Intermolecular Forces

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 Differentiate between the internal O-H covalent bond and the Hydrogen bonds between molecules. How many Hydrogen Bonds are shown in this picture?

7. Strongest Intermolecular Force Hydrogen Bonds

8. Hydrogen Bonds 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 (H2, N2, O2, F2, Cl2, Br2, I2) (footballs)

19. 2 More Easy categories of Nonpolar Molecules Hydrocarbons (CxHy) & Small symmetric molecules (CO2, CS2, CF4, CBr4, etc.)

20. What are the 7 diatomic elements? H2, N2, O2, F2, Br2, Cl2, & I2

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? • He Ne Ar Kr Xe Rn • F2 Cl2 Br2 I2 • CH4 C2H6 C3H8 C4H10 C5H12 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 50C 70C 20C

32. Evaporates easilyWeak 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. Which substance has the weakest IMF? The RED substance Pressure, atm Temperature, C

43. Relationship between IMF and physical properties?

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 … I2 and CO2 sublime Have high vapor pressure & weak IMF