Updated Nov.16/04 -- prepared by Dr.Bird... ----------------------------------------------------------------------------------------- NUMERICAL ANSWERS TO ASSIGNED TUTORIAL PROBLEM SETS FOR CHEM205 FROM KOTZ & TREICHEL'S CHEMISTRY & CHEMICAL REACTIVITY ----------------------------------------------------------------------------------------- NOTE: none of the answers from Ch.9 have been verified NOTE: "^" means "raised to the power of..." to indicate scientific notation... NOTE: I cannot represent superscripts/subscripts here -- so...for polyatomic ions, I have left a space between the ion's formula and its charge Ch. Q# Comments ----------------------------------------------------------------------------------------- 9 1 Li: 1 valence electron (e-) Ti: 4 valence e- Zn: 2 valence e- Si: 4 valence e- Cl: 7 valence e- 9 8 BCl3 has 6. In Cl3BNH3 it has 8. (see p342) 9 9 SeF4, BrF4 -, XeF4 9 13 Bond order = (3 pairs linking C to O)/(2 C to O links) = 3/2 9 17 The highter the bond order, the shorter the bond, and the larger the bond energy. 9 19 A polar bond such as H-Cl has a positive and a negative end. An example of a non-polar bond is in H-H. 9 21 Electronegativities generally increase from left to right across a period, and decrease down a group. 9 29 C: Group 14 (4A) 4 valence electrons (e-) Cl: Group 17 (7A) 7 valence e- Ne: Group 18 (8A) 8 valence e- Si: Group 14 (4A) 4 valence e- Se: Group 16 (6A) 6 valence e- Al: Group 13 (3A) 3 valence e- 9 31 P, Cl, Se, and Sn can accomodate more than 4 valence e-'s 9 33 MgCl (Mg typically forms Mg2+ ions) BaF3 (Ba typically forms Ba2+ ions) CsKr (Kr is unlikeley to form a Kr- ion) 9 37a NaCl (shorter distance) 37b MgO (shorter distance) 37c MgS (higher charge) 9 39 CS2 is like CO2, see page 333, NO2 - see page 363, BF4 - see Table 9.6, p339 Cl2SO is pyramidal, S central, with Cl-S single bonds, and an S=O double bond, or alternatively, with all single bonds. Both have one lone pair on S. 9 43 It is possible to draw 3 for each which obey the octet rule, but some are more reasonable than others. 9 45a BrF5: "electron pair geometry": octahedral, molecular geometry:square pyramidal. 45b IF3: "electron pair geometry": trigonal bipyramidal, molecular geometry: T-shaped. 45c IBr2 -: "electron pair geometry": trigonal bipyramidal, molecular geometry: linear. 45d BrF2 +: "electron pair geometry": tetrahedral, molecular geometry: bent. 9 49a SO2 (O-S=O) (pairs not shown) O = -1, S = 0, O = 0 49b SOCl2 (see Q39) First structure: all have zero formal charge, or, with all single bonds, Cl's = 0, S = +1, O = -1. 49c SO2Cl2 if drawn with all bonds single: Cl's = 0, S = +2, O's = -1. Alternative Lewis structures with one or 2 S=O double bonds can be drawn. (See 49b) 49d FSO3 - has S central and if drawn with all bonds single, F = 0, O = -1, S = +2. Again, alternatives with S=O bonds can be drawn, reducing the charge on S by 1 for each. 9 51 (a) N, (b) C, (c) Br, (d) O 9 53a All are polar. 53b The C=O bond is most polar, and the O is the more negative atom. 9 55a H3O +: Even though the formal charge on O is +1, and on H is zero, H is less electronegative. The three H atom therefore are likely to bear the positive charge. The O-H bonds are polar with H the positive end. 55b NH4 +: Even though the formal charge on N is +1, and on H is zero, H is less electronegative. The four H atom therefore are likely to bear the positive charge. The N-H bonds are polar with H the positive end. 55c NO2 +: The formal charge on N is +1 and on O it is zero, This conforms to the relative electronegativities. The bonds are polar, with O the negative end. 55d NF4 +: The formal charge on N is +1 and on F it is zero, This conforms to the relative electronegativities. The bonds are polar, with F the negative end. 9 57a Yes. Both have 24 valence electrons. 