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Unformatted text preview: 1/9/2008 Chem 106
Lecture 2 L t Chapter 13 Intermolecular Forces 1 From Last Time
Solids and liquids differ from gases Closer molecular packing Intermolecular forces Electronegativity and polarity Ion-dipole forces Ion+/-e Force = k(n+/-e)/d2 Demo that didn't work...
2 1 1/9/2008 Why did (or should) the can collapse? 3 IonIon-Ion Forces for comparison of magnitude
Na+--Cl i in salt l These are the strongest forces. Lead to solids with high melting temperatures. NaCl, mp = 800 NaCl, mp = 800 oC MgO, mp = 2800 MgO, mp = 2800 oC Ionic attractive forces 700-1100 kJ/mol 7004 2 1/9/2008 Covalent Bonding Forces for Comparison of Magnitude
C=C, 610 kJ/mol CC, 346 kJ/mol CH, 413 kJ/mol CN, 887 kJ/mol
5 IonIon-dipole Forces and Enthalpy of hydration - water dipole O H H + Water is highly polar and can interact h hl l d with positive ions to give hydrated ions in water.
H, Enthalpy of hydration Negative?
6 3 1/9/2008 Different ions have different H(hydration)
Attraction between ion and dipole depends on ion charge, Attraction between ion and dipole depends on i A i b i d di l d d ion charge, h iondipole distance and dipole magnitude. iondipole distance and dipole magnitude. Measured by H for M Measured by H for Mn+ + H2O > [M(H2O)x]n+ O > [M(H 8 4 1/9/2008 Which of the following have the greatest enthalpy of hydration? 9 Attraction Between Ions and Permanent Dipoles
Many metal ions are hydrated. Many metal ions are hydrated This is the reason metal salts dissolve in water. 10 5 1/9/2008 Not all intermolecular forces involve ions
Water boils at 100 C Liquid water must have strong intermolecular forces Water has no ionic charge Water has a permanent dipole 11 DipoleDipole-Dipole Forces
b d o ecu es a g pe a e t d po es to bind molecules having permanent dipoles to bind molecules having permanent dipoles to one another. 6 1/9/2008 DipoleDipole-Dipole Forces
Influence of dipoled po e o ces s see Influence of dipoledipole forces is seen in the ue ce o d po e t e boiling points of simple molecules. Compd Mol. Wt. Boil Point N2 28 196 oC CO 28 192 oC Br B2 160 59 59 oC ICl 162 97 oC DipoleDipole-Dipole Forces vaporization Liquid Vapor
heat energy absorbed by liquid Hvap = molar heat of vaporization 7 1/9/2008 DipoleDipole-Dipole Forces
Att Attraction b t ti between opposite poles it l Affect evaporation and condensation Polar compounds higher boiling point Affect solubility In a polar solvent, which would you l l h h ld expect to be more soluble, polar or nonpolar solutes?
15 Polar or Nonpolar HOHO-CH2CH2-OH Ethylene Glycol CH3CH2CH2CH2CH2CH2CH3 Heptane, Heptane, a saturated hydrocarbon 16 8 1/9/2008 Soluble in water at any ratio Insoluble in water, but soluble in nonpolar solvents.
Fig. 13-5, p.596 Molecules of Similar Size Size HOHO-CH2CH2-OH Ethylene Glycol, MW=62 Glycol Unlimited solubility in water CH3CH2CH2CH3 Butane, Butane MW=58 Solubility 6.1 mg/100mg 18 9 1/9/2008 DipoleDipole-Dipole Forces
Di l -di l interactions i DipoleDipole dipole i t ti increase b ili boiling point and Hvap. More energy is required to separate molecules with dipoles. Dipole-dipole interactions increase solubility. Dipole- p p y Nonpolar solutes typically less soluble in polar solvents. "Like dissolves like"
19 There must be more... No charge, no dipole. Why are they soluble?
20 10 1/9/2008 FORCES INVOLVING INDUCED DIPOLES
How can nonpolar molecules such as O2 and I2 dissolve How can non in water?
The water dipole INDUCES a dipole in the O2 electric cloud. DipoleDipole-induced dipole Allows weak attraction between O2 and H2O IM FORCES -- INDUCED DIPOLES
Consider I2 dissolving in ethanol, CH3CH2OH.
- I-I - O R H + The alcohol temporarily creates or INDUCES a dipole in I2. I-I + - O R H + 11 1/9/2008 FORCES INVOLVING INDUCED DIPOLES Process of inducing a g dipole is polarization Degree to which electron cloud of an electron cloud of an atom or molecule can be distorted in its polarizability. Most elements are not gases Molecules, not ions No permanent dipoles No permanent dipoles Why would these exist in condensed form? 24 12 1/9/2008 FORCES INVOLVING INDUCED DIPOLES
Formation of a dipole in two nonpolar I2 molecules.
Induced dipole-induced dipole dipoleAlso called "London Dispersion Forces" Induce Dipole/Induce Dipole Forces
The induced forces between I2 molecules are very weak, subl mes (g g so solid I2 sublimes (goes from a solid to gaseous molecules). 13 1/9/2008 FORCES INVOLVING INDUCED DIPOLES
The magnitude of the induced dipole depends on the tendency to be distorted. Higher molec. weight > larger induced dipoles. Higher molec. weight > larger induced dipoles. Molecule Boiling Point (oC) Boiling Point ( 161.5 CH4 (methane) 88.6 C2H6 (ethane) C3H8 (propane) (propane) 42.1 42 1 C4H10 (butane) 0.5 Boiling Points of Hydrocarbons H C
4 10 C3H8 C2H6 CH4 Note linear relation between bp and molar mass. 28 14 1/9/2008 IM forces thus far...
1. 2. 2 3. 4. Iondipole Dipoledipole Dipoleinduced dipole Induced dipoleinduced dipole (London dispersion) What IM forces are responsible for holding O2 in the liquid state? 29 What IM forces are responsible for holding BF3 in the liquid state? Planar Polar or Nonpolar? Nonpolar?
30 15 1/9/2008 For Friday Read chapter 13 Begin homework assignment on OWL 31 16 ...
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