Chemistry The Central Science 11 th edition Theodore

Chemistry, The Central Science, 11 th edition Theodore L. Brown, H. Eugene Le. May, Jr. , and Bruce E. Bursten Chapter 11 Intermolecular Forces, Liquids, and Solids John D. Bookstaver St. Charles Community College Cottleville, MO Intermolecular Forces © 2009, Prentice-Hall, Inc.

States of Matter The fundamental difference between states of matter is the distance between particles. Intermolecular Forces © 2009, Prentice-Hall, Inc.

States of Matter Because in the solid and liquid states particles are closer together, we refer to them as condensed phases. Intermolecular Forces © 2009, Prentice-Hall, Inc.

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. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Intermolecular Forces The attractions between molecules are not nearly as strong as the intramolecular attractions that hold compounds together. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Intermolecular Forces They are, however, strong enough to control physical properties such as boiling and melting points, vapor pressures, and viscosities. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Intermolecular Forces These intermolecular forces as a group are referred to as van der Waals forces. Intermolecular Forces © 2009, Prentice-Hall, Inc.

van der Waals Forces • Dipole-dipole interactions • Hydrogen bonding • London dispersion forces Intermolecular Forces © 2009, Prentice-Hall, Inc.

Ion-Dipole Interactions • Ion-dipole interactions (a fourth type of force), are important in solutions of ions. • The strength of these forces are what make it possible for ionic substances to dissolve in polar solvents. Intermolecular Forces © 2009, Prentice-Hall, Inc.

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 viceversa. – These forces are only important when the molecules are close to each other. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Dipole-Dipole Interactions The more polar the molecule, the higher is its boiling point. Intermolecular Forces © 2009, Prentice-Hall, Inc.

London Dispersion Forces While the electrons in the 1 s 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 Intermolecular same side of the atom. Forces © 2009, Prentice-Hall, Inc.

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. Intermolecular Forces © 2009, Prentice-Hall, Inc.

London Dispersion Forces Another helium nearby, then, would have a dipole induced in it, as the electrons on the left side of helium atom 2 repel the electrons in the cloud on helium atom 1. Intermolecular Forces © 2009, Prentice-Hall, Inc.

London Dispersion Forces London dispersion forces, or dispersion forces, are attractions between an instantaneous dipole and an induced dipole. Intermolecular Forces © 2009, Prentice-Hall, Inc.

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. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Factors Affecting London Forces • The shape of the molecule affects the strength of dispersion forces: long, skinny molecules (like n-pentane tend to have stronger dispersion forces than short, fat ones (like neopentane). • This is due to the increased surface area in n-pentane. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Factors Affecting London Forces • The strength of dispersion forces tends to increase with increased molecular weight. • Larger atoms have larger electron clouds which are easier to polarize. Intermolecular Forces © 2009, Prentice-Hall, Inc.

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 the dominating force. • If one molecule is much larger than another, dispersion forces will likely determine its physical properties. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Sample Exercise 11. 1 Comparing Intermolecular Forces The dipole moments of acetonitrile, CH 3 CN, and methyl iodide, CH 3 I, are 3. 9 D and 1. 62 D, respectively. (a) Which of these substances has greater dipole–dipole attractions among its molecules? (b) Which of these substances has greater London dispersion attractions? (c) The boiling points of CH 3 CN and CH 3 I are 354. 8 K and 315. 6 K, respectively. Which substance has the greater overall attractive forces? Solution (a) Dipole–dipole attractions increase in magnitude as the dipole moment of the molecule increases. Thus, CH 3 CN molecules attract each other by stronger dipole–dipole forces than CH 3 I molecules do. (b) When molecules differ in their molecular weights, the more massive molecule generally has the stronger dispersion attractions. In this case CH 3 I (142. 0 g/mol) is much more massive than CH 3 CN (41. 0 g/mol), so the dispersion forces will be stronger for CH 3 I. (c) Because CH 3 CN has the higher boiling point, we can conclude that more energy is required to overcome attractive interactions between CH 3 CN molecules. Thus, the total intermolecular attractions are stronger for CH 3 CN, suggesting that the energies resulting from dipole–dipole forces are decisive when comparing these two substances. Nevertheless, the attractive interactions due to dispersion forces play an important role in determining the properties of CH 3 I. Practice Exercise Of Br 2, Ne, HCl, HBr, and N 2, which is likely to have (a) the largest intermolecular dispersion forces, (b) the largest dipole–dipole attractive forces? Answer: (a) Br 2 (largest molecular weight), (b) HCl (largest polarity) Intermolecular Forces © 2009, Prentice-Hall, Inc.

