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CHEM 31 Introductory Chemistry EXAM #3 November 14, 2001 Name: Keye, Francis Scott SSN: Lab T.A.: INSTRUCTIONS: Read through the entire exam before you begin. Answer all of the questions. For questions involving calculations, show all of your work -- HOW you arrived at a particular answer is MORE important than the answer itself! Circle your final answer to numerical questions. The entire exam is worth a total of 150 points. Attached are a periodic table and a formula sheet jam-packed with useful stuff. Good Luck! Page 2 Possible Points 30 3 4 5 6 7 30 9 45 16 20 TOTAL: 150 Points Earned 30 30 9 45 16 20 150 1. For the Hydrogen atom: a. (10 pts) Calculate the energy change (in Joules) associated with an n=6 to n=1 electronic transition. ∆E = = = = = = RH(1/(ni)2 - 1/(nf)2) (2.1798741 x 10-18 J)(1/(6)2 – 1/(1)2) (2.1798741 x 10-18 J)(1/36 – 1) (2.1798741 x 10-18 J)(-0.9722222) -2.1193220 x 10-18 J -2.119322 x 10-18 J b. (5 pts) Is a photon emitted or absorbed as a result of this transition? EMITTED ABSORBED circle one c. (10 pts) Calculate the wavelength (in nm) of the photon having the same energy as the energy change associated with this transition. E = hc/λ → λ = hc/E λ = (6.62606876 x 10-34 J-s)(2.9979 x 108 m/s) 2.1193220 x 10-18 J = 9.37294623 x 10-8 m = 9.37294623 x 10-8 m x 109 nm m = 9.3729 x 101 nm or 93.729 nm d. (5 pts) Is the photon in the visible portion of the electromagnetic spectrum? YES NO circle one 2 2. (5 pts each) Write the ground state electron configurations for the following atoms and ions (use noble gas abbreviations for core electrons where appropriate): a. Cl: [Ne]3s23p5 b. Ti: [Ar]4s23d2 c. Ti2+: [Ar]3d2 d. Ag: [Kr]5s14d10 3. (10 pts) Give the values for the quantum numbers n, l, and ml for each orbital in the 4d subshell. n = 4 l = 2 ml = -2, -1, 0, 1, 2 3 4. (3 pts each) Circle the number next to the appropriate response for each of the following: a. The azimuthal quantum number (l) governs: 1. The energy of an orbital 2. The shape of an orbital 3. The color of an orbital 4. The spin of an electron in an orbital 5. The spatial orientation of an orbital b. Which of the following orbital diagrams shows the lowest energy electron configuration for Si? 1. [Ne] ↓↑ 3s 2. [Ne] ↓↑ 3s 3. [Ne] ↑_ 3s 4. [Ne] ↑↓↑ 3s 5. [Ne] ↑_ 3s ↓↑ __ __ 3p ↑_ ↑_ __ 3p ↑_ ↑_ ↑_ 3p ↑_ __ __ 3p ↓↑ ↑_ __ 3p c. Which ONE of the following statements is TRUE ? 1. First ionization energies are always exothermic. 2. Atom electron affinities are always endothermic. 3. Atomic size always increases with increasing atomic number. 4. Electronegativity values have no units. 5. The Born-Haber cycle describes the reproductive process for spineless invertebrates. 4 5. (5 pts each) For the following, circle the species in each row with the desired property: a. smallest atomic radius Rb K Li F Na Ca 2+ S2- Cl- Ar K+ c. greatest electronegativity O At Br I Cl d. greatest ionization energy Na Na + Mg e. greatest lattice energy NaF CrN MgCl2 f. smallest ion separation NaF NaCl NaBr b. largest radius Mg2+ Al LiF MgF2 NaI LiF 6. (15 pts) Using the information provided on the formula sheet, calculate the enthalpy change (kJ) for the combustion of 1 mol of ethanol (C2H5OH): C2H5OH (l) + 3O2 (g) → 2CO2 (g) + 3H2O (l) ∆ Ho = Σ n∆ ∆ Hof(products) - Σ n∆ ∆ Hof(reactants) ∆ Ho = [2(-393.5 kJ) + 3(-285.83 kJ)] – [(-235.1 kJ) + 3(0)] = [-787.0 kJ – 857.49 kJ] + 235.1 kJ = -1409.39 kJ = -1409.4 kJ 5 7. (16 pts) The Born-Haber Cycle is really just an application of Hess’s Law that enables us to calculate the lattice energy (∆Hlatt) for an ionic compound. Given the reactions below (and their associated enthalpy changes), calculate ∆Hlatt (kJ/mol) for MgBr2: MgBr2 (s) → Mg2+ (g) + 2Br- (g) ∆Hlatt = ? Here are the reactions you’ll need: Rxn 1: Rxn 2: Rxn 3: Rxn 4: Rxn 5: Rxn 6: ∆Hof = -524 kJ/mol ∆Hof = 147.1 kJ/mol ∆Hof = 111.8 kJ/mol IE1 = 738 kJ/mol IE2 = 1.45 x 103 kJ/mol EA = -325 kJ/mol Mg (s) + Br2 (l) → MgBr2 (s) Mg (s) → Mg (g) ½Br2 (l) → Br (g) Mg (g) → Mg+ (g) + e Mg+ (g) → Mg2+ (g) + e Br (g) + e - → Br- (g) NOTE: In order to receive full credit, you need to clearly show how you arrived at your answer! -(Rxn 1): Rxn 2: 2(Rxn 3): Rxn 4: Rxn 5: 2(Rxn 6): MgBr2 (s) → Mg (s) + Br2 (l) Mg (s) → Mg (g) Br2 (l) → 2Br (g) Mg (g) → Mg+ (g) + eMg+ (g) → Mg2+ (g) + e2Br (g) + 2e- → 2Br- (g) MgBr2 (s) → Mg2+ (g) + 2Br- (g) -(-524 kJ/mol) 147.1 kJ/mol 2(111.8 kJ/mol) 738 kJ/mol 1.45 x 103 kJ/mol 2(-325 kJ/mol) 2432.7 kJ/mol ∆ Hlatt = 2433. kJ/mol 6 8. (5 pts each) Draw the most probable Lewis structure for each of the following molecules. Indicate the formal charge on the central atom and the electrondomain geometry for each. a. SO2 6 + 12 = 18 e.. .. .. :O = S = O: AX3 – Trigonal Planar fc = 6 –(4 + 2) = 0 b. SF6 6 + 42 = 48 e.. .. :F: :F: .. \ / .. :F – S – F: .. / \ .. :F: :F: .. .. AX6 – Octahedral fc = 6 –(6) = 0 c. NH3 5 + 3 = 8 e.. H – N / \ H H AX4 – Tetrahedral fc = 5 –(3 + 2) = 0 d. XeF2 8 + 14 = 22 e.. .. .. :F – Xe – F: .. .. .. .. AX5 – Trigonal Bipyramidal fc = 8 –(2+6) = 0 7