9/2/10 TYPES OF INTERMOLECULAR INTERACTIONS

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1 Tro Chpt. 11 Liquids, solids and intermolecular forces Solids, liquids and gases - A Molecular Comparison Intermolecular forces Intermolecular forces in action: surface tension, viscosity and capillary action Vaporization and vapor pressure Sublimation and fusion Quantitative aspects of phase changes Phase Diagrams Skip sections Tro 11.2 Tro 11.3 INTERMOLECULAR vs. INTRAMOLECULAR INTERACTIONS TYPES OF INTERMOLECULAR INTERACTIONS 4 mol C-H bonds x 414 kjmol -1 or 1656 kj per mol of methane! 8.9 kj C + 4H + Non-covalent, intermolecular interactions between covalent molecules (1) London (dispersion) forces (2) Dipole-dipole forces (3) Hydrogen bonding 1 joule: * the energy required to lift a small apple (102 g) one meter against Earth's gravity. * one hundredth of the energy a person can get by drinking a single 5 mm diameter droplet of beer. * The amount of energy released if a can of beer is dropped from wait height 1

2 ION ION INTERACTION EXAMPLE: E att + d Q 1 Q 2 d Opposite charges attract Na pm Q is charge Cl 181 pm ION DIPOLE INTERACTION + d δ δ+ electrostatic interaction Q µ E att d 2 Na + : : O H H neutral polar molecule with dipole moment Q charge µ dipole moment ions in solution DIPOLE DIPOLE INTERACTION δ+ δ- δ δ+ attraction E att µ 4 d 6 More polar molecules have larger attractive force EXAMPLE: H Cl δ δ+ δ δ+ repulsion net effect, averaged over time Cl H DISPERSION INTERACTION LONDON INTERACTION Nonpolar molecules will liquify, so there must be attractive intermolecular forces Electrons are always moving in molecules At some times, there is an instantaneous dipole moment When this occurs, a dipole is induced in the adjacent atom and an attractive force is the result E att 1 d 6 Highly polarizable molecules are more subject to dispersion forces LDF increases with molecular size Molecular shape is also involved LDFs occur for all molecules 2

3 DISPERSION INTERACTION Molar Mass and Boiling Point Ar Ar LDF only intermolecular force for noble gases H Cl Cl H LDF in addition to dipole-dipole SIZE Ar BP = C = 87.5 K Xe BP = C = K atomic size related to polarizability Dipole Moment and Boiling Point Molecular Shape and Boiling Point 3

4 Practice Choose the Substance in Each Pair with the Highest Boiling Point Practice Choose the Substance in Each Pair with the Highest Boiling Point a) CH 4 CH 3 CH 2 CH 2 CH 3 a) CH 2 FCH 2 F CH 3 CHF 2 b.p. = -164 C b.p. = -0.5 C b.p. = 30.7 C b.p. = C b) CH 3 CH 2 CH=CHCH 2 CH 3 cyclohexane b) or b.p. trans: 47.5 C cis: 60.3 C b.p. = 66.4 C b.p. = 80.7 C A H bonded to a very electronegative atom (O, N, F) can interact with lone pair electrons on (O, N, F) on another molecule H-bonds are directional H-bond strength related to dipole moment of bond Properties of H 2 O are related to H-bonding in liquid and solid H-bond energies usually 4-25 kj/mol H bonds weak compared to covalent bonds H bonds strong compared to intermolecular forces 4

5 Practice Choose the substance in each pair that is a liquid at room temperature (the other is a gas) a) CH 3 OH CH 3 CHF 2 b) CH 3 -O-CH 2 CH 3 CH 3 CH 2 CH 2 NH 2 Practice Choose the substance in each pair that is more soluble in water a) CH 3 OH CH 3 CHF 2 b) CH 3 CH 2 CH 2 CH 3 CH 3 NH 2 5

6 Tro 11.4 Surface tension: Molecules at the liquid surface have a higher potential energy than those in the interior. As a result, liquids tend to minimize their surface area and the surface behaves like a membrane or skin. Surface tension allows a paper click to float on water! Viscosity Viscosity: the resistance of a liquid to flow 1 poise = 1 P = 1 g/cm s often given in centipoise, cp larger intermolecular attractions = larger viscosity higher temperature = lower viscosity Capillary Action the adhesive forces pull the surface liquid up the side of the tube, while the cohesive forces pull the interior liquid with it the liquid rises up the tube until the force of gravity counteracts the capillary action forces 6

7 Tro 11.5 Equilibrium nature of phase changes Water (dyed red) and mercury in a glass test tube Dynamic Equilibrium 7

8 Vapor Pressure Vapor Pressure as a function of temperature and intermolecular forces Clausius-Clapeyron Equation 8

9 Clausius-Clapeyron Equation Boiling Point Clausius-Clapeyron plot for diethyl ether Determining vapor pressure under different conditions The vapor pressure of ethanol is 115 torr at 34.9 C. If ΔH vap 40.5 kjmol -1, calculate the temperature (in C) when the vapor pressure is 760 torr. T 2 = 350 K or 77 C 9

10 Tro 11.6 Phase Changes - Sublimation and Deposition molecules in the solid have thermal energy that allows them to vibrate surface molecules with sufficient energy may break free from the surface and become a gas this process is called sublimation the capturing of vapor molecules into a solid is called deposition the solid and vapor phases exist in dynamic equilibrium in a closed container at temperatures below the melting point therefore, molecular solids have a vapor pressure Melting = Fusion solid sublimation deposition gas Tro 11.7 Quantitative aspects of phase changes: How much energy is released when 2.5 mol of water vapor at 130 C is cooled to ice at -40? Stage 1: H 2 O (g) [130 C] H 2 O (g) [100 C] q = n x C water(g) x ΔT q = (2.5 moles) x (33.1 Jmol -1 C -1 ) x ( C) q = J Stage 2: H 2 O (g) [100 C] H 2 O (l) [100 C] q = n x (-ΔH vap ) q = 2.5 x (-Δ40.7 kjmol -1 ) q = -102 kj 10

11 Stage 3: H 2 O (l) [100 C] H 2 O (l) [0 C] q = n x C water(l) x ΔT q = (2.5 moles) x (75.4 Jmol -1 C-1 ) x (0-100 C) q = J Stage 4: H 2 O (l) [0 C] H 2 O (s) [0 C] q = n x (-ΔH fus ) q = 2.5 x (-Δ6.02 kjmol -1 ) q = kj Wicked Weird Phases - Supercritical Fluid as a liquid is heated in a sealed container, more vapor collects causing the pressure inside the container to rise and the density of the vapor to increase and the density of the liquid to decrease at some temperature, the meniscus between the liquid and vapor disappears and the states commingle to form a supercritical fluid supercritical fluid have properties of both gas and liquid states Stage 5: H 2 O (s) [0 C] H 2 O (s) [-40 C] q = n x C water(s) x ΔT q = (2.5 moles) x (37.6 Jmol -1 C-1 ) x (-40-0 C) q = J Using Hess s law, sum of q for all 5 stages = -142 kj Supercritical n-pentane Tro 11.8 Typical phase diagram for most substances For most substances, increasing pressure will solidify a liquid (i.e. the solid is more dense than the liquid). The opposite is true for water (note the negative slope for the solid-liquid line) 11

12 Information extracted from phase diagrams Starting at the triple point, what phase exists when the pressure is held constant and the temperature is increased to 0.5 C gas! Starting at the triple point, what phase exists when the temperature is held constant and the pressure is increased to to 20 mm Hg? liquid! 12