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Chem 1 AA3 - Chapter 12 - Liquids, Solids, Intermolecular Forces.docx

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Jeff Landry

Chapter 12: Liquids, Solids, & Intermolecular Forces 12.1 Intermolecular Forces In pure susbtances: (van der waal forces) - London dispersion forces eg. Iodine (non-polar, no permanent dipoles) - Dipole-dipole forces: eg. Acetone (permanent dipoles) - Hydrogen-bonding forces: eg. Water In solutions we also see: - Ion-diple forces eg. Hydrated ions - Ionic interactions eg. Hydrated ions London Dispersion Forces - Also called induced dipole- induced – dipole forces - Spontaneous, temporary fluctuations in electron density that propagates throughout sample; creates a dipole that causes an attraction between two neighbouring molecules - Number of electrons available increases polarizability - Instantaneous dipoles propagate throughout the sample - Force strength depend son polarizability (tendency or charge separation to occur) - Force strength increase with: o Atomic number and size of the molecule (due to polarizability) Molecule Melting Point Cl2 172 K gas at room temp I2(much larger) 387 K solid at room temp o Linear vs branched structures (b/c of increased intermolecular contact area Pentane, zig-zag “straight” chain of 5 12, boiling point = 36.1C Neopentane is more spherical shaped Long chain form allows them to stack close and efficiently together, provides a lot of surface area for them to connect Spherical nature of neopentane doesn’t give a lot of surface area for it to connec tith other neopentane molecules Dipole-Dipole Forces - Occur in polar molecules (molecules with permanent dipoles) o Polar molecules have bond dipoles and asymmetric shape (eg. Acetone is polar, while carbon dioxide is not polar) - Being polar increases b.p. over dispersion forces alone o Eg. N2(77K), NO (121K), O (20K)  O2 has more electrons than N2, NO is polar Hydrogen Bonding - Between an h atom in a polar bond (ie. H-N, H-O, or H-F) and another electronegative atom (ie. N, O, F) - One H-bond Is small, but many are mighty - Need a H atom engaged in a really polar bond to N, O, F and it’s delta pos, and needs to connect to lone pair in another N, O, F - Baesi-pairing in DNA: H-N ----------O (has a lone pair) - Base pairing in ammonia NH3 --- NH3 Effects of Hydrogen Bonding - H2O, HF, NH3 all have hydrogen bonding so their BPs are all much higher than CH4 which doesn’t have H bonds - X-axis – increase in molecular mass displays increase in London dispersion therefore BP increases left to right - H2O vs HF: structure of H2O – 2 polar H atoms, 2 lone pairs to connect with neighbours; it`s balanced. F has 3 lone pairs on F but 1 H to accept; imbalanced, same with NH3, imbalance. More H bonds possible for H2O so higher BP - Cl still has reasonable electroneg bumping it`s BP up a bit so it`s not linear with other lines - Black, Blue, green lines- dipole-dipole bonds shifts everything up a bit over red line which are all symmetrical and lack dipole bonds - Why is group 17 lower than 16 : dipoles in 17 are stronger… electronegativity, shape.. Ionic Interactions - Ionic interactions are very favourable in solids, but are weaker in solution, and can even be unfavourable in water, where solvation of the individual ions by water can be more favourable than the ionic interactions - Solid NaCl, melting point = 801 C - Aqueous NaCl solution : Na + and Cl- separately surrounded by 6 water molecules in hydration circle? o Pos dipole of water attracts to Cl -, neg dipole of water attracts to Na+ - Solutions of polyanions and polycations - Take substrate dip it in polyanion solution and multiple ion-ion connections stick polyanion on, rines it off, dip it in polycation, can do 2000 layers - Layer-by-layer polyelectrolyte deposition - Analogous polymer shells may one day be used to protect transplanted donor cells from the hosts’ immune syste: encapsulated isltes of Langerhans for Type I diiabetics; they can produce and release insulin but are protected for immune rejection Relative Strength of IMFs: A Summary - All molecules have dispersion (London) forces - The range of strength for each force type overlaps, but approximately: Dispersion < Dipole < H-bonding < Ionic/Covalent Interaction Interaction Energy (kJ/mol) Dispersion 0.05-40 Dipole 5-25 Hydrogen Bonding 10-40 per bond Ionic 400-4000 CC- Covalent bond 350 per bond 12.2 Impact of IMF Strength in Liquids Physical property Effect of increasing IMF Boiling point (b.p.) increase Vapour pressure decrease Melting point (m.p.) increase Viscosity increase Vapour pressure: tendency
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