How Do We Explain This? • The nonpolar series (Sn. H 4 to CH 4) follow the expected trend. • The polar series follows the trend from H 2 Te through H 2 S, but water is quite an anomaly. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Hydrogen Bonding • The dipole-dipole interactions experienced when H is bonded to N, O, or F are unusually strong. • We call these interactions hydrogen bonds. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Hydrogen Bonding • Hydrogen bonding arises in part from the high electronegativity of nitrogen, oxygen, and fluorine. • . . Media GalleryAnimationsHydro gen. BondingHydrogen. Bon ding. html Also, when hydrogen is bonded to one of those very electronegative elements, the hydrogen nucleus is exposed. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Sample Exercise 11. 2 Identifying Substances that Can Form Hydrogen Bonds In which of the following substances is hydrogen bonding likely to play an important role in determining physical properties: methane (CH 4), hydrazine (H 2 NNH 2), methyl fluoride (CH 3 F), or hydrogen sulfide (H 2 S)? Solution Analyze: We are given the chemical formulas of four substances and asked to predict whether they can participate in hydrogen bonding. All of these compounds contain H, but hydrogen bonding usually occurs only when the hydrogen is covalently bonded to N, O, or F. Plan: We can analyze each formula to see if it contains N, O, or F directly bonded to H. There also needs to be a nonbonding pair of electrons on an electronegative atom (usually N, O, or F) in a nearby molecule, which can be revealed by drawing the Lewis structure for the molecule. Solve: The criteria listed above eliminate CH 4 and H 2 S, which do not contain H bonded to N, O, or F. They also eliminate CH 3 F, whose Lewis structure shows a central C atom surrounded by three H atoms and an F atom. (Carbon always forms four bonds, whereas hydrogen and fluorine form one each. ) Because the molecule contains C—F a bond and not a H—F bond, it does not form hydrogen bonds. In H 2 NNH 2, however, we find bonds. If we draw the Lewis structure for the molecule, we see that there is a nonbonding pair of electrons on each N atom. Therefore, hydrogen bonds can exist between the molecules as depicted below. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Sample Exercise 11. 2 Identifying Substances that Can Form Hydrogen Bonds Solution (continued) Check: While we can generally identify substances that participate in hydrogen bonding based on their containing N, O, or F covalently bonded to H, drawing the Lewis structure for the interaction, as shown above, provides a way to check the prediction. Practice Exercise In which of the following substances is significant hydrogen bonding possible: methylene chloride (CH 2 Cl 2), phosphine (PH 3), hydrogen peroxide (HOOH), or acetone (CH 3 COCH 3)? Answer: HOOH Intermolecular Forces © 2009, Prentice-Hall, Inc.

Summarizing Intermolecular Forces © 2009, Prentice-Hall, Inc.

Sample Exercise 11. 3 Predicting the Types and Relative Strengths of Intermolecular Attractions List the substances Ba. Cl 2, H 2, CO, HF, and Ne in order of increasing boiling points. Solution Analyze: We need to relate the properties of the listed substances to boiling point. Plan: The boiling point depends in part on the attractive forces in the liquid. We need to order these according to the relative strengths of the different kinds of intermolecular attractions. Solve: The attractive forces are stronger for ionic substances than for molecular ones, so Ba. Cl 2 should have the highest boiling point. The intermolecular forces of the remaining substances depend on molecular weight, polarity, and hydrogen bonding. The molecular weights are H 2 (2), CO (28), HF (20), and Ne (20). The boiling point of H 2 should be the lowest because it is nonpolar and has the lowest molecular weight. The molecular weights of CO, HF, and Ne are roughly the same. Because HF can hydrogen bond, however, it should have the highest boiling point of the three. Next is CO, which is slightly polar and has the highest molecular weight. Finally, Ne, which is nonpolar, should have the lowest boiling point of these three. The predicted order of boiling points is therefore H 2 < Ne < CO < HF < Ba. Cl 2 Check: The actual normal boiling points are H 2 (20 K), Ne (27 K), CO (83 K), HF (293 K), and Ba. Cl 2 (1813 K)—in agreement with our predictions. Practice Exercise (a) Identify the intermolecular attractions present in the following substances, and (b) select the substance with the highest boiling point: CH 3, CH 3 OH, and CH 3 CH 2 OH. Answer: (a) CH 3 has only dispersion forces, whereas the other two substances have both dispersion forces and hydrogen bonds; (b) CH 3 CH 2 OH Intermolecular Forces © 2009, Prentice-Hall, Inc.

Intermolecular Forces Affect Many Physical Properties The strength of the attractions between particles can greatly affect the properties of a substance or solution. Intermolecular Forces © 2009, Prentice-Hall, Inc.

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. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Surface Tension Surface tension results from the net inward force experienced by the molecules on the surface of a liquid. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Phase Changes Intermolecular Forces © 2009, Prentice-Hall, Inc.

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. Intermolecular Forces © 2009, Prentice-Hall, Inc.

