CHEMISTRY 107 Section 501 Exam #3 Version A November 15, 2017 Dr. Larry Brown

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1 NAME: (print) UIN #: CHEMISTRY 107 Section 501 Exam #3 Version A November 15, 2017 Dr. Larry Brown This is a 50-minute exam and contains 22 multiple-choice questions and 3 free response problems. Point values for the different questions are as indicated. There should be 10 numbered pages, including this one. There is also a periodic table attached to the back of the exam; you may tear the periodic table page off, and you do not need to turn that page in. Questions 1 22 should be answered on the scantron provided to you. Problems should be answered on the exam paper, with ALL of your work shown as clearly as possible. This will help us award partial credit. Answers without supporting work may not receive credit. You may use a calculator for this exam, but you may NT retrieve or use any alphanumeric information or algorithms that might be stored in your calculator s memory. Please PRINT your name and UIN number above, and SIGN the honor code statement below. Also, please put your name on every page of the exam, in case a page gets detached from the exam. Potentially Useful Information PHYSICAL CNSTANTS CNVERSIN FACTRS Avogadro s Number N A = mol 1 1 atm = 760 torr = 14.7 lb in 2 Gas Constant R = L atm mol 1 K 1 1 atm = 101,324 N m 2 = J mol 1 K 1 0 C = K = L torr mol 1 K 1 1 m = 10 9 nm = 10 6 µm Planck s Constant h = J s 1 L = 1,000 ml Speed of light c = m s 1 1 kj = 1,000 J 1 MJ = 1,000 kj "n my honor, as an Aggie, I have neither given nor received unauthorized aid on this exam." SIGNATURE: PLEASE D NT PEN YUR EXAM UNTIL TLD T D S. Exam 2017 L.S. Brown A1

2 Determine whether each of the following should be positive, negative, or zero. (2 points each) 1. w for a gas that expands from a volume of 0.5 L to 2.3 L 2. q for 275 g of copper that is heated from 5 C to 75 C 3. H for the dimerization of N2 to form N24 (Both substances are gases, with the structures shown below. ther resonance forms exist for both gases, but only one is shown here.) N + N N N 4. S for the dimerization of N2 to form N24 (see above) 5. Hf for N2(g) 6. S for N2(g) 7. H for a reaction in which two solutions at room temperature are mixed together and the resulting solution reaches a temperature of 8 C A L.S. Brown

3 NAME: Consider the following Lewis structures, and assume that they are all drawn correctly. Then use the structures to answer the questions that follow. Structure A Structure B Structure C Cl P Cl Cl F F Br F F Structure D N Structure E H C H (3 pts) 8. ne of these structures is an ion. Which one? (A) Structure A (B) Structure B (C) Structure C (D) Structure D (E) Structure E (3 pts) 9. For how many of these structures would additional resonance forms be important? (A) 1 (B) 2 (C) 3 (D) 4 (E) 5 (3 pts) 10. How many of these structures represent planar species? (Meaning all of the atoms must be coplanar.) (A) 1 (B) 2 (C) 3 (D) 4 (E) 5 (3 pts) 11. How many pi bonds would be present in structure A? (A) 1 (B) 3 (C) 6 (D) 14 (E) 15 (4 pts) 12. What is the correct shape for the species represented by structure C? (A) tetrahedral (B) trigonal pyramidal (C) see-saw (D) octahedral (E) square planar 2017 L.S. Brown A3

4 Draw Lewis structures for C2, CH3H, and C3 2, and use your structures to answer questions (CH3H contains an H group.) (4 pts) 13. What is the correct hybridization for the carbon atom in CH3H? (A) s (B) sp (C) sp 2 (D) sp 3 (E) p (4 pts) 14. What is the correct molecular geometry for C3 2? (A) tetrahedral (B) trigonal pyramidal (C) trigonal planar (D) trigonal bipyramidal (E) see-saw (4 pts) 15. What is the approximate C bond angle in C2? (A) 45 (B) 90 (C) (D) 120 (E) 180 (4 pts) 16. All 3 species contain carbon-oxygen bonds, and each one has a different C bond length. Rank them in order of increasing C bond lengths. (From shortest bond length to longest bond length.) (A) C2 < CH3H < C3 2 (B) CH3H < C2 < C3 2 (C) C2 < C3 2 < CH3H (D) CH3H < C3 2 < C2 (E) C3 2 < C2 < CH3H A L.S. Brown

