Chem Chapter 6 Final Review

  1. Chapter 6: Intermolecular Forces: Section 6.1
  2. Types of Intermolecular Forces
    Intermolecular forces – attractive forces between molecules

    Weaker than any bond within a molecule (covalent, ionic)

    Arise from interaction of δ+ and δ- regions on separate molecules
  3. London (Dispersion) Forces
    The WEAKEST intermolecular force

    Attraction formed from temporary (“induced”) dipoles on molecules

    Occurs momentarily in all molecules when e- become unevenly distributed over a molecule’s surface
  4. London Forces
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  5. Important Notes on London Dispersion
    • Occur in ALL molecules
    • –Only significant w/nonpolar molecules because these are the ONLY intermolecular forces in which nonpolar molecules can participate

    • Temporary
    • –Electrons are constantly in motion. They will re-disperse around the molecule.
  6. Dipole-Dipole Interactions
    Stronger than London dispersion

    Attraction formed between permanent dipoles (polar molecules)

    Do not exist between nonpolar molecules

    •Polar molecules also exhibit London forces
  7. Dipole-Dipole Interactions
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  8. Hydrogen Bonding
    A special kind of dipole-dipole interaction

    The STRONGEST intermolecular force

    Occurs when a hydrogen attached to a highly electronegative atom (N, O, F) comes close to a lone pair of e- on N, O, or F on another molecule.
  9. Hydrogen Bonding
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  10. Hydrogen bond donor
    Hydrogen bond donor – molecule w/ a H atom covalently bonded to an O, N, or F
  11. Hydrogen bond acceptor
    Hydrogen bond acceptor – molecule w/ a lone pair of e- on an O, N, or F
  12. Intermolecular Forces
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  13. Ion-Dipole Interactions
    Technically not intermolecular because it involves ions

    Stronger than H-bonding

    Occurs when an ion comes close to an opposite partial charge on a molecule
  14. Ion-Dipole Interactions
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  15. Ion-Dipole Interactions
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  16. Intermolecular Forces
    Play a large role in solubility of substances in one another

    Affect changes of state (solid, liquid, gas)
  17. Section 6.2: Intermolecular Forces and Solubility
  18. Intermolecular Forces and Solubility
    Golden Rule: Like dissolves like

    –Polar substances dissolve polar substances

    –Nonpolar dissolve nonpolar
  19. Nonpolar Compounds
    Triglycerides – dietary oils (e.g., cooking oil)

    –Formed via condensation rxn of three fatty acids with glycerol (esterification rxn)

  20. Esterification Reaction
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  21. Esterification Reaction
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  22. Nonpolar Compounds
    In order to dissolve, molecules must interact.

    Nonpolar oil (London forces) will not interact with polar water (H-Bonding).

    Oils won’t dissolve in water.
  23. Polar Compounds
    Sucrose – a carbohydrate (table sugar)

    –Multiple hydroxyl groups make it polar (capable of dipole-dipole interactions)

    –OH groups give it ability to H-bond

    Polar sucrose dissolves in polar water.
  24. Ionic Compounds
    Able to interact with water via ion-dipole interactions

    Ionic compounds dissolve in water

    Water molecules surround ions in a process called hydration.
  25. Hydration
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  26. Soap
    Salt – an ionic compound

    • Soap – composed of fatty acid salts
    • –Fatty acids are nonpolar (carboxylic acid group), but fatty acid salts are polar (carboxylate group).
    • –Polar “head,” nonpolar “tail”

    Amphipathic – compounds with polar and nonpolar parts
  27. Soap
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  28. Soap
    • Amphipathic compounds won’t dissolve in water.
    • –Tails are hydrophobic (water-fearing)
    • –Heads are hydrophilic (water-loving)

    –Water interacts only with hydrophilic heads, forming a micelle.
  29. Micelle
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  30. Soap
    Most stains, including dirt and grease, are nonpolar.

    They are attracted to the hydrophobic tails of soap molecules.

    Stains become trapped in micelles and can be washed away with water.
  31. Section 6.3: Intermolecular Forces and Changes of State
  32. Intermolecular Forces and Changes of State
    • Heat is a form of energy.
    • –Causes molecules to move faster.

    Intermolecular forces are strongest when molecules move slowly.

    As heat increases, I.F. decrease.
  33. Heat and Intermolecular Forces
    • Solids have slow-moving molecules.
    • –Strong intermolecular forces

    Liquids are faster.

