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Unformatted text preview: 121 Liquids and Solids 13 13-1 Dispersion forces are caused by the polarization of the electron cloud of an atom or molecule by the close approach of another atom, molecule, or ion. The weak mutual attraction of these temporary induced dipoles accounts for dispersion forces. These attractions are important only over extremely short distances. Polarizability (ease of distortion of the electron cloud) increases with increasing sizes of molecules and with increasing numbers of electrons. Therefore, dispersion forces are stronger for molecules that are larger or have more electrons. 13-3 Dispersion forces are the only important intermolecular forces of attraction operating between the nonpolar molecules of (a) GeH 4 (tetrahedral) and (b) molecular MgCl 2 (linear). The other two molecules, PI 3 (trigonal pyramidal) and F 2 O (angular), are polar, so they would have intermolecular dipole-dipole forces. 13-5 (a) GeH 4 and (b) MgCl 2 . 13-7 (a) There is a bigger electronegativity difference between O and H than between S and H, so the hydrogen bonding would be stronger in H 2 O . (b) dichloromethane fluoroamine Cl H C H Cl H N F H Dichloromethane has no hydrogen atoms bonded to any very electronegative element, so it cannot form significant hydrogen bonds. Fluoroamine does have hydrogen atoms bonded to a more electronegative element (N H) and also has lone pairs (on N, F) to which hydrogen bonding can occur. Thus the hydrogen bonding would be stronger in fluoroamine . (c) acetone ethyl alcohol H C | H | H C || : O : C | H | H H H C | H | H C | H | H O .. .. H Acetone has no hydrogen atoms bonded to any very electronegative element, so it cannot hydrogen-bond. Ethyl alcohol does have hydrogen atoms bonded to a more electronegative element (OH), and also has lone pairs (on O) to which hydrogen bonding can occur. Thus the hydrogen bonding would be stronger in ethyl alcohol . 122 13-9 (a) BrF 5 is square pyramidal, Br F F F F F . The bond dipoles due to the four equatorial Br F bonds cancel in pairs, but the Br F dipole and the opposite dipole due to the lone pair do not cancel. Hence the BrF 5 molecules are polar, and exhibit strong dipole-dipole forces . (b) The Lewis structure of acetone is shown in the answer to Exercise 13-7(c). This molecule is polar due to the C O bond, so it exhibits dipole-dipole forces . It does not hydrogen-bond, as explained in Exercise 13-7(c). (c) Formaldehyde, H C || : O : H, is quite polar, so it would exhibit dipoledipole forces . 13-11 The intermolecular attractions between ethylene glycol molecules are less significant than those between glycerine molecules, but greater than between those of ethyl alcohol molecules. Glycerine has three OH groups which can participate in hydrogen bonding, while ethylene glycol has two, and ethyl alcohol has only one....
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