Chapter13 - WHY ? Why is water usually a liquid and not a...

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Unformatted text preview: WHY ? Why is water usually a liquid and not a gas? Why does liquid water boil at such a high temperature for such a small molecule? Why does ice float on water? Why do snowflakes have 6 sides? Why is I2 a solid whereas Cl2 is a gas? Why are NaCl crystals little cubes? Intermolecular Forces Intramolecular forces - the forces holding atoms together to form molecules. Now turn to forces between molecules intermolecular forces. Forces between molecules, between ions, or between molecules and ions. Ion-Ion Forces Na+ Cl- in salt These are the strongest forces. Lead to solids with high melting temperatures. NaCl, mp = 800 oC MgO, mp = 2800 oC Covalent Bonding Forces C=C, 610 kJ/mol CC, 346 kJ/mol CH, 413 kJ/mol CN, 887 kJ/mol Attraction Between Ions and Permanent Dipoles Water is highly polar and can interact with positive ions to give hydrated ions in water. This is the reason metal salts dissolve in water. water -H di O H + Attraction Between Ions and Permanent Dipoles Attraction between ions and dipoles depends on ion charge and ion-dipole distance. Measured by H for Mn+ + H2O --> [M(H2O)x]n+ -1922 kJ/mol Mg Cs Na H - - - O + H + + + + 2+ -405 kJ/mol -263 kJ/mol Dipole-Dipole Forces Such forces bind molecules having permanent dipoles to one another. Dipole-Dipole Forces Influence of dipole-dipole forces is seen in the boiling points of simple molecules. Compd 2 Mol. Wt. 28 28 BP o N CO -196 C o -192 C Hydrogen Bonding A special form of dipole-dipole attraction, which enhances dipole-dipole attractions. H-bonding is strongest when X and Y are N, O, or F H-Bonding Btwn Methanol and Water H-bond - + - Hydrogen Bonding in H2O H-bonding is especially strong in water because The O--H bond is very polar There are 2 lone pairs on the O atom Accounts for many of water's unique properties. Hydrogen Bonding in H2O Ice has open lattice-like structure. Ice density is < liquid and so it floats on water. Hydrogen Bonding in H2O H bonds -> the high specific heat capacity of water This is the reason water is used to put out fires, it is the reason lakes/oceans control climate, and is the reason thunderstorms release huge energy. BP's of H-Containing Compounds H bonds leads to the high bp of water. FORCES INVOLVING INDUCED DIPOLES How can non-polar molecules such as O2 and I2 dissolve in water? Water induces a dipole in the O2 electron cloud. Solubility increases with mass the gas due to an increase in polarizability. INDUCED DIPOLES The induced forces between I2 molecules are very weak, it sublimes. - I-I - O R H + The alcohol temporarily creates or INDUCES a dipole in I2. I-I + - O R H + INDUCED DIPOLES The magnitude of the induced dipole depends on the tendency to be distorted. Higher molec. weight ---> larger induced dipoles. Molecule CH4 (methane) C2H6 (ethane) C3H8 (propane) C4H10 (butane) - 0.5 Boiling Point (oC) - 161.5 - 88.6 - 42.1 Boiling Points of Hydrocarbons C4H10 C3H8 C2H6 CH4 Linear relationship between BP and MW. Intermolecular Forces Liquids Molecules are in constant motion There are intermolecular forces Molecules close together Liquids are almost incompressible Liquids do not fill the container Liquids The two key properties we need to describe are evaporation and its opposite--condensation evaporation---> LIQUID Add energy VAPOR break IM bonds make IM bonds Remove energy <---condensation Vapor Pressure - pressure exerted by a vapor when: rate of evaporation = the rate of condensation Liquids Equilibrium Vapor Pressure VP as a function of T. 1. The curves show all conditions of P and T where liquid and vapor are in equilibrium. 2. The VP rises with T. 3. When VP = external P, the liquid boils. This means that BP's of liquids change with altitude. 