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Chapter 9 Chemical Bonding I: PART 2

Chapter 9 Chemical Bonding I: PART 2. Ionic Bonding Model vs. Reality. Lewis theory: attractions btwn ions are strong ionic compounds should have high MPs & BPs (breaking down crystal should require lots of E ) stronger attraction (larger lattice E), higher the MP.

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Chapter 9 Chemical Bonding I: PART 2

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  1. Chapter 9Chemical Bonding I:PART 2

  2. Ionic Bonding Model vs. Reality • Lewis theory: • attractions btwn ions are strong • ionic compounds should have high MPs & BPs (breaking down crystal should require lots of E) • stronger attraction (larger lattice E), higher the MP Tro: Chemistry: A Molecular Approach, 2/e

  3. Ionic Bonding Model vs. Reality • Reality: • ionic compounds have high MPs & BPs • MP generally > 300 °C • all ionic compounds solids at room temp Tro: Chemistry: A Molecular Approach, 2/e

  4. Melting an ionic solid Properties of Ionic Compounds • Hard and brittle crystalline solids • all are solids at room temperature • Melting points generally > 300 C • The liquid state conducts electricity • the solid state does not conduct electricity • Many are soluble in water • the solution conducts electricity well Tro: Chemistry: A Molecular Approach, 2/e

  5. Which ionic compound below has the highest MP? • KBr (734 ºC) • CaCl2 (772 ºC) • MgF2(1261 ºC) • KBr • CaCl2 • MgF2 Tro: Chemistry: A Molecular Approach, 2/e

  6. Ionic BondingModel vs. Reality • Lewis theory: • positions of ions in crystal lattice are critical to stability of structure • predicts moving ions out of position should be difficult. • ionic solids should be hard Tro: Chemistry: A Molecular Approach, 2/e

  7. Streak test: • rub two materials together - see which “streaks” or cuts the other • harder one cuts or does not streak • Ionic solids are relatively hard compared to most molecular solids (except diamond) Tro: Chemistry: A Molecular Approach, 2/e

  8. Ionic solids are relatively hard compared to most molecular solids (except diamond) • Directly related to how tightly the ions • are held together electrostatically 8 Tro: Chemistry: A Molecular Approach, 2/e

  9. - - - - + + + + + - - - - - + + + + - - - - + + + + + - - - - - + + + + Ionic Bonding Model vs. Reality • Lewis theory • if ions are displaced from position in crystal lattice, repulsive forces occur • crystal becomes unstable & breaks apart. • ionic solids will be brittle. Tro: Chemistry: A Molecular Approach, 2/e

  10. - - - - - - - - + + + + + + + + + + - - - - - - - - - - + + + + + + + + - - - - + + + + + - - - - - + + + + Ionic Bonding Model vs. Reality Ionic solids arebrittle. When struck they shatter. Tro: Chemistry: A Molecular Approach, 2/e

  11. Ionic Bonding Model vs. Reality • To conduct electricity: charged particles must be able to flow. • ionic solid:ions are locked in position – can’t move around. • ionic solids should not conduct electricity Ionic solids donot conduct electricity Tro: Chemistry: A Molecular Approach, 2/e

  12. Ionic Bonding Model vs. Reality Lewis theory liquid state or dissolved in water, ions will move around  should conduct electricity ionic compounds conduct electricity in liquid state or dissolved in water Tro: Chemistry: A Molecular Approach, 2/e

  13. in NaCl(s), the ions are stuck in position and not allowed to move to the charged rods in NaCl(aq), the ions are separated and allowed to move to the charged rods Conductivity of NaCl Tro: Chemistry: A Molecular Approach, 2/e

  14. Lewis Theory of Covalent Bonding • another way atoms can achieve 8 valence e-’s is to share valence e-’s with other atoms • Shared e-’s count toward each atom’s octet • Sharing of valence electrons is called covalent bonding Tro: Chemistry: A Molecular Approach, 2/e

