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New unit 8 IMFAs. IMFA: intermolecular forces of attraction “bricks”— individual atoms, ions, or molecules of a solid “mortar”— holds the separate pieces.

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Presentation on theme: "New unit 8 IMFAs. IMFA: intermolecular forces of attraction “bricks”— individual atoms, ions, or molecules of a solid “mortar”— holds the separate pieces."— Presentation transcript:

1 new unit 8 IMFAs

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

3 IMFA: intermolecular forces of attraction

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

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

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

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+ 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) non-polar molecules (lower boiling points) ultra-polar molecule (much higher boiling point) hydrogen bonds (between molecules, not within them)

12 hydrogen bonding (or ultra-dipole attractions) HH O HH O HH O HH O Beware!! These are not hydrogen bonds. They are normal covalent bonds between hydrogen and oxygen. 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 3. + repels + – repels – 4. shifted layer shatters away from rest of crystal

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  SiO 2 (quartz or sand)  other synthetic combinations averaging 4 e – per atom: SiC (silicon carbide), BN (boron nitride)

17 m.p. = ~1600°C m.p. = 3550°C C 60 buckminsterfullerine “bucky ball”

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

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 thus at room temperature:  F 2 (g)  C ℓ 2 (g)  Br 2 ( ℓ )  I 2 (s) °C°C –250 –200 –150 –100 – ex: halogen family all are non-polar (London force) lowest to highest m.p. and b.p. matches lightest to heaviest – F 2 (38) meltboil – C ℓ 2 (71) – 7.2 Br 2 (160) I 2 (257) – F 2 (38) – C ℓ 2 (71) Br 2 (160) I 2 (257)

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

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

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

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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