1. Intermolecular Forces Forces Between Molecules

2. Why are intermolecular forces important? They determine the phase of a substance at room temperature. (Competition with kinetic energy.)

3. Solids Strong Intermolecular Forces

4. Gases Weak Intermolecular Forces

5. 3 Types of Intermolecular Forces Dispersion or Van der Waals Dipole-dipole Hydrogen Bonds

6. 0 1 2 3 4 How many Hydrogen Bonds are shown in this picture? Differentiate between the internal O-H covalent bond and the Hydrogen bonds between molecules.

7. Strongest Intermolecular Force Hydrogen Bonds

8. Occur between molecules!

9. 0 1 2 3 4 How many Hydrogen Bonds are shown in this picture?

10. When do Hydrogen bonds occur? Between molecules containing F, O, N bonded to a H atom. F, O, & N are all small & electronegative. So F-H, O-H, & N-H bonds are extremely polar.

11. Strongest hydrogen bonds? Between molecules containing F-H bonds

12. Weakest hydrogen bonds? Between molecules containing N-H bonds

13. What effect does H-bonding have on the properties of the substance? Hydrogen bonding leads to substantial increases in the expected boiling point.

14. Bucking the trends! Expected boiling points High boiling points attributed to hydrogen bonding!

15. Weakest Intermolecular Force Dispersion or Van der Waals forces

16. When do dispersion forces occur? Occur between all molecules. Most important between nonpolar molecules

17. Nonpolar No Poles! No separation of charge! The molecule is symmetric! Cannot tell 1 end from the other.

18. 2 Easy categories of Nonpolar Molecules Monatomic Gases (Column 18) (kickballs) & Diatomic Elements (H 2, N 2, O 2, F 2, Cl 2, Br 2, I 2 ) (footballs)

19. 2 More Easy categories of Nonpolar Molecules Hydrocarbons (C x H y ) & Small symmetric molecules (CO 2, CS 2, CF 4, CBr 4, etc.)

20. What are the 7 diatomic elements? H 2, N 2, O 2, F 2, Br 2, Cl 2, & I 2

21. What can you say about dispersion forces as a function of molecular size? The larger the molecules the stronger the dispersion forces!

22. Which molecule in each series has the strongest dispersion forces? 1.He Ne Ar Kr Xe Rn 2.F 2 Cl 2 Br 2 I 2 3.CH 4 C 2 H 6 C 3 H 8 C 4 H 10 C 5 H 12 Sometimes they ask which has the weakest dispersion forces!

23. When do dipole-dipole forces occur? Between polar molecules. Permanent separation of charge. 1 side  - (electron rich) vs. 1 side  + (electron poor)

24. Polar Has Poles! The molecule is NOT symmetric! Can tell 1 end from the other.

25. What’s with F-H, N-H or O-H covalent bonds that leads to hydrogen bonding? F, O, & N are all very small & very electronegative. F-H, O-H, & N-H bonds have a particularly large separation of charge which leads to extremely strong dipoles.

26. Vapor Gas phase of a substance that is normally a liquid at room temperature.

27. Vapor Pressure The partial pressure of a vapor above its liquid.

28. What does vapor pressure depend on? (Hint: see Table H) Vapor pressure depends on the temperature of the liquid (but not on how much, as long as a tiny bit is present, it’s okay).

29. What happens to the vapor pressure as the temperature of the liquid is increased? The vapor pressure always increases with temperature.

30. Does the vapor pressure depend on the amount of liquid present? No. A thimbleful is as good as an ocean.

31. Which has the highest v.p.? 90  C 20  C 50  C 70  C

32. Evaporates easily Weak IMF volatile

33. What are some differences between evaporation & boiling? Evaporation occurs at all temperatures. Boiling occurs at a definite temperature. Evaporation occurs at the surface. Boiling occurs throughout the liquid.

34. Heat of fusion Amount of energy required to convert 1 gram of a pure solid to the liquid phase at its melting point.

35. Heat of vaporization Amount of energy required to convert 1 gram of a pure liquid to the gas phase at its boiling point.

36. What happens to the boiling point as the intermolecular forces increase? The stronger the intermolecular forces, the higher the boiling point.

37. What happens to the melting point as the intermolecular forces increase? The stronger the intermolecular forces, the higher the melting point.

38. What happens to the heat of fusion as the intermolecular forces increase? The stronger the intermolecular forces, the higher the heat of fusion.

39. What happens to the heat of vaporization as the intermolecular forces increase? The stronger the intermolecular forces, the higher the heat of vaporization.

