Liquid–Vapor Equilibrium Goal 8 Describe and explain the equilibrium between a liquid and its own vapor and the process by which it is reached. In Section 4.5 we discussed how temperature is a measure of the average kinetic energy of the particles in a sample. The range of kinetic energies at a certain tem- perature is shown in Figure 15.19. Kinetic energy is plotted horizontally, and the fraction, or percentage, of the sample having a given kinetic energy is plotted verti- cally. The area beneath the curve represents all of the particles in the sample (100% of the sample). This type of graph is called a kinetic energy distribution curve. To evaporate or vaporize, a molecule must be at the surface of a liquid. It also must have enough kinetic energy to overcome the attractions of other molecules that would hold it in the liquid state. If E in Figure 15.19 represents this minimum amount of kinetic energy—we will call it the escape energy—only those surface mol- ecules having that energy or more can get away. The fraction, or percentage, of all surface molecules having that much energy is given by the area beneath the curve to the right of E. Copyright 2021 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some