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. Intermolecular Forces © 2009, Prentice-Hall, Inc.

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. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Sample Exercise 11. 4 Calculating ΔH for Temperature and Phase Changes Calculate the enthalpy change upon converting 1. 00 mol of ice at – 25 °C to water vapor (steam) at 125 °C under a constant pressure of 1 atm. The specific heats of ice, water, and steam are 2. 03 J/g-K, 4. 18 J/g-K, and 1. 84 J/g-K, respectively. For H 2 O, ΔHfus = 6. 01 k. J/mol and ΔHvap = 40. 67 k. J/mol. Solution Analyze: Our goal is to calculate the total heat required to convert 1 mol of ice at -25 °C to steam at 125 °C. Plan: We can calculate the enthalpy change for each segment and then sum them to get the total enthalpy change (Hess’s law, Section 5. 6). Solve: For segment AB in Figure 11. 19, we are adding enough heat to ice to increase its temperature by 25 °C. A temperature change of is 25 °C the same as a temperature change of 25 K, so we can use the specific heat of ice to calculate the enthalpy change during this process: For segment BC in Figure 11. 19, in which we convert ice to water at 0 °C, we can use the molar enthalpy of fusion directly: The enthalpy changes for segments CD, DE, and EF can be calculated in similar fashion: Intermolecular Forces © 2009, Prentice-Hall, Inc.

Sample Exercise 11. 4 Calculating ΔH for Temperature and Phase Changes Solution (continued) The total enthalpy change is the sum of the changes of the individual steps: Check: The components of the total energy change are reasonable in comparison with the lengths of the horizontal segments of the lines in Figure 11. 19. Notice that the largest component is the heat of vaporization. Practice Exercise What is the enthalpy change during the process in which 100. 0 g of water at 50. 0 °C is cooled to ice at – 30. 0 °C ? (Use the specific heats and enthalpies for phase changes given in Sample Exercise 11. 4. ) Answer: – 20. 9 k. J – 33. 4 k. J – 6. 09 k. J = – 60. 4 k. J Intermolecular Forces © 2009, Prentice-Hall, Inc.

Vapor Pressure • At any temperature some molecules in a liquid have enough energy to escape. • As the temperature rises, the fraction of molecules that have enough energy to escape increases. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Vapor Pressure As more molecules escape the liquid, the pressure they exert increases. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Vapor Pressure The liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Vapor Pressure • The boiling point of a liquid is the temperature at which it’s vapor pressure equals atmospheric pressure. • The normal boiling point is the temperature at which its vapor pressure is 760 torr. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Phase Diagrams Phase diagrams display the state of a substance at various pressures and temperatures and the places where equilibria exist between phases. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Phase Diagrams • The circled line is the liquid-vapor interface. • It starts at the triple point (T), the point at which all three states are in equilibrium. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Phase Diagrams It ends at the critical point (C); above this critical temperature and critical pressure the liquid and vapor are indistinguishable from each other. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Phase Diagrams Each point along this line is the boiling point of the substance at that pressure. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Phase Diagrams • The circled line in the diagram below is the interface between liquid and solid. • The melting point at each pressure can be found along this line. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Phase Diagrams • Below the triple point the substance cannot exist in the liquid state. • Along the circled line the solid and gas phases are in equilibrium; the sublimation point at each pressure is along this line. Intermolecular Forces © 2009, Prentice-Hall, Inc.

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. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Phase Diagram of Water • The slope of the solidliquid line 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. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Phase Diagram of Carbon Dioxide Carbon dioxide cannot exist in the liquid state at pressures below 5. 11 atm; CO 2 sublimes at normal pressures. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Phase Diagram of Carbon Dioxide The low critical temperature and critical pressure for CO 2 make supercritical CO 2 a good solvent for extracting nonpolar substances (like caffeine) Intermolecular Forces © 2009, Prentice-Hall, Inc.

Sample Exercise 11. 6 Interpreting a Phase diagram Referring to Figure 11. 28, describe any changes in the phases present when H 2 O is (a) kept at 0 °C while the pressure is increased from that at point 1 to that at point 5 (vertical line), (b) kept at 1. 00 atm while the temperature is increased from that at point 6 to that at point 9 (horizontal line). Solution Analyze: We are asked to use the phase diagram provided to deduce what phase changes might occur when specific pressure and temperature changes are brought about. Plan: Trace the path indicated on the phase diagram, and note what phases and phase changes occur. Solve: (a) At point 1, H 2 O exists totally as a vapor. At point 2 a solid–vapor equilibrium exists. Above that pressure, at point 3, all the H 2 O is converted to a solid. At point 4 some of the solid melts and equilibrium between solid and liquid is achieved. At still higher pressures all the H 2 O melts, so only the liquid phase is present at point 5. (b) At point 6 the H 2 O exists entirely as a solid. When the temperature reaches point 4, the solid begins to melt and equilibrium exists between the solid and liquid phases. At an even higher temperature, point 7, the solid has been converted entirely to a liquid. A liquid–vapor equilibrium exists at point 8. Upon further heating to point 9, the H 2 O is converted entirely to the vapor phase. Check: The indicated phases and phase changes are consistent with our knowledge of the properties of water. Practice Exercise (Using Figure 11. 27(b), describe what happens when the following changes are made in a CO 2 sample: (a) Pressure increases from 1 atm to 60 atm at a constant temperature of – 60 °C. (b) Temperature increases from – 60 °C to – 20 °C at a constant pressure of 60 atm. Answer: (a) CO 2(g) → CO 2(s); (b) CO 2(s) → CO 2(l) Intermolecular Forces © 2009, Prentice-Hall, Inc.