  15. Covalent Bonding:Bonding and Lone Pair Electrons • e-’s shared by atoms = bonding pairs (BP’s) • e-’s NOT shared by atoms but belong to one atom = lone pairs (LP’s) • akanon-bonding pairs Tro: Chemistry: A Molecular Approach, 2/e

  16. . . . . . . . . . . . . . . . . . . O S O Lone pairs Bonding pairs Covalent Bonding:Bonding and Lone Pair Electrons Tro: Chemistry: A Molecular Approach, 2/e

  17. F F •• •• •• •• • • F F •• • • • • H H O •• •• •• •• •• •• •• •• •• F F •• H •• H O •• •• •• Single Covalent Bonds • When 2 atoms share one pair of e-’s = single covalent bond • 2 electrons • One atom may use > one single bond  octet • to different atoms • H only duet Tro: Chemistry: A Molecular Approach, 2/e

  18. •• •• • • • • O O •• •• •• •• O •• •• •• •• O Double Covalent Bond • When two atoms share two pairs of e-’s = a double covalent bond • 4 electrons Tro: Chemistry: A Molecular Approach, 2/e

  19. •• •• • • • • N N • • N N •• •• •• •• •• Triple Covalent Bond • When two atoms share three pairs of e-’s = a triple covalent bond • 6 electrons Tro: Chemistry: A Molecular Approach, 2/e

  20. Covalent Bonding Model vs. Reality • Predicted by Lewis theory: • H & Halogens all diatomic molecular elements • Oxygen forms either two singlebonds or • one double bondin molecular compounds • (some stable O compounds have one single bond & another that has a triple bond, but it still has an octet) Tro: Chemistry: A Molecular Approach, 2/e

  21. Predictions of Molecular Formulas by Lewis Theory H is more stable when it is singly bonded to another atom H2 + + HCl Tro: Chemistry: A Molecular Approach, 2/e

  22. Oxygen is more stable when it is singly bonded to two other atoms + + H2O or doubly bonded to one other atom O2 + Tro: Chemistry: A Molecular Approach, 2/e

  23. Lewis theory of covalent bonding: • attractions btwn atoms are directional • shared e-’s most stable • btwn bonding atoms • covalently bonded compounds will be found • as individual molecules • Reality check: • Compounds of non-metals are made of individual molecule units Tro: Chemistry: A Molecular Approach, 2/e

  24. Lewis theory: • MP & BP of molecular compounds should be • relatively low • involves breaking attractions btwn molecules, • but not bonds btwn atoms • covalent bonds are strong, • but attractions btwn molecules • are generally weak 24 Tro: Chemistry: A Molecular Approach, 2/e

  25. Reality check: • Molecular compounds have low MPs & BPs • MP generally < 300 °C • molecular compounds are found in • all 3 states at room temp Tro: Chemistry: A Molecular Approach, 2/e

  26. Intermolecular Attractions vs. Bonding Tro: Chemistry: A Molecular Approach, 2/e

  27. Lewis theory: • Hardness & brittleness of molecular • compounds should vary depending on • strength of IM attractive forces • Type & strength of IM attractions varies • based on many factors • Some molecular solids are brittle and hard, but many are soft and waxy Tro: Chemistry: A Molecular Approach, 2/e

  28. Lewis theory: • neither molecular solids nor liquids should conduct electricity (no charged particles) • Molecular compounds • do not conduct electricity • in solid or liquid state • Molecular acids doconduct electricity • when dissolved in H2O, • but not in solid or liquid state, • (are ionized by the water) Tro: Chemistry: A Molecular Approach, 2/e

  29. Lewis theory: the more e-’s two atoms share, the stronger the bond should be Bond strengthis measured by how much E must be addedinto bond to break it in half. Tro: Chemistry: A Molecular Approach, 2/e

  30. In general: • triple bonds are stronger than double bonds, double bonds are stronger than single bonds. • Lewis theorypredicts • double bonds are 2x as strong • as single bonds, • but they are < 2x as strong 31 Tro: Chemistry: A Molecular Approach, 2/e

