Ch13_IMForces - WHY? • Why is water usually a liquid and...

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