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Exam 2-solutions - Version 100 Exam 2 mccord(50970 This...

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Version 100 – Exam 2 – mccord – (50970) 1 This print-out should have 34 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. McCord CH301 001 10.0 points The lattice enthalpy of calcium oxide is the energy change for the reaction 1. CaO(s) Ca(g) + O(g) 2. Ca(s) + 1 2 O 2 (g) CaO(s) 3. CaO(s) Ca(g) + 1 2 O 2 (g) 4. CaO(s) Ca 2+ (g) + O 2 (g) correct 5. Ca(g) + O(g) CaO(g) Explanation: Lattice enthalpy is the energy change at constant pressure when a 1 mole of an ionic solid is broken into ions of its component elements in gaseous form at 298 K, 1 atm. 002 10.0 points Which of the following compounds is non- polar? 1. NH 3 2. PH 3 3. O 3 4. PCl 3 5. BH 3 correct Explanation: NH 3 , PH 3 and PCl 3 are tetrahedral with one lone pair and are therefore not symmet- ric; their polar bonds do not cancel, and the molecule is polar. O 3 is bent, with a lone pair on the central oxygen and is thus polar. BH 3 is trigonal planar. The polar B H bonds cancel and the molecule is not polar. 003 10.0 points Which of the following is the best represen- tation of the compound magnesium oxide? 1. 2 Mg + , bracketleftbigg O bracketrightbigg 2 2. 3 Mg 2+ , 2 bracketleftbigg O bracketrightbigg 3 3. None is appropriate because magnesium oxide is a covalent compound. 4. Mg 2+ , 2 bracketleftbigg O bracketrightbigg 5. Mg + , bracketleftbigg O bracketrightbigg 6. Mg 2+ , bracketleftbigg O bracketrightbigg 2 correct Explanation: The best drawing will show the valence elec- trons, the charges, and the appropriate ratio of ions for MgO. 004 10.0 points Which of the following has bond angles of exactly 120 ? 1. NO 2 2. S 2 3 3. O 3 4. CS 2 3 correct 5. HO 2 Explanation: While CS 2 3 and NO 2 both have three re- gions of electron density around the central atom, in NO 2 one of these regions is a lone pair which will repel the other regions more strongly, making the bond angle less than 120 . Only in CS 2 3 would the bond angle be exactly 120 .
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Version 100 – Exam 2 – mccord – (50970) 2 005 10.0 points Which of the following symbols is used to depict a partial positive charge? 1. + 2. δ + correct 3. ψ + 4. σ + 5. γ + 6. λ + Explanation: 006 10.0 points Which of the following molecules is non- polar? 1. O 3 2. XeF 4 correct 3. PH 3 4. H 2 O 2 5. H 2 O Explanation: XeF 4 is symmetrical and therefore non- polar. Ozone (O 3 ) is the only polar molecule composed of non-polar bonds. The other molecules are all asymmetrical and therefore polar. 007 10.0 points In the molecular orbital theory of HCl, the ex- pected bonding molecular orbital (MO) would be 1. MO = H(1 s ) + Cl(3 s ) 2. MO = H(3 p ) + Cl(3 p ) 3. MO = H(1 s ) - Cl(3 s ) 4. MO = H(1 s ) + Cl(3 p ) correct 5. MO = H(1 s ) - Cl(3 p ) Explanation: H has 1 s valence electron and Cl has 3 p va- lence electrons. Bonding orbitals result from positive (in-phase) overlap of the atomic or- bitals.
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