Practice Exam II Answers
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Practice Exam II Answers

Course: CHEM 027, Spring 2008

School: Pacific

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2 Answer Key for Exam A Multiple Choice questions. 48 Points Choose the best answer from the list of choices. Use your scantron sheet and mark each answer clearly. Be sure to fully erase all changes. 1. An orbital is a reference to: I) a region of high electron density. II) a region in an atom where an electron is likely to be found. III) a wave function resulting from specific values assigned to quantum numbers...

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Key 2 Answer for Exam A Multiple Choice questions. 48 Points Choose the best answer from the list of choices. Use your scantron sheet and mark each answer clearly. Be sure to fully erase all changes. 1. An orbital is a reference to: I) a region of high electron density. II) a region in an atom where an electron is likely to be found. III) a wave function resulting from specific values assigned to quantum numbers in wave equations. IV) a spherical region around a nucleus where an electron can be found. (a) II only (b) (c) (d) (e) (a) (b) (c) (d) (e) (a) (b) (c) (d) (e) (a) (b) (c) (d) (e) (a) (b) (c) (d) (e) I and II II and III I, II, and III I, II, III, and IV An electron produces an electric field. The electron's magnetic field can be oriented in two dimensions. Paired electrons produce no net magnetic field. An experiment with silver atoms passing through a magnetic field seems to prove electron spin exists. Electron spin only exists when m = 0. they are in the same period they have filled subshells they have high atomic masses electrons are added to the core of the atoms rather than the outer shell none of these answers fits K < Mg < Ne < Na < F Ne < Mg < K < Na < F Ne < F < Mg < Na < K Mg < F < Ne < Na < K K < Na < Mg < F < Ne electron affinity becomes more negative as radii increases electron affinity becomes more negative as radii decreases electron affinity becomes positive as radii decreases electron affinity becomes positive as radii increases there is no relationship 2. Which of the following statements about electron spin is false? 3. The transition metals are similar because: 4. List in order of increasing atomic size: F, Na, K, Mg, Ne. 5. What is the general relationship between electron affinity and atomic radii? 3 6. From the following data, what is the estimated boiling point of liquid chlorine? argon/BP= 87 K; bromine/BP= 332 K; fluorine/BP= 86 K; sulfur/BP= 718 K. (a) (b) (c) (d) (e) (a) (b) (c) (d) (e) (a) (b) (c) (d) (e) (a) (b) (c) (d) (e) (a) (b) (c) (d) (e) 525 K 402 K 209 K 148 K 123 K HCN CHCl3 H2 O HClO4 BCl3 in the 1st and 2nd period only in families IA, IIA, and IIIA only anywhere except period I and II in all families except IA in the 3rd and 4th period only phosphorus oxygen nitrogen carbon barium 1 3 5 8 10 7. Which of the following molecules is nonpolar? 8. An expanded octet may occur: 9. Which of the following is least likely to form multiple bonds? 10. How many "dots" should be shown in the Lewis symbol for phosphorus? 11. Which of the pairs of molecules below havae the same hybridization on the central atom in each molecule (The central atom is underlined.) (a) (b) (c) (d) (e) HOCl, ClF3 H2 O, HNO HCN, CO2 BeH2 , NH3 none of these 4 12. The double covalent bond between two carbon atoms in ethylene (C2 H4 ): (a) (b) (c) (d) (e) is free to rotate because of delocalized electrons is a sigma bond consists of one sigma and one pi bond has a low electron density includes a "lone pair" of electrons on one of the carbons 13. How would replacing a benzene C and its H with an N atom (C6 H6 C5 H5 N) affect the delocalized electrons? (a) (b) (c) (d) (e) The molecule would not be stable. The N may contribute to delocalization like C. Only the five C atoms would be included in the delocalized electrons. There would be no delocalized electrons, but a double bond between the N and a C. There would be no delocalized electrons but a double bond between the N and the C on each side of the N. . only atoms may be moved. electrons and atoms may both be moved. only electrons may be moved. neither electrons nor atoms may be moved. electrons must be added. a triple bond is longer than a single bond. a double bond is stronger than a triple bond. a single bond is stronger than a triple bond. a double bond is longer