57b CO3 2- has three reasonable resonance structures. It is possible to draw a fourth for BO33- bearing in mind that B sometimes has only 6 electrons, e.g. BF3. 57c CO3 2-: C = 0, double bonded O's = 0, single bonded O's = -1. BO3 3-: B = -1 for the three structures like CO32-, and 0 for the 4th which has all B-O sinfle bonds. 57d The H+ would attach to an oxygen. 9 61a CN -: one C to N triple bond. 61b CH3CN: three C-H single bonds, one C-C single bond, one C to N triple bond. 61c SO3: two S-O single bonds, one S=O double bond. 61d CH3CH=CH2: six C-H single bonds, one C-C single bond, one C=C double bond. 9 63 (a) Si-O, (b) C-O, (c) C-F, (d) C to N triple bond. 9 65 HCO2 -: one C-O single bond, one C=O double bond, and two resonance structures. Average C to O bond order = 3/2. CH3OH: has a C-O single bond. CO3 2-: one C=O double bond and two C-O single bonds. Three resonance structures, with average C to O bond order 4/3. Therefore, CH3OH has the longest C to O bond, and HCO2 - the shortest. 9 73a ClF2 +: "electron pair geometry": tetrahedral, molecular geometry: bent 73b SnCl3 -: "electron pair geometry": tetrahedral, molecular geometry: trigonal pyramidal 73c PO4 3-: "electron pair geometry": tetraheral, molecular geometry: tetraheral 73d CS2: "electron pair geometry: linear, molecular geometry: linear 9 75a CO3 2-: "electron pair geometry": trigonal, molecular geometry: trigonal 75b NO3 -: "electron pair geometry": trigonal, molecular geometry: trigonal 75c SO3 2-: "electron pair geometry": tetraheral, molecular geometry: trigonal pyramidal 75d ClO3 -: "electron pair geometry: tetrahedral, molecular geometry: trigonal pyramidal 9 77a SiF6 2-: "electron pair geometry": octahedral, molecular geometry: octahedral 77b PF5: "electron pair geometry": trigonal bipyramidal, molecular geometry: trigonal bipyramidal 77c SF4: "electron pair geometry": trigonal bipyramidal, molecular geometry: "see-saw" 77d XeF4: "electron pair geometry: octahedral, molecular geometry: square planar 9 79a SCl2: 109 deg 79b NNO: 180 deg 79c CH2CHOH: angle 1: 120 deg, angle 2: 120, angle 3: 109 9 83a CH4: electroneg. diff. 2.5 - 2.1 = 0.4 83b NCl3: electroneg. diff. 3.0 - 3.0 = 0 83c BF3: electroneg. diff. 4.0 - 2.0 = 2.0 83d CS2: electroneg. diff. 2.5 - 2.5 = 0 83i The B-F bonds in BF3 are most polar 83ii None of the molecules are polar. (Although NCl3 has a trignonal pyramidal geometry, the N-Cl bonds are not, so the molecule would not be significantly polar.) 9 87 The N to O bonds in NO2 - have abond order of 1.5, while in NO2 + the bond order is 2. The shorter bonds (110 pm) correspond to the species with the higher bond order, i.e. NO2 + 9 89 ClF2 +: "electron pair geometry": tetrahedral, molecular geometry: bent, bond angle 109 deg. ClF2 -: "electron pair geometry": trigonal bipyramidal, molecular geometry: linear, bond angle 180 deg. 9 91 SO3 2-: "electron pair geometry": tetrahedral, molecular geometry: trigonal pyramidal, negative charge on O atoms, so H+ will attach to O. 9 95a (i) C to N triple bond, N-O single bond, formal charges: C = -1, N = +1, O = -1. (ii) C=N double bond, N=O double bond, formal charges: C = -2, N = +1, O = 0. (iii) C-N single bond, N to O triple bond, formal charges: C = -3, N = +1, O = +1. 95b The first resonance structure is the most reasonable since it places the negative charge on the most electronegative atom, and minimizes charge separation. 95c Carbon, the least electronegative element has to be assigned a negative charge, and all structures have an unfavorable charge distribution. 9 97(a) Angle 1 = 120 deg, angle 2 = 109 deg, angle 3 = 120 deg. 97b The C=O bond is the shortest C to O bond in the molecule. 97c The O-H bond is the most polar: the electroneg. diff. 3.5 - 2.1 = 1.4.