Name ______________________________________ Extra Practice Problems: Molecular Geometry & IMF 1. CH3CH2OH boils at 78 °C and CH3OCH3 boils at - 24 °C, although both compounds have the same composition. This difference in boiling points may be attributed to a difference in a. molecular mass b. density c. specific heat d. hydrogen bonding e. heat of combustion 2. Which of the following molecules has a dipole moment of zero? a. C6H6 (benzene) nonpolar b. NO polar c. SO2 polar d. NH3 polar e. H2S polar 3. Of the following molecules, which has the largest dipole moment? a. CO b. CO2 c. O2 d. HF largest electronegativity difference e. F2 4. X = CH3-CH2-CH2-CH2-CH3 nonpolar Y = CH3-CH2-CH2-CH2-OH polar but only one site for hydrogen bonding Z = HO-CH2-CH2-CH2-OH polar with TWO sites for hydrogen bonding Based on concepts of polarity and hydrogen bonding, which of the following sequences correctly lists the compounds above in the order of their increasing solubility in water? a. Z < Y < X b. Y < Z < X c. Y < X < Z d. X < Z < Y e. X < Y < Z 5. The compounds SF4 and CsF react to form an ionic compound according to the following equation. SF4 + CsF CsSF5 (i) Draw a complete Lewis structure for the SF5– anion in CsSF5. (ii) Identify the type of hybridization exhibited by sulfur in the SF5– anion. sp3d2 (iii) Identify the geometry of the SF5– anion that is consistent with the Lewis structure drawn in part (b)(i). square pyramid (iv) Identify the oxidation number of sulfur in the compound CsSF5. 4+ 6. Using principles of chemical bonding and/or intermolecular forces, explain each of the following. a. Xenon has a higher boiling point than neon has. Xenon has more mass which will lead to stronger dispersion forces which leads to a higher boiling point. b. Solid copper is an excellent conductor of electricity, but solid copper chloride is not. Electrons are free to move in metallic bonds and therefore can conduct electricity. In an solid ionic molecule electrons are locked in place in a lattice crystal and therefore cannot conduct electrity. c. SiO2 melts at a very high temperature, while CO2 is a gas at room temperature, even though Si and C are in the same chemical family. SiO2 is held together with network covalent bonds which is the strongest intermolecular force. CO2 only has dispersion forces holding the CO2 molecules together since it is nonpolar. Dispersion forces are significantly weaker than network covalent bonds therefore SiO2 should have much higher melting point than CO2. d. Molecules of NF3 are polar, but those of BF3 are not. BF3 is a symmetrical molecule with a geometry of trigonal planar where NF3 is asymmetrical with a geometry of trigonal pyramid. The lone pair on N makes it asymmetrical and therefore polar. 7. Answer the following questions that relate to chemical bonding (a) In the boxes provided, draw the complete Lewis structure (electron-dot diagram) for each of the three molecules represented below. CF4 PF5 SF4 (b) On the basis of the Lewis structures drawn above, answer the following questions about the particular molecule indicated. (i) What is the F-C-F bond angle in CF4? 109.5 (ii) What is the hybridization of the valence orbitals of P in PF5? sp3d (iii) What is the geometric shape formed by the atoms in SF4? See saw 8. Use principles of atomic structure, bonding and/or intermolecular forces to respond to each of the following. Your responses must include specific information about all substances referred to in each question. (a) At a pressure of 1 atm, the boiling point of NH3(l) is 240 K, whereas the boiling point of NF3(l) is 144 K. (i) Identify the intermolecular forces(s) in each substance. NH3 – dispersion, dipole-dipole, hydrogen bonding NF3 –dispersion, diole-dipole (ii) Account for the difference in the boiling points of the substances. Because NH3 can form hydrogen bonds it has a higher boiling point because it takes more energy to separate the molecules. (b) The melting point of KCl(s) is 776˚C, whereas the melting point of NaCl(s) is 801˚C. (i) Identify the type of bonding in each substance. Ionic bonds (ii) Account for the difference in the melting points of the substances. Na+ is a smaller ion than K+ there for there is a greater attraction between Na and Cl. The lattice energy is higher in NaCl which makes the melting point higher.