lecture 3 - Which types of intermolecular forces are at...

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Unformatted text preview: Which types of intermolecular forces are at work when CH3CH2CH3 interacts with itself? 1. Ion-Dipole 2. Dipole-Dipole 3. Hydrogen Bonding 4. Dipole-Induced Dipole 5. Induced Dipole-Induced Dipole ... 1 Why are all “lower” IMFs present? ... 2 What are London Dispersion Forces? 1. The army that forces crowds of rioters to disperse. 2. The force that drives induced dipole interactions. 3. Dipole-dipole forces. 4. The heat required to vaporize (disperse) 1 liter of water into the air. ... 3 Questions to consider: Why does ice float? g What holds the DNA double helix together? What happens to the molecules of a liquid when they are heated? ... 4 Mystery of Water ... 5 Mystery of Water ... 6 Hydrogen Bonding A special form of dipole-dipole attraction, dipolewhich enhances dipole-dipole dipoleattractions. H-bonding is strongest when X and Y are N, O, or F ... 7 Which is the strongest intermolecular force present when NH3 interacts with ethanol CH3CH2OH? 1. Ion-Dipole 2. Dipole-Dipole 3. Hydrogen Bonding 4. Dipole-Induced Dipole 5. Induced Dipole-Induced Dipole ... 8 Hydrogen Bonding in H2O H-bonding is especially strong in water because the O—H bond is O— very polar there are 2 lone pairs on the O atom Accounts for many of water’s unique properties. ... 9 Hydrogen Bonding in H2O Ice has open latticelattice-like structure. Ice density is < liquid. And so solid floats on water. Snow flake: www.snowcrystals.com ... 10 What is this? ... UMass Amherst Biochemistry Teaching Initiative Discovering the Double Helix Rosalind Franklin 1920-1958 James Watson and Francis Crick, 1953 Maurice Wilkins 1916 - 2004 ... 12 Hydrogen bonding in biology: DNA ... 13 BaseBase-Pairing through H-Bonds H- ... 14 Hydrogen Bonds control proteins too! Protein data bank ... 15 Phase changes Temp Gas Liquid Solid Heat Added ... UMass Amherst Biochemistry Teaching Initiative Solids, liquids, and gases Solids have little molecular motion Liquids have some molecular motion Gases have extensive molecular motion ... 17 Liquids • Molecules are in constant motion • There are appreciable intermolecular forces • Molecules are close together • Liquids are almost Li id l t incompressible • Liquids do not fill the container ... 18 Liquids Molecules at surface behave differently than those in the interior. Molecules at surface experience net INWARD force of attraction. This leads to SURFACE TENSION — the energy required to break the surface. 19 ... Liquids: Capillary Action Intermolecular forces also lead to CAPILLARY action and to the existence of a concave meniscus for a water column. concave meniscus H2 O in glass tube ADHESIVE FORCES between water and glass COHESIVE FORCES between water molecules ... 20 Capillary Action Movement of water up a piece of paper depends on H-bonds between HH2O and the OH groups of the cellulose in the paper. ... 21 Liquids The two key properties we need to describe are EVAPORATION and its opposite, CONDENSATION evaporation---> evaporation---> LIQUID Add energy VAPOR break IM bonds make IM bonds Remove energy <---condensation ---condensation ... 22 What you know instinctively: Evaporation is endothermic ΔHovap is Condensation is exothermic ... 23 Liquids—Distribution of Energies ... 24 Liquids—Evaporation To evaporate, molecules must have sufficient energy to break IM forces. Breaking IM forces requires energy. The process of evaporation is endothermic. ... 25 Liquids HEAT OF VAPORIZATION is the heat required (at constant P) to vaporize the liquid liquid. Is related to the Intermolecular forces! Liquid + heat ---> Vapor Compound H2O SO2 Xe ∆Hvap (kJ/mol) IM Force p 40.7 (100 oC) H-bonds 26.8 (-47 oC) dipole (12.6 (-107 oC) induced dipole (... 26 Example: How much energy (heat) is required to vaporize 1L of H2O? What do you need to solve this problem? ... UMass Amherst Biochemistry Teaching Initiative Liquids: Surface Tension SURFACE TENSION also leads to spherical liquid droplets. ... 28 ...
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