    • Gases are fastest.
    • –Weak intermolecular forces
  34. Changes of State
    • Changes of state (“phase transitions”)
    • –Solid > Liquid = melting
    • –Liquid > gas = evaporating

    Adding heat to a substance causes molecules to move faster, causing these changes.
  35. Boiling Points and Alkanes
    Boiling point – the temperature at which all molecules of a substance change from a liquid to a gas

    Heat disrupts intermolecular forces.
  36. Boiling Points and Alkanes
    Octane’s B.P. is higher than pentane’s B.P.

    Octane is larger, so molecules have more chances to interact.

    –Stronger intermolecular forces (London Dispersion)
  37. Boiling Points and Alkanes
    A molecule with larger surface area (like octane) has more surface contact with other molecules and more e- to disturb.

    In straight-chain alkanes, the more C atoms, the stronger the attraction between molecules.

    –Leads to higher boiling points
  38. Boiling Points and Alkanes
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    For alkanes with the same number of C atoms, straight-chain alkanes have higher boiling points than do branched alkanes.

    –“Spaghetti-and-meatball” interactions
  39. The Unusual Behavior of Water
    H2O has three atoms, propane has 11 (C3H8).

    –Expectation: propane’s boiling point is higher

    –Reality: Water’s B.P.: 100oC; propane’s: -42oC.
  40. The Unusual Behavior of Water
    • Why?
    • –Propane: London dispersion forces
    • –Water: Hydrogen bonding

    Stronger I.F. à more energy required to disrupt à higher boiling point
  41. Summary
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  42. Section 6.4: Fats, Oils, and Margarine – Solid to Liquid and Back Again
  43. Fat
    Fat – a lipid molecule composed of three fatty acids joined to a glycerol backbone. Solid at room temperature. (AKA, triglyceride)
  44. Oils
    Oils have the same structure, but are liquids at room temp. (AKA, triglyceride)
  45. Fats and Oils
    •Why is fat solid and oil liquid? Intermolecular forces.

    Mostly saturated hydrocarbon tails > Fats

    –Saturation creates more surface area > stronger I.F.

    –Only London forces > low melting point
  46. Fats and Oils
    Fatty acid tails of oils contain more double bonds.

    –Multiple double bonds à “polyunsaturated”

    –Double bonds create “kinks” in fatty acid tails, making interaction more difficult à weaker I.F.

    –Natural fatty acids contain only cis- double bonds
  47. Trans Fats
    Partial hydrogenation – the saturation of some double bonds, while others are left intact

    –Allows for manipulation of London forces

    Butter is saturated. Margarine is partially saturated, but still has double bonds, making margarine more spreadable.
  48. Trans Fats
    Hydrogenation is a difficult reaction to control.

    Sometimes causes the formation of trans- double bonds, instead of cis-.

    Trans- fats have been shown to have negative health effects (Heart disease, cancer).
  49. Section 6.5: Intermolecular Forces and the Cell Membrane
  50. Intermolecular Forces and the Cell Membrane
    Cell membranes are selectively permeable.

    Selective permeability – ability to allow certain substances through while denying other substances

    Cell membranes are composed of phospholipids.
  51. Phospholipids
    Phospholipids – glycerol backbone with two fatty acids and a phosphate-containing group

    Phosphate group is ionic (polar); fatty acid tails are nonpolar.

    –Overall phospholipid is amphipathic.
  52. Phospholipids
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  53. Phospholipids
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  54. Phospholipids
    Cartoon highlights strongly polar head with two nonpolar tails

    –Similar to soap, which has just one nonpolar tail

    Two tails > does NOT form a micelle
  55. Phospholipid Bilayer
    Recall: nonpolar = hydrophobic

    –Outside the cell: watery. Inside the cell: aqueous.

    –How does the hydrophobic portion escape?

    Phospholipids form a bilayer – a double layer of phospholipids
  56. Phospholipid Bilayer
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  57. Other Components of the Bilayer
    Phospholipids provide structure.

    Proteins provide function.

    –Allow molecules to move into or out of the cell.

    –Can protrude through the bilayer (integral membrane proteins) or associate with one polar surface (peripheral membrane proteins)
  58. The Fluid Mosaic Model
    Fluid Mosaic Model – the current model for how the cell membrane works

    Implies that components of the bilayer are able to move freely within the bilayer
  59. The Fluid Mosaic Model
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  60. Cholesterol
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    • Polar end: -OH
    • Nonpolar: rest of
    • molecule

    • OH protrudes into the surrounding
    • (aqueous) environment; rigid rings sit inside the bilayer, itself
  61. Cholesterol
    Determines fluidity/rigidity of the cell membrane

    • More cholesterol > more rigid bilayer
    • –Cholesterol interacts with phospholipid tails via London forces
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Chem Chapter 6 Final Review
Chem Chapter 6 Final Review from Power Points