Tutorial - Review Solutions

Tutorial - Review Solutions - CHEM 1AA3 REVIEW TUTORIAL...

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CHEM 1AA3 REVIEW TUTORIAL January 7-8, 2010 SOLUTIONS ________________________________________________________________________ Page 1 of 6 1. Lewis structures and resonance. For each of the following: (i) Write Lewis structures (charge-minimized); include formal charges in the structure. (ii) Include resonance structures where possible. (iii) Give the VSEPR class at each central atom; name the shape about each central atom; list all bond angles. (a) SO 4 2 The sulfate ion has 6 resonance structures: The anion is AX 4 (tetrahedral) at sulfur, and all O-S-O bond angles are 109.5 ° . (b) NO 3 The nitrate anion has 3 resonance structures: The anion is AX 3 (trigonal planar) at nitrogen, and all O-N-O bond angles are 120 ° . **Recall for nitrate that the octet rule cannot be exceeded at nitrogen, so charges cannot be minimized further. (c) ClO 4 The perchlorate anion has 4 resonance structures: The anion is AX 4 (tetrahedral) at chlorine, and all O-Cl-O bond angles are 109.5 ° . (d) CH 4 Methane is an AX 4 molecule (tetrahedral). The H-C-H bond angles are 109.5 ° . (e) NH 3 Ammonia is an AX 3 E molecule (trigonal pyramidal). The H-N-H bond angles are less than 109.5 ° (approx. 107 ° ).
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CHEM 1AA3 REVIEW TUTORIAL January 7-8, 2010 SOLUTIONS ________________________________________________________________________ Page 2 of 6 (f) formaldehyde (H 2 CO, C central atom, all atoms bonded to C) The molecule is an AX 3 molecule (trigonal planar). The H-C-H and H-C-O bond angles are not exactly 120 ° (ideal). Qualitatively we can say that the angles are non-ideal, because of the influence of the double bond to oxygen. The actual values: H-C-H angle is 116 ° and the H-C-O angles are 122 ° . (g) methanol (CH 3 OH, 3 H bonded to C, O bonded to C, 1 H bonded to O) This molecule has more than one “central atom”. The shape at the C atom is AX 4 , tetrahedral (angles 109.5 ° ), and the shape at the O atom is AX 2 E 2 (bent or V-shaped); H-O-C bond angle will be less than 109.5 ° (non-ideal) because of the lone electron pairs (likely close to about 105 ° ). (Note: for parts (b), and (d)-(g) above the given Lewis structures given also indicate the correct molecular shape, although the Lewis structure does not have to show that shape. The wedged line indicates an atom coming forwards out of the plane, and a dashed line indicates a line going backwards behind the plane). 2.
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Tutorial - Review Solutions - CHEM 1AA3 REVIEW TUTORIAL...

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