than a triple bond. a single bond is stronger than a double bond. 14. To convert from one resonance structure to another, (a) (b) (c) (d) (e) (a) (b) (c) (d) (e) 15. Of the possible bonds between carbon atoms (single, double, and triple), 16. According to VSEPR theory, if there are three pairs of electrons the in valence shell of an atom, they will be arranged in a(n) geometry. (a) (b) (c) (d) (e) (a) (b) (c) (d) (e) octahedral linear tetrahedral trigonal planar trigonal bipyramidal is tetrahedral. CBr4 PH3 CCl2 Br2 XeF4 all of these except XeF4 17. The electron-pair geometry of 5 18. The Cl--Si--Cl bond angle in the SiCl2 F2 molecule is approximately (a) (b) (c) (d) (e) 90 109.5 120 180 60 nonbonded electron pairs and 19. The central iodine atom in the ICl- ion has 4 bonded electron pairs in its valence shell. (a) (b) (c) (d) (e) 2, 2 3, 1 1, 3 3, 2 2, 4 20. For molecules of the general formula AXn , n may be greater than four (4) for . (a) (b) (c) (d) (e) (a) (b) (c) (d) (e) for any element A. only when A is an element from the third period or beyond in the periodic table. only when A is boron or beryllium. only when A is carbon. only when A is Xe. is polar. CCl4 CH4 SeF4 SiCl4 BF4 electron- 21. Of the molecules below, only 22. Generally, a molecule in which the central atom is sp-hybridized will have a(n) pair geometry. (a) (b) (c) (d) (e) octahedral tetrahedral trigonal planar trigonal bipyramidal linear 23. When three atomic orbitals are mixed to form hybrid orbitals, how many hybrid orbitals are formed? (a) (b) (c) (d) (e) one six three four five 6 24. The nitrogen-nitrogen bond in HNNH consists of (a) (b) (c) (d) (e) (a) (b) (c) (d) (e) one and one bond. one and two bonds. two bonds and one bond. two and two bonds. one bond and no bonds. one MO and one MO. one MO and one MO. one MO and one MO. one MO and one MO or one MO and one MO. two MOs, two MOs, one MO, and one MO. 25. According to MO theory, overlap of two p AOs produces Problems. 50 Points Show all work on these longer problems for full credit. Partial credit will be given if it is logically presented. Please box or otherwise clearly indicate answers. 26. (a) Arrange the following bonds from least to most polar (polar covalent): S-H, O-Cl, Al-O, As-O. (5 points) Answer: (most polar) Al--O, S--H, As--O, O--Cl (least polar) (b) Which of the following molecules would you expect to be polar (more than one is possible): HCN, SO3 , CS2 , SiF4 . (5 points) Answer: Only HCN is polar (SO3 has a single double bond, with resonances). 27. Determine the structure of the molecule BrF3 , including the hybridization on the Br, the bond angles, and the overall molecular geometry. (10 points) Answer: This molecule is T-shaped! The electron-pair geometry is trigonal bipyramidal. Putting the lone pairs equatorial ("trigonal"), yields 4 90 LP-BP interactions, while putting them axial (which would result in a trigonal planar geometry) yields 6 90 LP-BP interactions. F Br F The hybridization on the bromine is dsp3 -- five -type bonds (LP and BP). F F Br F F 7 28. Consider the molecules NH3 and NF3 . The dipole moments of the two molecules point in opposite directions (one towards N; the other away from N). Further, One molecule's dipole moment is much larger than the other. Assign these characteristics to each molecule using what you know about electronegativity and geometry. (10 points) Answer: Both of these molecules have a tetrahedral EP geometry, trigonal pyramidal molecular geometry. However, F is much more EN than H, and is more EN than N, so the dipole moment is reversed (towards the fluorines) in NF3 with respect to NH3 (towards the nitrogen). N H H H F N F F 29. Using Lewis and VSEPR theories, describe the molecular geometry of the ClO- ion. Is this ion 3 polar or non-polar? (10 points) Answer: Lewis symbol: Cl O O O The overall electron-pair geometry is tetrahedral; as chlorate has one lone pair, the molecular geometry is trigonal pyramidal. This ion is polar (in addition to being an ion). 30. Describe the bonding in O2 using MO theory. Indicate the occupation of each of the valence (bonding) electrons in this molecule. What is the bond order? Is oxygen para- or diamagnetic? (15 points) Answer: dioxygen has a total of 16 electrons, or 12 valence electrons. Its MO diagram is: !" #" # ! !" !" ! 2 2 2 2 2 4 2 or 1s 1s 2s 2s 2p 2p This molecule is paramagnetic and has a bond order of two.

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