42. Estimate  H o for forming 2 mol ammonia from molecular nitrogen and molecular hydrogen. Is this reaction exothermic or endothermic? (N
2
has a triple bond.)

43. Estimate  H o for the conversion of 1 mol carbon monoxide to carbon dioxide by combination with molecular oxygen. Is this reaction exothermic or endothermic? (CO has a triple bond.)
44. Which of the four molecules HF, HCl, HBr, and m has the strongest chemical bond? Using bond energies, estimate the
 H o for the reaction of molecular hydrogen with each of the gaseous molecular halogens: fluorine, chlorine, bromine, and iodine. Which is the most exothermic reaction?

48. The molecule below is urea, a compound used in plastics and fertilizers.
O
NH
2
H
2
N urea
(a) Which bonds in the molecule are polar and which are nonpolar?
(b) Which is the most polar bond in the molecule? Which atom is the partial negative end of this bond?
52. Write correct Lewis structures and assign a formal charge to each atom.
(a) SO
3
(b) C
2
N
2
(atoms bonded in the order NCCN)
(c) NO
2
-

53. Write correct Lewis structures and assign a formal charge to each atom.
(a) OCS
(b) HNC (atoms bonded in that order)
(c) CH3 -
54. Write correct Lewis structures and assign a formal charge to each atom.
(a) CH 3 CHO
(b) N
3
-
(c) CH
3
CN
60. These have two or more resonance structures. Write all the resonance structures for each.
(a) Nitric acid
O
HO
N
+
O
nitric acid

(b) Nitrate ion NO
3
-
61. These have two or more resonance structures. Write all the resonance structures for each molecule or ion.
(a) SO
3
(b) SCN -

Exceptions to the Octet Rule
67. Write the Lewis structure for each of these molecules or ions.
(a) BrF5
(b) lF5
(c) IBr
2
-
68. Write the Lewis structure for each of these molecules or ions.
(a) BrF
3
(b) I
3
-
(c) XeF
4

69. Which of these elements can form compounds with five or six pairs of valence electrons surrounding their atoms?
(a) C
(b) P
(c) °
(d) F
(e) Cl
(f) B
(g) Se
(h) Sn
71. All carbon-to-carbon bond lengths are identical in benzene.
Does this argue for or against the presence of C=C bonds in benzene? Explain.
72. Three dibromobenzenes are known. Write the Lewis structure and name for each compound.
74. Write the structural formula for 1,2-diiodobenzene (also known as ortho-diiodobenzene). Write the structural formulas for the meta and para isomers as well.

Biological Periodic Table
90. Make a list of the top ten most abundant essential elements needed by the human body.
91. Which types of compounds contain the majority of the oxygen found in the human body?
92. (a) How are metals found in the body, as atoms or as ions?
(b) What are two uses for metals in the human body?
93. Distinguish between macrominerals and microminerals.
94. Which minerals are essential at smaller concentrations but toxic at higher concentrations?