Chapter 10 & 11

Chapter 10 & 11 - General Chemistry I Fall 2007 Joann...

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Unformatted text preview: General Chemistry I Fall 2007 Joann S. Monko Chemistry 9th ed. Raymond Chang Molecular Polarity Molecules will be polar if a) bonds are polar AND b) the molecule is NOT "symmetric" 0 2 All above are NOT polar! Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Polar or Nonpolar? 2 2 0 3 Compare CO and H O. Which one is polar? CO is NOT polar even 2 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. -0.75 +1.5 -0.75 though the CO bonds are polar. CO is symmetrical. Polar or Nonpolar? 3 3 2 0 4 Consider AB molecules: BF , Cl CO, and NH . 3 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 5 Clicker Question Consider the molecule SO2 1. Polar bonds, polar molecule, 1.5, S = +1 2. Polar bonds, nonpolar molecule, 1.5, S = -1 3. Polar bonds, nonpolar molecule, 1.5, S = +1 4. Nonpol. bonds, nonpol. molecule, 1.5, S = -1 5. Polar bonds, nonpolar molecule, 2, S = +1 6. Polar bonds, polar molecule, 2, S = -1 7. Polar bonds, polar molecule, 2, S =+1 Molecular Polarity, BF & HBF F B F F 3 0 6 B is + atoms are - 2 F B--F bonds are polar molecule is symmetrical NOT polar B is + & F atoms are but H H B F F B--F & B--H bonds are polar. The molecule is NOT symmetrical & IS polar. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 7 Is Methane, CH , Polar? 0 4 Methane is symmetrical & is NOT polar. Is C H F Polar? 3 C--F bond is very polar. Molecule is not symmetrical & is polar. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Substituted Ethylene 0 8 C--F bonds are MUCH more polar than C --H bonds. Because both C--F bonds are on same side of molecule, molecule is POLAR. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Substituted Ethylene 0 9 C--F bonds are MUCH more polar than C--H bonds. Because both C--F bonds are on opposing ends of molecule, molecule is NOT POLAR. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 0 10 Consequences of H2O Polarity Microwave oven Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Chemistry, 8th ed. Chang Inter-molecular Forces 11 0 B4 studied INTRAmolecular forces -- the forces holding atoms together to form molecules. Now look at between molecules -- INTERmolecular forces. Forces between molecules, between ions, or between molecules and ions. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 12 0 Ion-Ion Forces for 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 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Attraction Between Ions & Permanent Dipoles water - dipole O H H + 13 0 Water is highly polar & can interact with positive ions to give hydrated ions. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Attraction Between Ions & Permanent Dipoles Many metal ions are hydrated. This is the reason metal salts dissolve in water. 14 0 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Attra c tio n Be twe e n Io ns & P e rm a ne nt Dip o le s Mg2+ - H O H + 15 0 Na + - H O H + - H O H + -1922 kJ/mol -405 kJ/mol Cs+ -263 kJ/mol Attraction between ions and dipole depends on ion charge and ion-dipole distance. Measured by H for Mn+ + H2O --> [M(H2O)x]n+ Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 16 0 Dip o le Dip o le Fo rc e s Such forces bind molecules having permanent dipoles to one another. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 17 0 Dip o le Dip o le Fo rc e s Influence of dipole-dipole forces is seen in the boiling points of simple molecules. Compd Mol. Wt. Boil Point N 28 -196 C Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Hydrogen Bonding 18 0 A special form of dipole-dipole attraction, which enhances dipole-dipole attractions. Hb o nd ing is s tro ng e s t wh e n X a nd Y a re N, O , o r F Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Me th a no l (C H eO H) & ing b e twe e n... Hyd ro g n Bo nd HO 2 3 19 0 T wo m e th a no l m o le c ule s - + - - Hydrogen bond - + Hydrogen bond Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 20 Clicker Question Arrange the following in order of increasing boiling points: RbF, CO2, CH3OH, CH3Br 1. RbF < CO2 < CH3OH < CH3Br 2. CH3OH < CO2 < CH3Br < RbF 3. CO2 < CH3Br < CH3OH < RbF 4. CH3Br < CH3OH < RbF < CO2 5. CH3Br < RbF < CO2 < CH3OH Hyd ro g e n Bo nd ing b e twe e n Am m o nia (NH ) & H O 3 2 21 0 - + - H-bond This H-bond leads to the formation of NH4+ and OHKotz &Treichel Chemistry & Chemical Reactivity 5th ed. Hydrogen Bonding in H O 2 22 0 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. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Hydrogen Bonding in H O 2 Ice has open lattice-like structure. Ice density is 23 0 < liquid. And so solid floats on water. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 24 Boiling Points of Simple Hydrocarbon Compounds H bonds leads to abnormally high boiling point of water. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Forces Involving Induced Dipoles 25 How can non-polar molecules such as O2 and I2 dissolve in water? The water dipole INDUCES a dipole in the O2 electric cloud. Dipole-induced dipole Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Forces Involving Induced Dipoles 26 Solubility increases with increased mass of the gas Kotz &Treichel Chemistry & Chemical Reactivity Forces Involving Induced Dipoles Consider I2 dissolving in alcohol, CH3CH2OH. - I-I The alcohol temporarily - O R H creates or + INDUCES a dipole in I2. I-I + - O R H + 27 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Forces Involving Induced Dipoles Formation of a dipole in two nonpolar I2 molecules. Induced dipole-induced dipole 28 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. The induced forces between I2 are very weak, -solid I2 sublimes (goes from a solid to gaseous molecules). Forces Involving Induced Dipoles 29 The magnitude of the induced dipole depends on the tendency to be distorted. Higher molec. weight ---> larger induced dipoles Molecule Boiling Point (oC) CH4 (methane) - 161.5 C2H6 (ethane) - 88.6 C3H8 (propane) - 42.1 C4H10 (butane) - 0.5 CH4 Note linear relation between bp & molar mass. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. C4H10 C3H8 C2H6 Intermolecular Forces Summary 30 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Liquids In a liquid molecules are in constant motion 31 there are appreciable intermolecular forces molecules close together Liquids are almost incompressible Liquids do not fill the container Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Liquids 32 The two key properties we need to describe are EVAPORATION and its oppositeCONDENSATION evaporation---> LIQUID Add energy break IM bonds make IM bonds Remove energy VAPOR <---condensation Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. ...
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