Question to ponder Why do some solids dissolve

Question to ponder… ¿Why do some solids dissolve in water but others do not? ¿Why are some substances gases at room temperature, but others are liquid or solid? ¿What gives metals the ability to conduct electricity, what makes non-metals brittle? The answers have to do with …

ion-dipole hydrogen bond dipole-dipole london dispersion intermolecular forces!

ion-dipole Between an ion and the partial charge on a polar molecule

hydrogen bond Between hydrogen and a more electronegative neighbor

dipole-dipole hydrogen bond Between oppositely charged regions of polar molecules

london dispersion Between nonpolar molecules when a temporary dipole occurs

ion-dipole Strongest hydrogen bond dipole-dipole london dispersion Weakest

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Testing concepts 1. Which attractions are stronger: intermolecular or intramolecular? 2. What evidence is there that nonpolar molecules attract each other? 3. Suggest some ways that the dipoles in London forces are different from the dipoles in dipole-dipole attractions. 4. A) Which would have a lower boiling point: O 2 or F 2? Explain. B) Which would have a lower boiling point: NO or O 2? Explain. 5. Which would you expect to have the higher melting point (or boiling point): C 8 H 18 or C 4 H 10? Explain. 6. What two factors causes hydrogen bonds to be so much stronger than typical dipole-dipole bonds? 7. So far we have discussed 4 kinds of intermolecular forces: What kind(s) of intermolecular forces are present in the following substances: a) NH 3, b) SF 6, c) PCl 3, d) Li. Cl (aq), e) HBr, f) CO 2

1. 2. 3. 4. 5. 6. 7. Intramolecular are stronger. The molecules gather together as liquids or solids at low temperatures. London forces • • • Are present in all compounds Can occur between atoms or molecules Are due to electron movement not to ∆ EN Are transient in nature (dipole-dipole are more permanent). London forces are weaker A) F 2 would be lower because it is smaller. Larger atoms/molecules can have their electron clouds more easily deformed and thus have stronger London attractions and higher melting/boiling points. B) O 2 because it has only London forces. NO has a small ∆ EN, giving it small dipoles. C 8 H 18 would have the higher melting/boiling point. This is a result of the many more sites available for London forces to form. 1) a large ∆EN, 2) the small sizes of atoms. a) NH 3: Hydrogen bonding (H + N), London. b) SF 6: London only (it is symmetrical). c) PCl 3: DEN=2. 9 -2. 1. Dipole-dipole, London. d) Li. Cl: DEN=2. 9 -1. 0. Ion-dipole, (London). e) HBr: DEN=2. 8 -2. 1. Dipole-dipole, London. f) CO 2: London only (it is symmetrical)