CHEM 31 Introductory Chemistry EXAM #3 November 13, 2002 Name: Anne Surkee 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 Possible Points Points Earned 2 20 20 3 20 20 4 30 30 5 20 20 6 15 15 7 35 35 8 10 10 TOTAL: 150 150
1. (10 pts) As you may recall from the demo in class, magnesium metal burns quite vigorously in solid carbon dioxide, forming magnesium oxide and carbon. The balanced reaction looks like this: 2Mg (s) + CO 2 (s) 2MgO (s) + C (s) Using the enthalpy data provided on the formula sheet, calculate the enthalpy change for this reaction ( H o rxn) in kj. H o = Σ n H o f(products) - Σ n H o f(reactants) H o = [2(-601.8 kj) + 0 kj] [(-443.53 kj) + 2(0 kj)] = -1203.6 kj + 443.53 kj = -760.07 kj = -760.1 kj 2. (10 pts) The standard molar enthalpy of formation ( H o f) for NaCl(s) is 410.9 kj/mol, whereas the lattice energy ( H latt ) for NaCl is +788 kj/mol. Write reaction equations for each of these processes. For the standard molar enthalpy of formation: Na(s) + ½ Cl 2 (g) NaCl (s) H o f = -410.9 kj/mol For the lattice energy: NaCl(s) Na + (g) + Cl - (g) H latt = +788 kj/mol 2
3. As I demonstrated in class, we can detect the presence of mercury in fluorescent bulbs by observing the bulb s emission spectrum. a. (7 pts) Mercury has a very intense emission line at 365.0 nm. Calculate the energy (J) of a single photon of this radiation. E = hc/λ = (6.62606876 x 10-34 J-s)(2.9979 x 10 8 m/s) 365.0 x 10-9 m = 5.44227165 x 10-19 J = 5.442 x 10-19 J b. (3 pts) Is this photon in the visible portion of the electromagnetic spectrum? YES NO (circle one) 4. The energy from radiation can be used to cause the rupture of chemical bonds. A minimum energy of 495 kj/mol is required to break the oxygen-oxygen bond in O 2. a. (5 pts) Calculate the minimum energy (J) required to break a single oxygen-oxygen bond. 495 kj x 1000 J x 1 mol O 2 = 8.2197 x 10-19 J/bond mol O 2 kj 6.02214 x 10 23 bonds = 8.22 x 10-19 J/bond b. (5 pts) If this energy were provided by a photon, calculate the wavelength (nm) of the photon having this minimum energy. E = hc/λ λ = hc/e λ = (6.62606876 x 10-34 J-s)(2.9979 x 10 8 m/s) 8.2197 x 10-19 J = 2.41667 x 10-7 m = 2.41667 x 10-7 m x 10 9 nm = 241.667 nm = 2.42 x 10 2 nm m 3
5. (10 pts) The electron microscope has been widely used to obtain highly magnified images of biological and other types of materials. When a electron is accelerated through a particular potential field, it attains a speed of 5.93 x 10 6 m/s. What is the characteristic wavelength (nm) of this electron? (NOTE: mass of electron = 9.10938188 x 10-28 g) λ = h = 6.62606876 x 10-34 J-s mv (9.10938188 x 10-31 kg)(5.93 x 10 6 m/s) = 1.226626 x 10-10 m = 1.226626 x 10-10 m x 10 9 nm = 0.1226626 nm = 0.123 nm 1 m 6. (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. Mg: [Ne] 3s 2 b. Cu: [Ar] 4s 1 3d 10 c. Cu + : [Ar] 3d 10 d. Br - : [Ar] 3d 10 4s 2 4p 6 OR [Kr] 4
7. (5 pts each) Draw the most probable Lewis structures for the following molecules and molecular ions. Calculate the formal charge on the central atom for each. Show resonance structures if appropriate. a. CO 2 4 + 12 = 16 e -.... :O = C = O: fc = 4 (4) = 0 b. NO 2-5 + 12 + 1 = 18 e -...... -...... - :O = N O: :O - N = O:.... fc = 5 (3+2) = 0 c. HCN 1+ 4 + 5 = 10 e - H C N: fc = 4 4 = 0 d. XeF 4 8 + 28 = 36 e -.. :F:...... :F Xe F:...... :F:.. fc = 8 (4+4) = 0 5
8. (5 pts each) Circle the number next to the most appropriate response for each of the following: a. The Bohr model of the atom: 1. was first proposed by Balmer more than 30 years before Bohr 2. accurately predicts the line emission spectrum for the hydrogen atom 3. is based on the wave properties of the electron in a hydrogen atom 4. was originally made out of balsa wood using simple hand tools in Bohr s garage 5. is also known as the plum pudding model of the atom b. Which one of the following orbital diagrams show an electron configuration for P that is not allowed? 1. [Ne] 3s 3p 2. [Ne] _ _ _ 3s 3p 3. [Ne] _ _ _ 3s 3p 4. [Ne] _ _ _ 3s 3p 5. [Ne] _ _ _ 3s 3p c. The generalized electron configuration [noble gas]ns 2 np 5 applies to elements found in which one of the following categories: 1. transition metals 2. noble gases 3. halogens 4. chalcogens 5. land mammals 6
d. Which one of the following atoms/ions are isoelectronic with Ar? 1. Al 3+ 2. Xe 3. K + 4. Ca 5. Na + e. The numerical values of the quantum numbers n, l, and m l for one of the 3d orbitals can be (expressed as: n, l, m l ): 1. 2, 2, 2 2. 2, 1, 2 3. 3, 1, -1 4. 3, 2, -3 5. 3, 2, 0 9. (5 pts each) For the following, circle the species in each row with the desired property: a. largest atomic radius Na Al S Ar K b. smallest radius Al 3+ O 2- F - Ne Na + c. greatest electronegativity F Cl Br N O d. most polar bond Li-F C-F H-H Na-I Na-F e. greatest lattice energy NaF KBr MgCl 2 LiF MgO 7
10. (10 pts) Calculate the standard enthalpy of formation ( H o f) of solid Mg(OH) 2, given the following information: Rxn 1: 2Mg (s) + O 2 (g) 2MgO (s) Rxn 2: Mg(OH) 2 (s) MgO (s) + H 2 O (l) Rxn 3: 2H 2 O (l) 2H 2 (g) + O 2 (g) H o = -1203.6 kj H o = +37.1 kj H o = +571.7 kj We need to get the reaction that describes the H o f for Mg(OH) 2 (s): Mg(s) + O 2 (g) + H 2 (g) Mg(OH) 2 (s) Combining the three reactions: Reverse Rxn 2: MgO (s) + H 2 O (l) Mg(OH) 2 (s) -(37.1 kj) ½ Rxn 1: Mg (s) + ½O 2 (g) MgO (s) ½ (-1203.6 kj) ½ Reverse Rxn 3: H 2 (g) + ½O 2 (g) H 2 O (l) ½ (-571.7 kj) Mg(s) + O 2 (g) + H 2 (g) Mg(OH) 2 (s) H o f H o f = -37.1 + ½(-1203.6 kj) + ½ (-571.7 kj) = -37.1 601.8 285.85 = -924.75 kj/mol = -924.8 kj/mol 8