1. John C. Kotz State University of New York, College at Oneonta John C. Kotz Paul M. Treichel John Townsend http://academic.cengage.com/kotz Chapter 12 Intermolecular Forces and Liquids

2. 2 © 2009 Brooks/Cole - Cengage Important – Read Before Using Slides in Class Instructor: This PowerPoint presentation contains photos and figures from the text, as well as selected animations and videos. For animations and videos to run properly, we recommend that you run this PowerPoint presentation from the PowerLecture disc inserted in your computer. Also, for the mathematical symbols to display properly, you must install the supplied font called “Symb_chm,” supplied as a cross-platform TrueType font in the “Font_for_Lectures” folder in the "Media" folder on this disc. If you prefer to customize the presentation or run it without the PowerLecture disc inserted, the animations and videos will only run properly if you also copy the associated animation and video files for each chapter onto your computer. Follow these steps: 1.Go to the disc drive directory containing the PowerLecture disc, and then to the “Media” folder, and then to the “PowerPoint_Lectures” folder. 2.In the “PowerPoint_Lectures” folder, copy the entire chapter folder to your computer. Chapter folders are named “chapter1”, “chapter2”, etc. Each chapter folder contains the PowerPoint Lecture file as well as the animation and video files. For assistance with installing the fonts or copying the animations and video files, please visit our Technical Support at http://academic.cengage.com/support or call (800) 423-0563. Thank you. http://academic.cengage.com/support

3. 3 © 2009 Brooks/Cole - Cengage WHY? Why is water usually a liquid and not a gas? Why does liquid water boil at such a high temperature for such a small molecule? Why does ice float on water? Why do snowflakes have 6 sides? Why is I 2 a solid whereas Cl 2 is a gas? Why are NaCl crystals little cubes?

4. 4 © 2009 Brooks/Cole - Cengage Intermolecular Forces and Liquids Chap. 12

5. 5 © 2009 Brooks/Cole - Cengage Inter- molecular Forces Have studied INTRA molecular forces—the forces holding atoms together to form molecules. Now turn to forces between molecules — INTER molecular forces. Forces between molecules, between ions, or between molecules and ions.

6. 6 © 2009 Brooks/Cole - Cengage Ion-Ion Forces for comparison of magnitude Na + —Cl - in salt These are the strongest forces. Lead to solids with high melting temperatures. NaCl, mp = 800 o C MgO, mp = 2800 o C

7. 7 © 2009 Brooks/Cole - Cengage Covalent Bonding Forces for comparison of magnitude C–H, 413 kJ/mol C=C, 610 kJ/mol C–C, 346 kJ/mol CN, 887 kJ/mol

8. 8 © 2009 Brooks/Cole - Cengage Attraction Between Ions and Permanent Dipoles Water is highly polar and can interact with positive ions to give hydrated ions in water.

9. 9 © 2009 Brooks/Cole - Cengage Attraction Between Ions and Permanent Dipoles Water is highly polar and can interact with positive ions to give hydrated ions in water. PLAY MOVIE

10. 10 © 2009 Brooks/Cole - Cengage Attraction Between Ions and Permanent Dipoles Many metal ions are hydrated. This is the reason metal salts dissolve in water.

11. 11 © 2009 Brooks/Cole - Cengage Attraction between ions and dipole depends on ion charge and ion-dipole distance. Measured by ∆H for M n+ + H 2 O f [M(H 2 O) x ] n+ Attraction Between Ions and Permanent Dipoles

12. 12 © 2009 Brooks/Cole - Cengage Dipole-Dipole Forces Dipole-dipole forces bind molecules having permanent dipoles to one another. PLAY MOVIE

13. 13 © 2009 Brooks/Cole - Cengage Dipole-Dipole Forces Influence of dipole-dipole forces is seen in the boiling points of simple molecules. CompdMol. Wt.Boil Point N 2 28-196 o C CO28-192 o C Br 2 16059 o C ICl16297 o C

