Ch13_IMForces

Ch13_IMForces - WHY? INTERMOLECULAR FORCES Chap. 13 • Why...

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Unformatted text preview: WHY? INTERMOLECULAR FORCES Chap. 13 • 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? Ion-Ion Forces for comparison of magnitude Table 13.1: summary of forces and their relative strengths. Attraction Between Ions and Permanent Dipoles Attraction Between Ions and Permanent Dipoles •• - •• water dipole O H H+ - Lewis acid •• •• Na+—Cl- in salt These are the strongest forces. Lead to solids with high melting temperatures. NaCl, mp = 800 oC MgO, mp = 2800 oC Have studied INTRAmolecular INTRA 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. Water is highly polar and can interact with positive ions to give hydrated give hydrated ions in water. Page 1 Lewis base O water dipole H H+ Water is highly polar and can interact with positive ions to give hydrated give hydrated ions in water. InterIntermolecular molecular Forces Forces Attraction Between Ions and Permanent Dipoles Attraction Between Ions and Permanent Dipoles • Many metal ions are hydrated. This is the reason metal salts dissolve in water. Dipole-Dipole Forces Attraction between ions and dipole depends on ion charge and ion-dipole distance . ion-dipole Measured by H for Mn+ + H2O --> [M(H 2O)x]n+ -H • • •O 2+ Mg Co(H2O)62+ 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 -H • • •O H + Na+ -1922 kJ/ mol Such forces bind molecules having permanent dipoles to one another. C + O - C + O C + - O - -H • • •O H + H + Cs+ -405 kJ/mol -263 kJ/mol See Example 13.1, page 588. Hydrogen Bonding Hydrogen Bonding A special form of dipole-dipole attraction, which enhances dipole-dipole attractions. - X—H ••••• Y— + - H-Bonding Between Methanol and Water - H-bond + Hydrogen bonding in HF H-bonding is strongest when X and Y are N, O, or F Page 2 - H-Bonding Between Two Methanol Molecules H-Bonding Between Ammonia and Water - + + - - H-bond H-bond This H-bond leads to the formation of NH4+ and OH- Hydrogen Bonding in Biology 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 —adenine —guanine with cytosine —guanine Hydrogen Bonding in Biology Hydrogen Bonding in Biology AMP = Adenosine monophosphate Hydrogen bonding and base pairing in DNA. Adenine Thymine Page 3 See Screen 13.6 Hydrogen Bonding in H2O 2 Hydrogen Bonding in H2O 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. Figure 13.10 Hydrogen Bonding in H2O Hydrogen Bonding in H2O H bonds ---> abnormally high specific heat capacity of water (4.184 g/K•mol). 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. Snow flake: http://www.its.caltech.edu /~atomic/snowcrystals/sno w3x.jpg Ice has open lattice-like structure. Ice density is < liquid. And so solid floats on water. Hydrogen Bonding Hydrogen Bonding H bonds leads to abnormally high boiling point of water. Hydrogen Bonding in H2O Hydrogen Bonding in H2O Ice has open lattice-like structure. Ice density is < liquid and so solid floats on water. Methane Hydrate http://www.gsj.go.jp/dMG/hydrate/MH.burn.gif See Screen 13.7 Page 4 Methane Clathrate Methane Clathrate FORCES INVOLVING INDUCED DIPOLES FORCES INVOLVING INDUCED DIPOLES • How can non-polar molecules such as Br 2, I2, and N2 condense to form liquids and solids? • Consider I2 dissolving in alcohol, CH 3CH2OH. Water induces a dipole in nonpolar O2 molecules, and so O 2 can dissolve in water. Dipole-induced dipole I-I -O R H + http://www.gtionline.org/ref/open/organ/us/doe/ Hydrates/about/molecule.gif FORCES INVOLVING INDUCED DIPOLES The induced forces between I 2 molecules are very weak, so solid I 2 sublimes (goes from a solid to gaseous molecules). ROH dipole distorts or polarizes the I2 electron cloud The alcohol temporarily creates or INDUCES a creates INDUCES dipole in I2. I-I + -O R H + FORCES INVOLVING INDUCED DIPOLES The magnitude of the induced dipole depends on the tendency to be distorted. Higher molec. weight ---> larger induced dipoles. Molecule Boiling Point (oC) Molecule Boiling CH4 (methane) - 161.5 CH 161.5 C2H6 (ethane) - 88.6 C3H8 (propane) - 42.1 42.1 C4H10 (butane) - 0.5 0.5 Page 5 Formation of a dipole in two nonpolar I2 molecules. Induced dipoleinduced dipole Boiling Points of Hydrocarbons C4H10 C3H8 C2H6 CH4 Note linear relation between bp and molar mass. ...
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