Additional practice problems, Exam 4 (Chapters 9, 10, 12, and maybe 13) 10/30/2006 The practice sheet is intended to be only one part of your preparations for the quiz. Expect to see problems and questions on the exam that are UNLIKE these. (In particular, some of the conceptual material not covered here may show up in multiple choice or short-answer format.) Please note my practice sheets tend be longer than the actual exams and often focus on the medium and "hard" questions. The exam is likely to be somewhat easier than the practice sheet, but you should be expecting some hard questions on the exam, too. The student who really understands how to do ALL the problems on the practice sheet and understands the concepts covered in class should be well-prepared for the exams (and also for future studies in the sciences). 1. Using the following framework as a guide, please write the Lewis structure(s) that best describe HNO 3. Please include all lone pairs. O N O H O 2. Please arrange the following bond angles in order of increasing size using the given letters (ie, A B C D E, if example A has the smallest angle and E has the largest). If two or more bond angles are equivalent, please use and equal sign (=) to illustrate that fact. A. angle Cl C Cl in CCl 4 (an AB 4 molecule) B. angle F Be F in BeF 2 (an AB 2 molecule) C. angle H O H in H 2 O (an AB 2 E 2 molecule) D. angle Cl Al Cl in AlCl 3 (an AB 3 molecule) E. angle F N F in NF 3 (an AB 3 E molecule) (smallest) (largest)
3. Which of the following is the best Lewis structure for HSeO 4 - (A, B, or C)? Please briefly explain your answer. A B C 4. In molecule A above (in question #3), how many sigma and pi bonds are there? Sigma bonds Pi bonds 5. For the following molecules, draw the best Lewis structure and indicate the formal charge on each atom. a. AsF 4 - b. XeO 3 6. Which of the following has the highest electronegativity: O, S, Se, or Te?
7a. Which of the following molecules would you expect to have the most polar bonds: BF 3 or NF 3? Explain briefly. (Here I am asking about individual bonds, not about the overall polarity of the molecule.) 7b. Which of those molecules would have a greater dipole moment (if any)? 8. If I ask whether HF is a polar molecule and whether it has a dipole moment, are those two different questions? 9. Write resonance structures for CO 3 -. 10a. You have a solution of CoCl 2, and you wish to know the concentration. Placing your solution in a cuvette with a path length of 1 cm, you find the absorbance at 510 nm to be 0.808. Given that the molar absorptivity of CoCl 2 solutions at 510 nm is 9.8 L/mol cm, what is the concentration of your solution? 10b. If I gave you a molar absorptivity at 490 nm, would you still be able to use that value with your absorbance at 510 nm? Why or why not?
11. How would you determine a molar absorptivity (hint: Beer s law plot) using a spectrophotometer and a stock solution of known concentration? Please describe the procedure you would follow. a. Experimental procedure: b. Data analysis. What would you plot? What equation would you fit your data to? c. How would that give you a molar absorptivity?
12a. Using the following information and the fact that the average C H bond energy is 414 kj/mol, estimate the standard enthalpy of formation of methane (CH 4 ). C(s) C(g) 2H 2 (g) 4H(g) ΔHº rxn = 716 kj/mol ΔHº rxn = 872.8 kj/mol Hint: remember what chemical equation describes bond energy (p 290 if you can t recall) 12b. What do we mean by average bond energy, as used in part a above? (For more practice on bond energy problems, see Chapter 9, #48, 49, 51, 80, 85, 88, 90) 13. What is a molecular orbital? What is meant by bonding orbitals and antibonding orbitals? Draw a picture if it helps your explanation.
14. ENORMOUS practice problem!! For the following molecules or ions, a) write the best Lewis dot structure, indicating formal charges if they are not zero and writing resonance structures where multiple structures are appropriate b) indicate the hybridization of the central atom c) draw the three-dimensional structure, including approximate bond angles in degrees d) write the name of the molecular shape (bent, linear, trigonal bipyramidal, etc) e) does the molecule have a dipole moment? H 2 O CO 2 SnF 2 ICl 3 SO 3 XeF 4 SF 4 CHCl 3 IF 5 SbCl 5 SF 6 HCN RnF 6 2+ IF 3 SO 2 BrF 5 PCl 4 - PCl 3 XeO 4 P.S. Don t do all of these if you don t need the practice you ll go crazy!! 15. Using Table 10.5 in your textbook as a guide (pg 334), explain why the bond order of N 2 is greater than that of N 2 +, but the bond order of O 2 is less than that of O 2 +.
16. Which would you expect to have a higher boiling point? Explain briefly why. a. HF or HCl? b. HCl or HBr? c. CH 4 or C 2 H 6? 17. Which would you expect to have a stronger interaction with H 2 O: Fe 2+ or Fe 3+? Why? 18. Which of the following molecules do you predict would be most soluble in water? Explain your reasoning briefly. CH 3 OH CH 3 CH 2 CH 2 OH CH 3 CH 2 CH 3 CH 3 CH 2 CH 2 CH 2 CH 3