4. If external P = 760 mm Hg, T of boiling is the normal boiling point 5. VP of a given molecule at a given T depends on IM forces. Here the VP's are in the order Liquids Heat of vaporization is the heat required (at constant P) to vaporize the liquid. LIQ + heat ---> VAP Compd. H2O SO2 Xe Hvap (kJ/mol) 40.7 (100 oC) 26.8 (-47 oC) 12.6 (-107 oC) IM Force H-bonds dipole induced dipole Equilibrium Vapor Pressure & the Clausius-Clapeyron Equation Clausius-Clapeyron equation -- used to find Hvap ln Pvap is proportional to Hvap, 1/T ln P = (Hvap/RT) + C Diethyl ether has a vapor pressure of 760 mm Hg at 34.6 C and a vapor pressure of 534 mm Hg at 25.0 C. What is the enthalpy of vaporization of diethyl ether? You don't need to convert P or T if they are the same units. P2 DH o vap 1 1 ln = - P1 R T2 T1 W 28.1 kJ/mol 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. Liquids Intermolecular forces also lead to capillary action and to the existence of a concave meniscus for a water column. adhesive forces - between water and glass cohesive forces - between water and itself Types of Solids TYPE Ionic EXAMPLE NaCl, CaF2 ZnS Metallic Molecular Ice Na, Fe I2 Metallic Dipole Induced Extended Covalent FORCE Ion-ion Network Diamond Graphite Network Solids A comparison of diamond (pure carbon) with silicon. Properties of Solids Molecules, atoms or ions locked into a crystal lattice Particles are close together Strong intermolecular forces Highly ordered, rigid ZnS, zinc sulfide Crystal Lattices Regular 3-D arrangements of equivalent lattice points in space. Lattice points define unit cells smallest repeating internal unit that has the symmetry characteristic of the solid. Cubic Unit Cells There are 7 basic crystal systems, this is cubic. All sides equal length 90 degree angles Atom Packing in Unit Cells Assume atoms are hard spheres and that crystals are built by packing of these spheres as efficiently as possible. Phase Diagrams Transitions Between Phases Lines connect all conditions of T and P where EQUILIBRIUM exists between the phases on either side of the line. (At equilibrium particles move from liquid to gas as fast as they move from gas to liquid, for example.) Phases Diagrams Important Points for H2O Normal boil point Normal freeze point Triple point Pcritical . T(C) P(mmHg) 100 760 0 760 0.0098 Note that line goes straight up 4.58 LIQUID Pressure Tcritical GAS High Temperature As P and T increase, you finally reach the critical T and P Solid-Liquid Equilibria In any system, if you increase P the density will go up. Therefore as P goes up, equilibrium favors phase with the larger density or smaller volume/gram. ICE favored at low P LIQUID H 2O favored at high P Solid-Liquid Equilibria ICE favored at low P LIQUID H 2O favored at high P Raising the pressure at constant T causes water to melt. The NEGATIVE SLOPE of the S/L line is unique to H2O. Almost everything else has positive slope. Solid-Liquid Equilibria The behavior of water under pressure is an example of - Le Chatelier's Principle At solid/liquid equilibrium, raising P squeezes the solid. It responds by going to phase with greater density, i.e., the liquid phase. P Solid H 2O 760 mmHg 0 C T Liquid H 2O Normal freezing point Solid-Vapor Equilibria The process by which a solid goes directly to a gas is called -- sublimation WHY ? Why is water usually a liquid and not a gas? Why does liquid water boil at such a high temperature for such a small molecule? Why does ice float on water? Why do snowflakes have 6 sides? Why is I2 a solid whereas Cl2 is a gas? Why are NaCl crystals little cubes? ...
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This note was uploaded on 04/07/2008 for the course CHEM 120 taught by Professor Tucker during the Spring '08 term at Siena College (Loudonville).

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