40. What happens to the evaporation rate as the intermolecular forces increase? The stronger the intermolecular forces, the lower the evaporation rate.

41. What happens to the vapor pressure as the intermolecular forces increase? The stronger the intermolecular forces, the lower the vapor pressure.

42. Pressure, atmTemperature,  C Which substance has the weakest IMF? The RED substance

43. Relationship between IMF and physical properties? IMFBPMPHfHf HvHv Ev.RtV.P. IMFBPMPHfHf HvHv EvRtVP

44. Boiling Point Temperature at which the vapor pressure of a liquid is equal to the external pressure.

45. Normal Boiling Point Temperature at which the vapor pressure of a liquid is equal to 1 atm or 101.3 kPa or 760 torr.

46. What happens to the boiling point when the external pressure is reduced? The boiling point is reduced.

47. What happens to surface tension and viscosity as IMF increase? They increase!

48. How can you reduce the viscosity? Raise the temperature!

49. Sublimation Solid to gas

50. Substances that sublime … Have high vapor pressure & weak IMF I 2 and CO 2 sublime

51. Deposition Gas to solid

52. Vaporization Liquid to Gas

53. Condensation Gas to liquid

54. Fusion Solid to liquid Or Melting

55. Freezing Liquid to solid Crystallization Solidification

56. Which phase changes are endothermic? Solid to liquid Liquid to Gas Solid to Gas GLSGLS

57. Which phase changes are exothermic? Gas to Liquid Liquid to Solid Gas to Solid G L S

58. Given 3 substances at room temperature: 1 solid, 1 liquid, & 1 gas, which has the strongest intermolecular forces? Solid > Liquid > Gas

59. Describe the mp & bp of gases. Low!

60. What happens to the temperature as heat is added to a substance at its melting or boiling point? Nothing, until the phase change is complete. The heat energy is going into the potential energy of the system.

61. HFHF HFHF -------- Hydrogen Bonding!

62. Ne -------- Van der Waals or dispersion forces

63. H-Cl -------- Dipole-dipole forces

64. H-N H-F -------- Hydrogen Bonding H H - - O H H - - -------- ---------

65. Time Temperature I IIIIIIVV Solid Solid & Liquid Liquid Liquid & Gas Gas K.E.  K.E.↔ P.E.↔ P.E.  Melt pt. Boil pt. Q = mC s  T Q = mC l  T Q = mC g  T Q = mH f Q = mH v

66. Equation for a pure substance in a single phase being warmed up or cooled down. Q = mC  T Q = energy in Joules m = mass in grams C = specific heat in J/g   T = temperature change

67. Equation for a substance melting or freezing at the freezing point. Q = mH f Q = energy in Joules m = mass in grams H f = heat of fusion

68. Equation for a substance boiling or condensing at the boiling point. Q = mH v Q = energy in Joules m = mass in grams H v = heat of vaporization

69. Temperature of the ice-water equilibrium. 0  C or 273K Freezing point Melting point

70. Temperature of the steam-water equilibrium. 100  C or 373K Boiling point Condensation point

71. Solid: high pressure & low temperature Gas: low pressure & high temperature Liquid: in between Triple Point: where 3 phase equilibria lines meet. 3 phases coexist at that one point

72. Go to 1 atm or 760 torr and go across until you hit the trace. At solid- liquid trace, drop down to find normal melting point. At liquid-gas trace, drop down to find normal boiling point. For this substance, the liquid phase is denser than the solid phase. The solid-liquid equilibrium line slopes downhill.

73. Line segment AB STOPS at point B, the critical point. No liquid can exist at T > T c. No gas exists above P c. Line segment AD goes forever. If it tilts uphill, the solid phase is denser than the liquid phase.

74. Some substances have more than one solid phase!

75. Crystalline solid Atoms or molecules are arranged in a repetitive, 3-D pattern.

76. Amorphous solid Atoms or molecules are randomly arranged.

77. As temperature , the “shoulder” on the high energy or high speed side of the curve becomes more pronounced. The high energy, high speed molecules are the molecules that can evaporate or escape from the surface. Maxwell Boltzmann distribution

78. The vapor pressure of a liquid  as its temperature  because … There is a bigger fraction of high speed molecules.

79. Why do substances with weaker IMF have higher vapor pressure? For substances with weaker IMF, the red line shifts to a lower energy so there are more molecules that can escape.

80. Why do substances with stronger IMF have lower vapor pressure? For substances with stronger IMF, the red line shifts to a higher energy so there are fewer molecules that can escape.