14. 14 © 2009 Brooks/Cole - Cengage Hydrogen Bonding A special form of dipole-dipole attraction, which enhances dipole-dipole attractions. H-bonding is strongest when X and Y are N, O, or F

15. 15 © 2009 Brooks/Cole - Cengage H-Bonding Between Methanol and Water H-bondH-bond ---- ++++ ----

16. 16 © 2009 Brooks/Cole - Cengage H-Bonding Between Two Methanol Molecules H-bondH-bond ---- ++++ ----

17. 17 © 2009 Brooks/Cole - Cengage H-Bonding Between Ammonia and Water H-bondH-bond ---- ++++ ---- This H-bond leads to the formation of NH 4 + and OH -

18. 18 © 2009 Brooks/Cole - Cengage Hydrogen Bonding in H 2 O H-bonding is especially strong in water because the O—H bond is very polarthe O—H bond is very polar there are 2 lone pairs on the O atomthere are 2 lone pairs on the O atom Accounts for many of water’s unique properties.

19. 19 © 2009 Brooks/Cole - Cengage Hydrogen Bonding in H 2 O Ice has open lattice-like structure. Ice density is < liquid. And so solid floats on water. Snow flake: www.snowcrystals.com www.snowcrystals.com

20. 20 © 2009 Brooks/Cole - Cengage Hydrogen Bonding in H 2 O Ice has open lattice-like structure. Ice density is < liquid and so solid floats on water. One of the VERY few substances where solid is LESS DENSE than the liquid. PLAY MOVIE

21. 21 © 2009 Brooks/Cole - Cengage A consequence of hydrogen bonding

22. 22 © 2009 Brooks/Cole - Cengage Hydrogen Bonding in H 2 O H bonds lead to abnormally high specific heat capacity of water (4.184 J/gK) This is the reason water is used to put out fires, it is the reason lakes/oceans control climate, and is the reason thunderstorms release huge energy.

23. 23 © 2009 Brooks/Cole - Cengage Hydrogen Bonding H bonds leads to abnormally high boiling point of water. See Screen 13.7 PLAY MOVIE

24. 24 © 2009 Brooks/Cole - Cengage Boiling Points of Simple Hydrogen- Containing Compounds See Active Figure 12.8

25. 25 © 2009 Brooks/Cole - Cengage Methane Hydrate

26. 26 © 2009 Brooks/Cole - Cengage Methane Clathrate

27. 27 © 2009 Brooks/Cole - Cengage Hydrogen Bonding in Biology H-bonding is especially strong in biological systems — such as DNA. DNA — helical chains of phosphate groups and sugar molecules. Chains are helical because of tetrahedral geometry of P, C, and O. Chains bind to one another by specific hydrogen bonding between pairs of Lewis bases. —adenine with thymine —adenine with thymine —guanine with cytosine —guanine with cytosine

28. 28 © 2009 Brooks/Cole - Cengage Portion of a DNA chain Double helix of DNA

29. 29 © 2009 Brooks/Cole - Cengage Base-Pairing through H-Bonds

30. 30 © 2009 Brooks/Cole - Cengage Double Helix of DNA

31. 31 © 2009 Brooks/Cole - Cengage Discovering the Double Helix James Watson and Francis Crick, 1953 Rosalind Franklin, 1920- 1958 Maurice Wilkins, 1916 - 2004

32. 32 © 2009 Brooks/Cole - Cengage Hydrogen Bonding in Biology Hydrogen bonding and base pairing in DNA. See ChemistryNow, Chapter 12 PLAY MOVIE

33. 33 © 2009 Brooks/Cole - Cengage FORCES INVOLVING INDUCED DIPOLES How can non-polar molecules such as O 2 and I 2 dissolve in water? The water dipole INDUCES a dipole in the O 2 electric cloud. Dipole-induced dipole PLAY MOVIE

34. 34 © 2009 Brooks/Cole - Cengage FORCES INVOLVING INDUCED DIPOLES Solubility increases with mass the gas

35. 35 © 2009 Brooks/Cole - Cengage FORCES INVOLVING INDUCED DIPOLES Process of inducing a dipole is polarizationProcess of inducing a dipole is polarization Degree to which electron cloud of an atom or molecule can be distorted in its polarizability.Degree to which electron cloud of an atom or molecule can be distorted in its polarizability.

