1. Lecture 27 © slg CHM 151 TOPICS: 1. Intermolecular Attractions 2. Solubilities and Boiling Points 3. Finals Week Overview

6. Group Work What volume of O 2, collected at 22.0 o C and 728 mm Hg would be produced by the decomposition of 8.15 g KClO 3, M = 122.5 g/mol? 2 KClO 3 (s) 2 KCl (s) + 3 O 2(g) 1. Calculate moles of O 2 from equation 2. Calculate V of O 2 from ideal gas law

8. New Material, Not Included, Test 5 But included, ACS Test....

9. Attractive or Intermolecular Forces, an Overview: Ion-ion (greatest, full charges, solid metal salts) Ion- dipole (very strong: hydrated ions) dipole-dipole (polar molecules) “induced dipole” - “induced dipole” (close packed non polar molecules) (small but increases with surface area)

10. ION- ION ION - DIPOLE DIPOLE - DIPOLE (H-Bonding, greatest) DIPOLE - INDUCED DIPOLE INDUCED DIPOLE - INDUCED DIPOLE, (* tiniest of attractions but becomes very significant when molecules are very large!) Strength of Attraction

11. H 2 O MOLECULE: STRONG DIPOLE X, O: 3.5 X, H: 2.1 X = 1.4

12. ION - DIPOLE ATTRACTIONS:Salts in water

13. Methyl Chloride: Dipole DIPOLE-DIPOLE ATTRACTIONS

14. Random Liquid DIPOLE - DIPOLE ATTRACTIONS

15. Hydrogen Bonding: Especially Strong Dipole-Dipole Attraction When H (X = 2.1) is bonded to F, O, N (X = 4.0, 3.5, 3.0), the bond is polarized strongly towards the more electronegative atom. Recall that the H atom consists of one proton (p + ) in the nucleus and one electron (e - ) outside the nucleus. Uneven sharing of that lone electron in a polarized covalent bond leave H as an exposed proton with no surrounding electronic cloud.

18. H’s lone electron Exposed positive charge

19. When hydrogen exists in this polarized bonding situation (or as H + ) it is often referred to as what it is: a “proton”. More importantly, when this occurs, the H atom forms a secondary bond to a pair of unshared electrons on another N, O or F as illustrated on the next slides. This secondary bond is described as “hydrogen bonding.” Biologically, this “hydrogen bond” is of greatest consequence: DNA and protein molecules are bound together by these fragile but essential attractions.

21. Hydrogen bonds

23. Water, H 2 O, exhibits the highest degree of hydrogen bonding of all molecules, having two hydrogens and two unshared pairs per molecule. When solid (ice), all the water molecules are locked into a rigid structure in which all the molecules are “tetrahedrally” joined to each other. In liquid water the attachment is more mobile: the particles can flow past each other, exchanging one attraction for another (higher energy state!). Escape to the gas (vapor) state requires especially high energy, and the boiling point for “hydrogen bonded” molecules is quite unusually high for the size of the molecule.

24. Special Reading Assignment To really appreciate the impact of hydrogen bonding on the properties of water we experience, be sure to read the insert in Kotz, p. 594 entitled: A CLOSER LOOK: The Unusual Properties of Water: A Consequence of Hydrogen Bonding

25. BP, Various Molecules H BONDING NON POLAR

26. Finally, the weakest set of attractions, “induced dipole- induced dipole”: When any molecule gets close enough to another molecule (liquid, solid state) so that the molecules are “touching”, induced polarity sets in. The electron clouds surrounding any set of bonds in the molecule will be repulsed by similar clouds on another molecule. The clouds will shift away from each other (“polarize”), and some positive attraction from the nuclei of one atom can attract electron clouds on a neighboring atom.

27. Induced dipole-dipole: surface attractions Exhibited by all molecules in liquid or solid state!

28. Effect of Intermolecular Attractions on BP’s A liquid will boil at a temperature at which all its particles have sufficient energy to escape into the gas state. Specifically, a liquid will boil when its “vapor pressure” is equal to the pressure on its surface, usually atmospheric. “Vapor Pressure” is the pressure exerted by molecules escaping from the surface of any liquid. “Normal Boiling Point” (H 2 0, 100 o C) is reached officially when the vapor pressure of the liquid reaches 1 atm.

30. a) mass of the particles involved (the bigger the more gravitational attraction) b) surface area (the more surface area, the more “induced dipole-induced dipole effect”) c) intermolecular attractions Boiling Points are a composite property, resulting from: By comparing particles of same general mass and size, one can deduce degree of intermolecular attraction. The four following molecules have about same M and surface area, but note the range in BP’s.

33. A word on attractions and solubilities: “Like dissolves like”: Hydrocarbon, Organic Solvent (acetone, ethers) soluble: slightly polar substances non polar substances fats, oils, creams, many paints and dyes, etc Water soluble: Many ionic solids (especially Na + NH 4 + salts) hydrogen bonded substances (alcohols, sugars) small polar substances (acetone)

34. Unit Five Test: will cover: a) shapes, bond and molecular polarity b) gas laws (just like homework) provided: PT’s, R values, all needed electronegativity values will not include chapter 13

35. “That’s All Folks!!!” Good Luck in your exams, future studies, and in finding a great professional career... if you need advising about other chemistry classes, or letters of recommendation, or want to chat: feel free to stop by my office....