  31. Bond length determined by measuring dist btwn nuclei of bonded atoms In general: triple bonds are shorter than double bonds, double bonds are shorter than single bonds Tro: Chemistry: A Molecular Approach, 2/e

  32. Polar Covalent Bonding • Covalent bonding btwnunlike atoms results in unequal sharing of e-’s: • one atom pulls e-’s in bond closer to its side • one end of bond has larger e- density than the other = polar covalent bond Tro: Chemistry: A Molecular Approach, 2/e

  33. Polar Covalent Bonding • Bond polarity: • end with larger e- density • gets a partial neg charge • end that is e- deficient • gets a partial positive charge 34 Tro: Chemistry: A Molecular Approach, 2/e

  34. HF • • H F EN 2.1 EN 4.0 d+ d- Tro: Chemistry: A Molecular Approach, 2/e

  35. Electronegativity Ability of an atom to attract bonding e-’s to itself = electronegativity Increases across period (left to right) Decreases down group (top to bottom) Tro: Chemistry: A Molecular Approach, 2/e

  36. Fluorine = most electroneg element • Francium = least electroneg element • Noble gas atoms  not assigned values • Opposite of atomic size trend 37 Tro: Chemistry: A Molecular Approach, 2/e

  37. Electronegativity Scale Tro: Chemistry: A Molecular Approach, 2/e

  38. Larger the diff in electronegativity, • the more polar the bond • Neg end toward more electroneg atom 39 Tro: Chemistry: A Molecular Approach, 2/e

  39. Percent Ionic Character 4% 51% 0 0.4 2.0 4.0 Electronegativity Difference Electronegativity Diff & Bond Type If diff in electroneg btwn bonded atoms is 0, bond is pure covalent  equal sharing If diff is 0.1 to 0.4, bond is nonpolar covalent If diff is 0.5 to 1.9, bond is polar covalent If diff is > or equal to 2.0, bond is ionic “100%” Tro: Chemistry: A Molecular Approach, 2/e

  40. ENCl = 3.0 3.0 − 3.0 = 0 Pure Covalent ENCl = 3.0 ENH = 2.1 3.0 – 2.1 = 0.9 Polar Covalent ENCl = 3.0 ENNa = 0.9 3.0 – 0.9 = 2.1 Ionic Bond Polarity Tro: Chemistry: A Molecular Approach, 2/e

  41. Water – a Polar Molecule stream of water attracted to a charged glass rod stream of hexane not attracted to a charged glass rod Tro: Chemistry: A Molecular Approach, 2/e 42

  42. Tro: Chemistry: A Molecular Approach, 2/e

  43. Bond Dipole Moments • Dipole moment, m, is a measure of • bond polarity • a dipole is a material with • a (+) and (−) end Tro: Chemistry: A Molecular Approach, 2/e

  44. Dipole moment, m is • directly a size of partial charges • and • directlyadistbtwn them: • = (q)(r) • not Coulomb’s Law! • measured in Debyes, D 45 Tro: Chemistry: A Molecular Approach, 2/e

  45. Dipole moment, m • = (q)(r) • Generally: • more e-’s two atoms share • & the larger the atoms are • the larger the dipole moment 46 Tro: Chemistry: A Molecular Approach, 2/e

  46. Dipole Moments Tro: Chemistry: A Molecular Approach, 2/e

  47. Percent Ionic Character • = % of a bond’s measured dipole moment compared to what it would be if e-’s were completely transferred • % ionic character indicates degree to which the electron is transferred Tro: Chemistry: A Molecular Approach, 2/e

  48. Determine whether an N―O bond is ionic, covalent, or polar covalent • Determine electroneg of each element N = 3.0; O = 3.5 • Subtract electroneg’s, • large minus small: (3.5) − (3.0) = 0.5 Tro: Chemistry: A Molecular Approach, 2/e

  49. If diff is > or = 2.0, then bond is ionic; • otherwise it’s covalent • If the diff is 0.5 to 1.9, • then bond is polar covalent; • otherwise it’s covalent • diff (0.5) is 0.5 to 1.9 = Tro: Chemistry: A Molecular Approach, 2/e

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