36. 36 © 2009 Brooks/Cole - Cengage IM FORCES — INDUCED DIPOLES Consider I 2 dissolving in ethanol, CH 3 CH 2 OH. O H -  +  I-I R -  +  O H +  -  R The alcohol temporarily creates or INDUCES a dipole in I 2.

37. 37 © 2009 Brooks/Cole - Cengage FORCES INVOLVING INDUCED DIPOLES Formation of a dipole in two nonpolar I 2 molecules. Induced dipole- induced dipole PLAY MOVIE

38. 38 © 2009 Brooks/Cole - Cengage FORCES INVOLVING INDUCED DIPOLES The induced forces between I 2 molecules are very weak, so solid I 2 sublimes (goes from a solid to gaseous molecules). PLAY MOVIE

39. 39 © 2009 Brooks/Cole - Cengage FORCES INVOLVING INDUCED DIPOLES The magnitude of the induced dipole depends on the tendency to be distorted. Higher molec. weight f larger induced dipoles. MoleculeBoiling Point ( o C) MoleculeBoiling Point ( o C) CH 4 (methane) - 161.5 CH 4 (methane) - 161.5 C 2 H 6 (ethane)- 88.6 C 2 H 6 (ethane)- 88.6 C 3 H 8 (propane) - 42.1 C 3 H 8 (propane) - 42.1 C 4 H 10 (butane) - 0.5 C 4 H 10 (butane) - 0.5

40. 40 © 2009 Brooks/Cole - Cengage Boiling Points of Hydrocarbons Note linear relation between bp and molar mass. CH 4 C2H6C2H6C2H6C2H6 C3H8C3H8C3H8C3H8 C 4 H 10

41. 41 © 2009 Brooks/Cole - Cengage Summary of Intermolecular Forces Ion-dipole forcesIon-dipole forces Dipole-dipole forcesDipole-dipole forces –Special dipole-dipole force: hydrogen bonds Forces involving nonpolar molecules: induced forcesForces involving nonpolar molecules: induced forces

42. 42 © 2009 Brooks/Cole - Cengage Intermolecular Forces Summary

43. 43 © 2009 Brooks/Cole - Cengage Intermolecular Forces See Figure 12.12

44. 44 © 2009 Brooks/Cole - Cengage Liquids Section 12.4 In a liquid molecules are in constant motionmolecules are in constant motion there are appreciable intermolec. forcesthere are appreciable intermolec. forces molecules close togethermolecules close together Liquids are almost incompressibleLiquids are almost incompressible Liquids do not fill the containerLiquids do not fill the container PLAY MOVIE

45. 45 © 2009 Brooks/Cole - Cengage Liquids The two key properties we need to describe are EVAPORATION and its opposite— CONDENSATION break IM bonds make IM bonds Add energy Remove energy LIQUID VAPOR r condensation Evaporation f

46. 46 © 2009 Brooks/Cole - Cengage Liquids—Evaporation To evaporate, molecules must have sufficient energy to break IM forces. Breaking IM forces requires energy. The process of evaporation is endothermic. PLAY MOVIE

47. 47 © 2009 Brooks/Cole - Cengage Liquids— Distribution of Energies Distribution of molecular energies in a liquid. KE is propor- tional to T. Distribution of molecular energies in a liquid. KE is propor- tional to T. 0 Number of molecules Molecular energy higher Tlower T See Figure 12.13 Minimum energy req’d to break IM forces and evaporate

48. 48 © 2009 Brooks/Cole - Cengage Distribution of Energy in a Liquid See Figure 12.13

49. 49 © 2009 Brooks/Cole - Cengage Liquids At higher T a much larger number of molecules has high enough energy to break IM forces and move from liquid to vapor state. High E molecules carry away E. You cool down when sweating or after swimming.

