Please sign the Gustavus honor code before turning in your exam:

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1 CHE Exam #4 11/17/06 Name Please sign the Gustavus honor code before turning in your exam: On my honor, I pledge that I have not given, received, nor tolerated others use of unauthorized aid in completing this work. Signed There are 9 questions (total 100 points) and one extra credit question on 5 pages. You are also given a periodic table and some potentially useful information, including Tables 9.2, 10.1, 10.2, and 10.4 and Figure 9.4 from your textbook. Check to make sure all pages are present before beginning. Instructions: Answer the following questions, clearly indicating your final answer. Show your work: additional pages are available if you need them. Remember to include units where appropriate and use scientific notation to present numbers in simplest form. You have until the end of the class period to work on the exam. 1. (6 points) In the Analysis of Copper by Spectrophotometry lab, you determined the molar absorptivity of a solution of [Cu(NH 3 ) 4 ] 2+ and used that value to calculate the percent copper in a penny. Dissolving the copper from a g piece of a penny into a final solution volume of 50 ml results in a [Cu(NH 3 ) 4 ] 2+ concentration of 4.7x10-3 M. Determine the percent copper in the penny.

2 2. (12 points) Given the bond energies in Table 9.2 and the following thermochemical equation for the reaction of one mole of methane (CH 4 ) with chlorine gas (Cl 2 ) to give methyl chloride (CH 3 Cl) and hydrogen chloride (HCl), determine the energy of a C-Cl bond (in kj/mol). CH 4(g) + Cl 2(g) CH 3 Cl (g) + HCl (g) ΔH rxn = -101 kj/mol 3. (16 points) The phosphate ion (PO 4 3- ) plays a number of important roles in biological systems. a. Draw a Lewis structure for the phosphate ion that satisfies the octet rule on all of the atoms. b. What is the formal charge on the phosphorus atom in phosphate according to the Lewis structure? c. What is the molecular geometry of phosphate? Refer to Table 10.1 and/or 10.2 if necessary. d. Determine the hybridization state of the phosphorus atom in phosphate. Refer to Table 10.4 if necessary. 2

3 4. (Multiple choice) (6 points) The bond in which of the following pairs of atoms would be the least polar (ie. lowest percent ionic character)? A. C Cl B. C C C. C H D. O C E. N C 5. (18 points) The following questions refer to arsenic trichloride (AsCl 3 ), a toxic compound that is used in the preparation of sneeze gas, which can be used by police to control crowds. a. Draw the most plausible Lewis structure of arsenic trichloride b. What is the arrangement of electron pairs around the central atom in AsCl 3? c. What is the molecular geometry of AsCl 3? d. Use the crossed arrow convention to indicate the direction of the net dipole moment, if any, on the AsCl 3 Lewis structure in part (a). e. How many sigma (σ) bonds are formed within one AsCl 3 molecule? f. How many pi (π) bonds are formed within one AsCl3 molecule? 3

4 6. (10 points) Consider two molecules: aluminum tribromide (AlBr 3 ) and phosphorus trifluoride (PF 3 ). Which of these molecules is more polar? Support your choice with Lewis structures and a brief answer. 7. (10 points) When ethene (C 2 H 4 ) reacts with hydrogen gas (H 2 ) to form ethane (C 2 H 6 ), does the distance between the carbon atoms become shorter, longer, or remain the same? Provide a brief rationale for your answer. 8. (12 points) Use the bond energies listed in Table 9.2 to determine the enthalpy change for the combustion of one mole of acetylene (C 2 H 2 ) to form carbon dioxide (CO 2 ) and water vapor (H 2 O). C 2 H 2(g) + O 2(g) CO 2(g) + H 2 O (g) ΔH rxn =? 4

5 9. (10 points) Two potential Lewis structures of methyl isocyanate are drawn below. a. Indicate the formal charges on each of the atoms in both structures. b. Circle the Lewis structure that would be preferred based on the formal charges. Extra Credit (3 points) The science of chemistry is traditionally organized into five divisions or sub-disciplines. For example, one of these sub-disciplines is analytical chemistry. List at least three of the remaining four chemical disciplines. (No partial credit. Sorry.) 5