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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?
Inter-molecular Forces Have studied 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 Forcesfor comparison of magnitude Na+—Cl- in salt These are the strongest forces. Lead to solids with high melting temperatures. NaCl, mp = 800 oC MgO, mp = 2800 oC
C=C, 610 kJ/mol C–C, 346 kJ/mol C–H, 413 kJ/mol CN, 887 kJ/mol Covalent Bonding Forcesfor comparison of magnitude
Attraction Between Ions and Permanent Dipoles Water is highly polar and can interact with positive ions to give hydratedions in water.
Attraction Between Ions and Permanent Dipoles Water is highly polar and can interact with positive ions to give hydratedions in water. PLAY MOVIE
Attraction Between Ions and Permanent Dipoles Many metal ions are hydrated. This is the reason metal salts dissolve in water.
Attraction Between Ions and Permanent Dipoles Attraction between ions and dipole depends on ion charge and ion-dipole distance. Measured by ∆H for Mn+ + H2O f [M(H2O)x]n+
Dipole-Dipole Forces Dipole-dipole forces bind molecules having permanent dipoles to one another. PLAY MOVIE
Dipole-Dipole Forces Influence of dipole-dipole forces is seen in the boiling points of simple molecules. Compd Mol. Wt. Boil Point N2 28 -196 oC CO 28 -192 oC Br2 160 59 oC ICl 162 97 oC
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-bond H-Bonding Between Methanol and Water - + -
H-Bonding Between Two Methanol Molecules - + - H-bond
H-Bonding Between Ammonia and Water - + - H-bond This H-bond leads to the formation of NH4+ and OH-
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 solid floats on water. Snow flake: www.snowcrystals.com
Hydrogen Bonding in H2O Ice has open lattice-like structure. Ice density is < liquid and so solid floats on water. PLAY MOVIE One of the VERY few substances where solid is LESS DENSE than the liquid.
Hydrogen Bonding in H2O H bonds lead to abnormally high specific heat capacity of water (4.184 J/g•K) 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.
Hydrogen Bonding H bonds leads to abnormally high boiling point of water. PLAY MOVIE See Screen 13.7
Boiling Points of Simple Hydrogen-Containing Compounds See Active Figure 12.8
Hydrogen Bonding in Biology H-bonding is especially strong in biological systems — such as DNA. DNA — helical chains of phosphate groups and sugar molecules. Chains are helical because of tetrahedral geometry of P, C, and O. Chains bind to one another by specific hydrogen bonding between pairs of Lewis bases. —adenine with thymine —guanine with cytosine
Double helix of DNA Portion of a DNA chain
Discovering the Double Helix Rosalind Franklin, 1920-1958 Maurice Wilkins, 1916 - 2004 James Watson and Francis Crick, 1953
Hydrogen Bonding in Biology Hydrogen bonding and base pairing in DNA. PLAY MOVIE See ChemistryNow, Chapter 12
FORCES INVOLVING INDUCED DIPOLES How can non-polar molecules such as O2 and I2 dissolve in water? The water dipole INDUCES a dipole in the O2 electric cloud. PLAY MOVIE Dipole-induced dipole
FORCES INVOLVING INDUCED DIPOLES Solubility increases with mass the gas
FORCES INVOLVING INDUCED DIPOLES • Process of inducing a dipole is polarization • Degree to which electron cloud of an atom or molecule can be distorted in its polarizability.
d - I-I I-I d The alcohol temporarily creates or INDUCES a dipole in I2. + d - d O - O R H R H d + d + IM FORCES — INDUCED DIPOLES Consider I2 dissolving in ethanol, CH3CH2OH.
FORCES INVOLVING INDUCED DIPOLES Formation of a dipole in two nonpolar I2 molecules. Induced dipole-induced dipole PLAY MOVIE
FORCES INVOLVING INDUCED DIPOLES The induced forces between I2 molecules are very weak, so solid I2sublimes (goes from a solid to gaseous molecules). PLAY MOVIE
FORCES INVOLVING INDUCED DIPOLES The magnitude of the induced dipole depends on the tendency to be distorted. Higher molec. weight f larger induced dipoles. Molecule Boiling Point (oC) CH4 (methane) - 161.5 C2H6 (ethane) - 88.6 C3H8 (propane) - 42.1 C4H10 (butane) - 0.5
C4H10 C3H8 C2H6 CH4 Boiling Points of Hydrocarbons Note linear relation between bp and molar mass.
Summary of Intermolecular Forces • Ion-dipole forces • Dipole-dipole forces • Special dipole-dipole force: hydrogen bonds • Forces involving nonpolar molecules: induced forces
Intermolecular Forces See Figure 12.12
LiquidsSection 12.4 In a liquid • molecules are in constant motion • there are appreciable intermolec. forces • molecules close together • Liquids are almost incompressible • Liquids do not fill the container PLAY MOVIE
LIQUID VAPOR Liquids The two key properties we need to describe are EVAPORATION and its opposite—CONDENSATION Evaporation f Add energy break IM bonds make IM bonds Remove energy r condensation
Liquids—Evaporation To evaporate, molecules must have sufficient energy to break IM forces. Breaking IM forces requires energy. The process of evaporation is endothermic. PLAY MOVIE
higher T lower T Number of molecules 0 Molecular energy Minimum energy req’d to break IM forces and evaporate Liquids—Distribution of Energies Distribution of molecular energies in a liquid. KE is propor-tional to T. See Figure 12.13
Distribution of Energy in a Liquid See Figure 12.13
Liquids At higher T a much larger number of molecules has high enough energy to break IM forces and move from liquid to vapor state. High E molecules carry away E. You cool down when sweating or after swimming.
Liquids When molecules of liquid are in the vapor state, they exert a VAPOR PRESSURE EQUILIBRIUM VAPOR PRESSURE is the pressure exerted by a vapor over a liquid in a closed container when the rate of evaporation = the rate of condensation. PLAY MOVIE