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new unit 8

new unit 8. IMFAs. IMFA: i nter m olecular f orces of a ttraction. “mortar” — holds the separate pieces together. (the IMFA). “bricks” — individual atoms, ions, or molecules of a solid. IMFA: i nter m olecular f orces of a ttraction. types of IMFA. strongest. occurs between.

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new unit 8

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  1. new unit 8 IMFAs

  2. IMFA: intermolecular forces of attraction “mortar”— holds the separate pieces together (the IMFA) “bricks”— individual atoms, ions, or molecules of a solid

  3. IMFA: intermolecular forces of attraction

  4. types of IMFA strongest occurs between covalent network atoms such as C, Si, & Ge (when in an extended grid or network) ionic bond cations and anions (metals with non-metals in a salt) metallic bond metal atoms hydrogen bond ultra-polar molecules (those with H–F, H–O, or H–N bonds) dipole-dipole attraction polar molecules van der Waals forces London forces non-polar molecules weakest

  5. details about each IMFA strongest covalent network ionic bond metallic bond hydrogen bond dipole-dipole attraction London forces weakest

  6. London (or dispersion) forces • non-polar molecules (or single atoms) normally have no distinct + or – poles • how can they attract each other enough to condense or freeze? • they form temporary dipoles • electron clouds are slightly distorted by neighboring molecules • sort of like water sloshing in a shallow pan

  7. London dispersion forces in action 1. temporary polarization due to any random little disturbance δ+ δ- 2. induced polarization caused by neighboring molecule 3. induced polarization spreads 4. induced polarization reverses non-polar molecules, initially with uniform charge distribution

  8. dipole-dipole attractions • polar molecules have permanent dipoles • the molecules’ partial charges (δ+, δ-) attract the oppositely-charged parts of neighboring molecules • this produces stronger attraction than the temporary polarization of London forces • therefore polar molecules are more likely to be liquid at a temperature where similar non-polar molecules are gases

  9. dipole-dipole attractions δ- δ+

  10. hydrogen bonding (or ultra-dipole attractions) • H—F, H—O, and H—N bonds are more polar than other similar bonds • these atoms are very small, particularly H • F, O, and N are the three most electronegative elements • these bonds therefore are particularly polar • molecules containing these bonds have much higher m.p. and b.p than otherwise expected for non-polar or polar molecules of similar mass • the geological and biological systems of earth would be completely different if water molecules did not H-bond to each other

  11. hydrogen bonding (or ultra-dipole attractions) ultra-polar molecule (much higher boiling point) hydrogen bonds (between molecules, not within them) non-polar molecules (lower boiling points)

  12. hydrogen bonding (or ultra-dipole attractions) Beware!! These are not hydrogen bonds. They are normal covalent bonds between hydrogen and oxygen. O O O O H H H H H H H H These are hydrogen bonds. They are between separate molecules (not within a molecule).

  13. metallic bonding • structure • nuclei arranged in a regular grid or matrix • “sea of electrons”—delocalized valence electrons free to move throughout grid • metallic “bond” is stronger than van der Waals attractions but generally is weaker than covalent bond since there are not specific e– pairs forming bonds • resulting properties • shiny surface • conductive (electrically and thermally) • strong, malleable, and ductile • alloy = mixture of metals

  14. ionic bonding (salts) • structure: orderly 3-D array (crystal) of alternating + and – charges • made of • cations(metals from left side of periodic table) • anions (non-metals from right side of periodic table) • properties • hard but brittle (why?) • non-conductive when solid • conductive when melted or dissolved

  15. why are salts hard but brittle? 1. apply some force 2. layer breaks off and shifts 4. shifted layer shatters away from rest of crystal 3. + repels + – repels –

  16. covalent networks • strong covalent bonds hold together millions of atoms (or more) in a single strong particle • properties • very hard, very strong • very high melting temperatures • usually non-conductive (except graphite) • examples • carbon (two allotropes: diamond, graphite) • pure silicon or pure germanium • SiO2 (quartz or sand) • other synthetic combinations averaging 4 e– per atom: • SiC (silicon carbide), BN (boron nitride)

  17. m.p. = 3550°C C60 buckminsterfullerine “bucky ball” m.p. = ~1600°C

  18. summary of properties strongest strength m.p. & b.p. conductive? network extremely hard very high usually not (except graphite) ionic hard but brittle medium to high if melted or dissolved (mobile ions) strong, malleable, ductile medium to high very (delocalized e–) metallic hydrogen dipole soft and brittle low no van der Waals forces London weakest

  19. consequences of IMFAs • melting points and boiling points rise with • strength of IMFA • increasing molar mass • substances generally mix best with other substances having the same or similar IMFAs • ”like dissolves like” • non-polar mixes well with non-polar • polar mixes well with polar • (polar also mixes well with ultra-polar and ionic) • other physical properties such as strength, conductivity, etc. are related to the type of IMFA

  20. predicting melting points, boiling points • stronger IMFAs cause higher m.p. and higher b.p. • when atoms/ions/molecules are more strongly attracted to each other, temperature must be raised higher to overcome the greater attraction • more polar molecules have higher m.p. and b.p. • atoms and molecules that are heavier and/or larger generally have higher m.p. and higher b.p. • larger/heavier atoms (higher molar mass) have more e– • larger e– clouds can be distorted (polarized) more by London or dipole forces, causing greater attraction • strategy to predict m.p. and b.p. • first sort atoms/molecules into the six IMFA categories • then sort those in each category from lightest to heaviest

  21. same IMFA: sort by molar mass melt boil • ex: halogen family • all are non-polar (London force) • lowest to highest m.p. and b.p. matches lightest to heaviest I2 (257) • thus at room temperature: • F2 (g) • Cℓ2 (g) • Br2 (ℓ) • I2 (s) +184.4 +150 I2 (257) +113.7 +100 Br2 (160) +58.8 +50 Br2 (160) –7.2 0 Cℓ2 (71) –34.04 –50 –100 Cℓ2 (71) –101.5 F2 (38) –150 –182.95 –200 F2 (38) –219.62 –250 °C

  22. same mass: sort by IMFA type ethylene glycol (can form twice as many H-bonds) • ex: organic molecules • all are ~60 g/mol • different types of IMFA +198 +150 +100 1-propanol (ultra-polar = H-bonds) +97.4 acetone (more polar) +56.2 +50 methyl ethyl ether (slightly polar) +10.8 0 butane (non-polar) –0.5 • the stronger the IMFA, the higher the boiling point –50 °C

  23. isomers (and an isobar) butane and 2-methylpropane glycerol and 1-propanol n- and neo pentane 1-propanol and 2-propanol 1-propanol and methyl ethyl ketone

  24. soaps and emulsifiers some molecules are not strictly polar or non-polar, but have both characteristics within the same molecule oil polar region soap or emulsifier water non-polar region this kind of molecule can function as a bridge between molecules that otherwise would repel each other

  25. soaps and emulsifiers with a soap or emulsifier present to surround it, a drop of non-polar oil can mix into polar water

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