50. 50 © 2009 Brooks/Cole - Cengage Liquids When molecules of liquid are in the vapor state, they exert a VAPOR PRESSURE EQUILIBRIUM VAPOR PRESSURE is the pressure exerted by a vapor over a liquid in a closed container when the rate of evaporation = the rate of condensation. PLAY MOVIE

51. 51 © 2009 Brooks/Cole - Cengage Measuring Equilibrium Vapor Pressure Liquid in flask evaporates and exerts pressure on manometer. See Active Figure 12.16

52. 52 © 2009 Brooks/Cole - Cengage Vapor Pressure See ChemistryNow, Chapter 12 PLAY MOVIE

53. 53 © 2009 Brooks/Cole - Cengage Equilibrium Vapor Pressure See Active Figure 12.17

54. 54 © 2009 Brooks/Cole - Cengage Liquids Equilibrium Vapor Pressure FIGURE 12.17: VP as a function of T. 1. The curves show all conditions of P and T where LIQ and VAP are in EQUILIBRIUM 2. The VP rises with T. 3. When VP = external P, the liquid boils. This means that BP’s of liquids change with altitude. This means that BP’s of liquids change with altitude.

55. 55 © 2009 Brooks/Cole - Cengage Boiling Liquids Liquid boils when its vapor pressure equals atmospheric pressure. Please open both movies at the same time. PLAY MOVIE

56. 56 © 2009 Brooks/Cole - Cengage Boiling Point at Lower Pressure When pressure is lowered, the vapor pressure can equal the external pressure at a lower temperature. PLAY MOVIE

57. 57 © 2009 Brooks/Cole - Cengage Consequences of Vapor Pressure Changes When can cools, vp of water drops. Pressure in the can is less than that of atmosphere, so can is crushed. PLAY MOVIE

58. 58 © 2009 Brooks/Cole - Cengage 4. If external P = 760 mm Hg, T of boiling is the NORMAL BOILING POINT 5. VP of a given molecule at a given T depends on IM forces. Here the VP’s are in the order C 2 H 5 H 5 C 2 H H 5 C 2 H H wateralcoholether increasing strength of IM interactions extensive H-bonds dipole- dipole O O O Liquids See Figure 12.17: VP versus T

59. 59 © 2009 Brooks/Cole - Cengage Liquids HEAT OF VAPORIZATION is the heat req’d (at constant P) to vaporize the liquid. LIQ + heat f VAP Compd.∆ vap H (kJ/mol) IM Force H 2 O40.7 (100 o C)H-bonds SO 2 26.8 (-47 o C)dipole Xe12.6 (-107 o C)induced dipole

60. 60 © 2009 Brooks/Cole - Cengage Equilibrium Vapor Pressure & the Clausius-Clapeyron Equation Clausius-Clapeyron equation — used to find ∆ vap H˚.Clausius-Clapeyron equation — used to find ∆ vap H˚. The logarithm of the vapor pressure P is proportional to ∆ vap H and to 1/T.The logarithm of the vapor pressure P is proportional to ∆ vap H and to 1/T. ln P = –(∆ vap H˚/RT) + Cln P = –(∆ vap H˚/RT) + C

61. 61 © 2009 Brooks/Cole - Cengage Liquids Molecules at surface behave differently than those in the interior. Molecules at surface experience net INWARD force of attraction. This leads to SURFACE TENSION — the energy req’d to break the surface.

62. 62 © 2009 Brooks/Cole - Cengage Surface Tension SURFACE TENSION also leads to spherical liquid droplets. PLAY MOVIE

63. 63 © 2009 Brooks/Cole - Cengage Liquids Intermolec. forces also lead to CAPILLARY action and to the existence of a concave meniscus for a water column. concave meniscus H 2 O in glass tube ADHESIVE FORCES between water and glass COHESIVE FORCES between water molecules

64. 64 © 2009 Brooks/Cole - Cengage Capillary Action Movement of water up a piece of paper depends on H-bonds between H 2 O and the OH groups of the cellulose in the paper. PLAY MOVIE