Dr. Lori S. Van Der Sluys Page 1 Chapter 11 Chapter 11 part 1: Intermolecular forces Read: BLB 11.1–2; 11.6 HW: BLB 11.9,15,21,23,25,27; Supplemental 11:1–4 Know: • intermolecular forces 1. electrostatic interactions 2. dispersion forces 3. dipole-dipole interactions 4. hydrogen bonds (H-bonds) What Bonus Deadlines are coming up?? ________________________________ When is Exam 2?? _________________ Where do I go for Exam 2?? __________ Which homework/reading am I finished with? __ Molecular Composition __ Organic __ Geometry, VSEPR, Polarity __ Hybrid orbitals, Bond Types, Resonance __ Intermolecular Forces Dr. Lori S. Van Der Sluys Page 2 Chapter 11 INTERMOLECULAR (IM) FORCES Noncovalent interactions • chemical bonds: intramolecular forces 150–1100 kJ/mol • intermolecular (IM) forces 1–50 kJ/mol • intermolecular interactions: forces that hold covalent molecules together in groups • general classes of IM interactions: 1. electrostatic interactions 2. dispersion forces 3. dipole-dipole interactions 4. hydrogen bonds (H-bonds)Dr. Lori S. Van Der Sluys Page 3 Chapter 11 Why doesn!t everything exist as a gas?? Kinetic Molecular Description of Liquids and Solids gas: Kinetic energy >> intermolecular forces liquid: Kinetic energy ! intermolecular forces solid: Kinetic energy << intermolecular forces Kinetic Energy " T Cooling: T #, KE # gas $ liquid $ solid Dr. Lori S. Van Der Sluys Page 4 Chapter 11 IMF = Non-covalent Interactions chemical bonds 150-1100 kJ/mol intermolecular (IM) forces 1-50 kJ/mol Relative strengths of intermolecular forces can be seen experimentally: • What kind of attraction? • What happens as intermolecular distances decrease? Strength of IM forces depends on: Q charge on ion µ dipole moment % polarizability Define Polarizability: % increases as _____________________ % increases as _____________________Dr. Lori S. Van Der Sluys Page 5 Chapter 11 TYPES OF INTERMOLECULAR FORCES Depends on Ion-Ion Q ionic bond Ion-Dipole Q, µ Ions in aqueous solutions of electrolytes Dipole-Dipole µ Ion-Induced Dipole Q, % ions in nonpolar solvents Dipole-Induced Dipole µ, % polar molecules in nonpolar solvents Dispersion % induced-dipole induced dipole Hydrogen Bonding directional must have H bonded dipole-dipole DIRECTLY to N, O, or F Dr. Lori S. Van Der Sluys Page 6 Chapter 11 ELECTROSTATIC FORCES Ion-Ion + & dQQE21! Ion-Dipole Q µ + & + Dipole-Dipole µ1 µ2 321dEµµ! 2dQEµ! + + & & & + + + &Dr. Lori S. Van Der Sluys Page 7 Chapter 11 621dE!!" LONDON DISPERSION FORCES (LDF) Induced Dipoles: + + )– )+ Na+ Ar + & + London Dispersion forces = Induced dipoles - induced dipoles forces )+ )– )+ )– Ease of electronic distortion is: What’s happening to the electrons? Is there a dipole moment? What happens to neighboring molecules? Dr. Lori S. Van Der Sluys Page 8 Chapter 11 ALL molecules have LDF Interactions Ar ||||| Ar H'Cl in addition ( to dipole-dipole Cl'H Dispersion forces are related to: 1. 2. Size is related to:Dr. Lori S. Van Der Sluys Page 9 Chapter 11 HYDROCARBONS ALKANES ALKENES ALKYNES H'C(C'H CH3'C(C'H AROMATIC HYDROCARBONS Are Alkanes polar???? What kind of IMF?? HC HHHHC CHHHHHHC CHHHHCHHHCHHHHCCHHHCH3CCCCCCCHHHHHH CCCCCCHHHHHHDr. Lori S. Van Der Sluys Page 10 Chapter 11 SHAPE Affects the amount that two molecules can interact (the more interaction, the more LDF): 5-Carbon Alkanes n-pentane BP = 36.2°C C'C'C'C'C (straight chain) neopentane BP = 9.5°C C * C'C'C * C (branched) 2-methyl butane is in between: BP = 28°C C'C'C'C * CDr. Lori S. Van Der Sluys Page 11 Chapter 11 Effect of Hydrogen “Bonding” on BP of water The effect of hydrogen bonding is also seen in the abnormally high boiling points of NH3 and HF Dr. Lori S. Van Der Sluys Page 12 Chapter 11 Hydrogen Bonding H bonded directly to a very electronegative atom (N, O, F) interacts with lone electron pairs on N, O, F of another molecule. • H bond energies are ~ 4-25kJ/mole • Strong IMF’s • Weak compared to covalent bonds, but are “directional” like covalent bonds. Hydrogen Bonding Explains: ! why ice floats on water ! double helix in DNA ! %-helix in proteins H OHH OHDr. Lori S. Van Der Sluys Page 13 Chapter 11 Importance of H bonding in DNA & RNA Polymers: high molecular wt. materials formed from many small molecules called _____________ (repeating unit). nucleotides: 4 organic bases: bind specifically in pairs DNA double helix is held together by hydrogen bonding interactions. " " Dr. Lori S. Van Der Sluys Page 14 Chapter 11 %-Helix structure for a protein is held together by hydrogen bonding interactions.Dr. Lori S. Van Der Sluys Page 15 Chapter 11 How do you know which has the strongest IM forces? • compare B.P. and M.P. higher B.P. (M.P.) means stronger IM forces • If substances are similar, follow trends. Example: inert gases Is shape different? What happens to BP as MW increases? • If trends compete, need BP or MP Eg. CH3Cl &24.2°C CH3Br 3.6°C CH3I 42.4°C µ increases size (polarizability) increases London Dispersion forces dominate here Dr. Lori S. Van Der Sluys Page 16 Chapter 11 Energies of Interaction Types Type Example +(kJ/mol) Covalent C'C 350 Electrostatic ion-ion Na+ ----Cl& 450 ion-dipole Na+ ---CF3H 33 dipole-dipole CF3H--- CF3H 2 London CF4---CF4 2 dispersion Hydrogen Bonding H2O---H2O 24Dr. Lori S. Van Der Sluys Page 17 Chapter 11 IMF Effect on BP Type Example BP(°C) Covalent C(graphite) 4827 Ion-ion NaCl 1413 H-bonding H2O 100 Dipole- CHF3 &82 Dipole London CF4 &129 Dispersion • What happens to all intermolecular interactions at the boiling point? • What happens to the BP if the interaction is strongest? Dr. Lori S. Van Der Sluys Page 18 Chapter 11 Figure 11.12 Flowchart for determining intermolecular forcesDr. Lori S. Van Der Sluys Page 19
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