5 Band diagrams for two different solid materials are shown below. NAME: (3 pts) 17. The band diagram on the right above represents what type of material? (A) metal (B) intrinsic semiconductor (C) p-type semiconductor (D) n-type semiconductor (E) superconductor (3 pts) 18. Which of the materials below might correspond to the band diagram on the right above? (A) Ge doped with Ga (B) Si doped with P (C) Si (D) Si doped with Ge (E) Rb For questions 19 & 20, choose the option that best fills in the blank(s) in the statement. (3 pts) 19. In these diagrams, the black stripes at the bottom represent and are referred to as the. (A) empty orbitals, valence band (B) occupied orbitals, conduction band (C) empty orbitals, conduction band (D) occupied orbitals, valence band (E) empty orbitals, band gap (3 pts) 20. Electrical resistance decreases as we move down the elements in Group 14 from carbon (diamond) to silicon to germanium. This means that the band gap in germanium is the band gap in diamond. (A) larger than (B) smaller than (C) the same as 2017 L.S. Brown A5

6 Use the following data as needed in problems 21 & 22. C(s, graphite) + 2(g) C2(g) H = kj 2 H2(g) + 2(g) 2 H2(g) H = kj C2H5H(!) + 3 2(g) 2 C2(g) + 3H2(g) H = kj (4 pts) 21. Find the standard molar enthalpy of formation (DHf ) for steam (H2(g)). (This part. should be easy if you remember how DHf is defined.) (A) 0 kj/mol (B) kj/mol (C) kj/mol (D) kj/mol (E) kj/mol (6 pts) 22. Calculate DH for the reaction below. 4 C(s, graphite) + 6 H2(g) + 2(g) 2 C2H5H(!) (A) 2112 kj (B) 5494 kj (C) 2346 kj (D) kj (E) kj A L.S. Brown

7 NAME: 23. Some thermodynamic data for isopropyl alcohol (C3H7H) are given below. Heat of fusion: DH fus = 5.41 kj/mol Heat of vaporization: DH vap = 43.3 kj/mol Heat capacity of liquid: CP = J/mol/K Heat capacity of vapor: CP = 108 J/mol/K Normal boiling point: Tboil = 62.4 C (8 pts) How much heat would be required to convert g of liquid isopropyl alcohol at 25.0 C into vapor at C? 2017 L.S. Brown A7

8 24. Consider the complete combustion of 1-butyne. 2 CH3CH2CCH(g) (g) 8 C2(g) + 6 H2(g) DH = 4929 kj (8 pts) (a). A small cylinder contains 8.18 L of CH3CH2CCH(g) at 0.29 atm and 21 C. How much heat would be released if all of that 1-butyne were burned according to the reaction shown above? A L.S. Brown

9 NAME: (Problem #24, continued) 2 CH3CH2CCH(g) (g) 8 C2(g) + 6 H2(g) DH = 4929 kj (8 pts) (b). Lewis structures for all of the molecules involved in the combustion of 1-butyne are shown below. Use the information in the following table to estimate the bond energy of a carbon-carbon triple bond (i.e., the missing entry in the table). H H H C C C C H C H H H H Average energy Average energy Type of bond Type of bond (kj/mol) (kj/mol) C C 345 CºC??? C 360 C= 800 = H 460 C H L.S. Brown A9

10 (10 pts) 25. Carbon tetrachloride (CCl4) is a liquid at room temperature and pressure. Use thermodynamic information from the table below to estimate the boiling point of CCl4. Hf (kj/mol) S (J/K/mol) Gf (kj/mol) CCl4(!) CCl4(g) This one requires G, so it would not be on your exam. (For Grading) Problem Score 23 (8) 24 (16) 25 (10) TTAL (34) A L.S. Brown