H 2.2 Chemistry 1A Spring 1999 Exam #4 KEY Chapters 9 & 10 Electronegativities Li Be B C N O F 0.98 1.57 2.04 2.55 3.04 3.44 3.98 Na Mg Al Si P S Cl 0.93 1.31 1.61 1.9 2.19 2.58 3.16 K Ca Ga Ge As Se Br 0.82 1.0 1.81 2.01 2.18 2.55 2.96 Rb Sr In Sn Sb Te I Xe 0.82 0.95 1.78 1.96 2.05 2.1 2.66 2.6 Cs Ba Tl Pb Bi Po 0.79 0.89 2.33 2.02 2.0 2.2 1
For each of the following, write the word, words, or number in each blank that best completes each sentence. (2 points each) 1. An atom s highest energy s and p electrons are called valence electrons. 2. Isomers are molecules that have the same molecular formula but a different structural formula. They have the same atoms but a different order of linking the atoms. 3. Draw a reasonable Lewis structure and a geometric sketch for each of the following molecules. Determine whether each is polar or nonpolar. (6 points each) a. C 2 H 6 b. BBr 3 nonpolar (no polar bonds) c. CF 4 nonpolar (symmetrical distribution of polar bonds) d. SCl 2 nonpolar (symmetrical distribution of polar bonds) polar (asymmetrical distribution of polar bonds) 2
e. CH 3 OH polar (asymmetrical distribution of polar bonds all alcohols have polar molecules) 4. Draw a reasonable Lewis structure for a nitric acid, HNO 3. Use the skeleton below. The structure is best described in terms of resonance, so draw all of its reasonable resonance structures and the resonance hybrid that summarizes these structures. (4 points) 5. Draw two reasonable Lewis structures for phosphoric acid, H 3 PO 4. (Your structures should not contain any O-O bonds.) One of the Lewis structures should emphasize the octet rule, and the other structure should minimize formal charges. If either Lewis structure is best described with resonance, draw all of its reasonable resonance structures and the resonance hybrid. (4 points) Obeys octet rule No formal charges 3
6. Methyl cyanoacrylate is the main ingredient in Super Glue. Using its Lewis structure below, do each of the following. (20 points) On the structure below, indicate the hybridization for each atom that has an arrow pointing toward it. Describe each of the bonds with an arrow pointing at it as either a sigma (σ) or a pi (π) bond, and list the atomic orbitals that overlap to form the bond. Write the name of the electron group geometry around each atom that has an arrow pointing to it. 4
Draw the geometric sketch of the molecule, including bond angles. Write the name of the molecular group geometry around each atom that has an arrow pointing to it. There are many ways to sketch the molecule and still show the correct geometry. The following is just one example. 7. For each of the following, write the name of the type of attraction holding these particles in the solid and liquid form. Indicate the formula in each pair that represents the substance that you would expect to have the stronger attractions that hold the particles in the liquid or solid form. (1 ½ points each blank) a. Potassium phosphate, K 3 PO 4 (used to make liquid soaps) type of attraction ionic bonds or HF (additive to liquid rocket propellants) type of attraction Hydrogen bonds stronger attractions K 3 PO 4 Ionic bonds are generally stronger than hydrogen bonds, so the ionic bonds in K 3 PO 4 are expected to be stronger. b. Butanoic acid, CH 3 (CH 2 ) 2 CO 2 H (used to make perfumes and pharmaceuticals) type of attraction hydrogen bonds and London forces or stearic acid, CH 3 (CH 2 ) 16 CO 2 H (used to makes soap, pharmaceuticals, and cosmetics) type of attraction hydrogen bonds and London forces stronger attractions stearic acid The larger stearic acid molecules have stronger London forces. 5
c. Titanium, Ti (structural material in aircraft, jet engines, and surgical instruments) type of attraction metallic bonds or hydrogen cyanide, HCN (used to manufacture rodenticides and pesticides) type of attraction dipole-dipole attractions and London forces stronger attractions Ti Metallic bonds are generally stronger than dipole-dipole attractions and London forces, so the metallic bonds in Ti are expected to be stronger. d. Boron tribromide, BBr 3 (a catalyst for the formation of diborane) type of attraction London forces or boron trifluoride, BF 3 (a catalyst) type of attraction London forces stronger attractions BBr 3 Both have nonpolar molecules held together by London forces. The larger BBr 3 molecules have stronger London forces. e. hydrogen peroxide, H 2 O 2 (used in rocket fuels) type of attraction hydrogen bonds or fluorine, F 2 (used to produce fluorocarbons) type of attraction London forces stronger attractions H 2 O 2 For molecules that are about the same size, hydrogen bonds are stronger than London forces, so H 2 O 2 has the stronger attractions. 6
Answer the following in short answer form. (6 points each) 8. The Lewis structure for phosphorus pentafluoride, PF 5, is below. With reference to the assumptions of the valence bond model, write an explanation for how phosphorus atoms are able to form five equivalent covalent bonds in molecules like PF 5. Only the highest energy electrons participate in bonding. The phosphorus atom is surrounded by five electron groups (five bonds), so we predict that it is sp 3 d hybridized. To form these bonds, it is as if an electron is promoted from a 3s orbital to an empty 3d orbital, and then the 3s, the three 3p, and the one 3d orbital blend to form sp 3 d hybrid orbitals. Covalent bonds form to pair unpaired electrons, so we get five equivalent bonds. 9. At room temperature and pressure, fluorine, F 2, and chlorine, Cl 2, are gases, bromine, Br 2, is a liquid, and iodine, I 2, is a solid. In each case, the attractions that could hold the particles together are London forces. Write an explanation for why the London forces between halogen molecules get stronger as one moves down the periodic table. A larger molecule has more electrons and a greater chance of having its electron cloud distorted from its idealized shape. Thus, instantaneous dipoles are more likely to form in larger molecules. The electron clouds in larger molecules are also larger, so the average distance between the nuclei and the electrons is greater; as a result, the electrons are less tightly held and shift more easily to create a dipole. The increased strength of London forces with increased size of molecules explains a trend in the properties of halogens. Fluorine, F 2, and chlorine, Cl 2, which contain relatively small molecules, are gases at room temperature and pressure. Bromine, Br 2, with larger molecules and therefore stronger London forces between them, is a liquid. Iodine, I 2, with the largest molecules of all the halogens, is a solid. 7