Solids • We can think of solids as falling into two groups: – crystalline, in which particles are in highly ordered arrangement. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Solids • We can think of solids as falling into two groups: – amorphous, in which there is no particular order in the arrangement of particles. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Attractions in Ionic Crystals In ionic crystals, ions pack themselves so as to maximize the attractions and minimize repulsions between the ions. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Crystalline Solids Because of the ordered in a crystal, we can focus on the repeating pattern of arrangement called the unit cell. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Crystalline Solids There are several types of basic arrangements in crystals, like the ones depicted above. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Crystalline Solids We can determine the empirical formula of an ionic solid by determining how many ions of each element fall within the unit cell. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Ionic Solids • What are the empirical formulas for these compounds? – (a) Green: chlorine; Gray: cesium – (b) Yellow: sulfur; Gray: zinc – (c) Gray: calcium; Blue: fluorine� (a) (b) Cs. Cl Zn. S (c) Ca. F 2 Intermolecular Forces © 2009, Prentice-Hall, Inc.

Types of Bonding in Crystalline Solids Intermolecular Forces © 2009, Prentice-Hall, Inc.

Covalent-Network and Molecular Solids • Diamonds are an example of a covalentnetwork solid, in which atoms are covalently bonded to each other. – They tend to be hard and have high melting points. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Covalent-Network and Molecular Solids • Graphite is an example of a molecular solid, in which atoms are held together with van der Waals forces. – They tend to be softer and have lower melting points. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Metallic Solids • Metals are not covalently bonded, but the attractions between atoms are too strong to be van der Waals forces. • In metals valence electrons are delocalized throughout the solid. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Sample Integrative Exercise Putting Concepts Together The substance CS 2 has a melting point of – 110. 8 °C and a boiling point of 46. 3 °C. Its density at 20 °C is 1. 26 g/cm 3. It is highly flammable. (a) What is the name of this compound? (b) List the intermolecular forces that CS 2 molecules would have with each other. (c) Predict what type of crystalline solid CS 2(s) would form. (d) Write a balanced equation for the combustion of this compound in air. (You will have to decide on the most likely oxidation products. ) (e) The critical temperature and pressure for CS 2 are 552 K and 78 atm, respectively. Compare these values with those for CO 2 (Table 11. 5), and discuss the possible origins of the differences. (f) Would you expect the density of CS 2 at 40 °C to be greater or less than at 20 °C? What accounts for the difference? Solution (a) The compound is named carbon disulfide, in analogy with the naming of other binary molecular compounds such as carbon dioxide. (Section 2. 8) (b) Only London dispersion forces affect CS 2; it does not have a dipole moment, based upon its molecular shape, and obviously cannot undergo hydrogen bonding. (c) Because CS 2(s) consists of individual CS 2 molecules, it will be a molecular solid. (d) The most likely products of the combustion will be CO 2 and SO 2. (Sections 3. 2 and 7. 8) Under some conditions SO 3 might be formed, but this would be the less likely outcome. Thus, we have the following equation for combustion: CS 2(l) + 3 O 2(g) → CO 2(g) + 2 SO 2(g) (e) The critical temperature and pressure of CS 2 (552 K and 78 atm) are both higher than those given for CO 2 in Table 11. 5 (304 K and 73 atm). The difference in critical temperatures is especially notable. The higher values for CS 2 arise from the greater London dispersion attractions between the CS 2 molecules compared with CO 2. These greater attractions are due to the larger size of the sulfur compared to oxygen and therefore its greater polarizability. Intermolecular Forces © 2009, Prentice-Hall, Inc.

Sample Integrative Exercise Putting Concepts Together Solution (continued) (f) The density would be lower at the higher temperature. Density decreases with increasing temperature because the molecules possess higher kinetic energies. Their more energetic movements result in larger average distances between molecules, which translate into lower densities. Intermolecular Forces © 2009, Prentice-Hall, Inc.