Outline for Today Wednesday, Nov. 28 • Chapter 11: Intermolecular Forces and Liquids • Intermolecular Foces • Properties of Liquids • Vapor Pressure • Phase Changes � 1
Intermolecular Forces • London Dispersion : Attraction between momentary or instantaneous dipoles in molecules. • Dipole-Dipole : Attraction between partial charges due to permanent dipoles • Hydrogen-Bonding : Attraction between partial charges on H-F , H-O, or H-N bonds. • Ion-Dipole : Attraction between ions and partial charges of dipoles
Review of London Dispersion
Dipole-Dipole • Dipole-Dipole: Attraction between partial charges due to permanent dipoles. • Example: Is there a dipole on CH 3 CN? • How do the dipoles interact?
Dipole-Dipole Interactions in Solids
Dipole Dipole Interactions in Liquids
The larger the dipole, the stronger the dipole-dipole interactions! O O N Increasing Polarity Increasing Dipole-Dipole Interactions Increasing Boiling Point
Hydrogen-Bonding is a special type of dipole interaction • Hydrogen-Bonding : Attraction between partial charges on H-F , H-O, or H-N bonds. • H-F , H-N, and H-O are especially polar bonds! • They interact strongly with small electronegative ions or atoms (like F , O, or N of another molecule). • That interaction is called hydrogen bonding.
NOT A Hydrogen Bond H H H C H H C H H H Yes, there are hydrogens bonded to stu ff … BUT there are no intermolecular hydrogen bonding interactions . There is no hydrogen bond.
Hydrogen Bonding Is Critical for Understanding the Properties of Water!
Predict the relative strengths of the dipole-dipole interactions • Order the following by increasing boiling point (based on strength of the intermolecular interactions) O O c) 2-Methylpropane a) Butane b) Propyl alcohol d) Isopropyl alcohol
Intermolecular Forces with Charged Particles • Ion-Dipole Forces : Attraction (or Repulsion) between an ion and the partial charge on the end of a molecule
Try this chart with: Ammonia, Methane, Acetic Acid, Hydrochloric Acid
Comparing IMFs for different molecules • When molecules of two substances have similar molecular weights : • Their dispersion forces are about equal. • Use dipole-dipole interactions to determine IMF strength. • Example: CH 3 COOH vs. CH 3 CH 2 CH 2 OH H O H H H H H C O H vs. H C C C O C H H H H Boiling 118 o C 97 o C Temperature:
Hydrogen H Bond #1 O H H O C H C O C H C H H O H Boiling O 118 o C Temperature: H H H C Acetic Acid can form 2 O C hydrogen bonds H Hydrogen It can be a hydrogen bond Bond #2 donor and a hydrogen bond acceptor
H H C H H H H C H H C H H H C O H C C O H H H H H H H 97 o C H C O H C C H Propanol can only form 1 Just 1 Hydrogen Bond H H hydrogen bond. Its IMFs are weaker than acetic acid’s therefore its boiling point is lower.
Comparing IMFs for different molecules • When molecules of two substances have very di ff erent molecular weights and there is no hydrogen bonding: • Dispersion forces tend to be more important in determining strength of IMF . • The higher the molecular weight the stronger the IMFs. • Example: CCl 4 vs. C 8 H 18 CCl 4 C 8 H 18 154 g/mol 257 g/m Molecular Mass 76 o C 135 o C Boiling Point
Comparing IMFs for different molecules • Because C 8 H 18 has a higher molecular mass, it has more electrons. • More electrons means it is more polarizable than the CCl 4 • Thus the London Dispersion Forces will be larger.
Properties of Liquids: Viscosity • Viscosity : A substance’s resistance to flow • Stronger intermolecular forces = More Viscous • Example: Pitch Drop Experiment
Properties of Liquids: Surface Tension • Surface Tension: Energy required to increase surface area by a given amount • The stronger the intermolecular forces, the greater the surface tension.
Properties of Liquids: Capillary Action • Cohesive Forces: intermolecular forces that bind a substance to itself • Adhesive Forces: intermolecular forces that bind a substance to a surface • Capillary Action occurs when adhesive force is greater than cohesive force.
Examples of Capillary Action • Shape of a meniscus in a glass test tube
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