Chem 301

Chem 301 Spring 2004

Exam 2 March 24, 2004

Name

KEY

This exam has 6 questions on 6 pages. 1.

(25 pts.) Draw the best Lewis structures for the following species. Show all valence electrons and formal charges for all atoms. Provide the information requested. a.

SnCl3– molecular shape trigonal pyramid hybridization of central atom sp

b.

26e–

0

Sn

Cl

Cl

Cl

0

0

3

NO2– molecular shape bent

-

18e–

0

0 0

O

N

O

-

hybridization of central atom sp2

c.

SeOCl2 molecular shape trigonal pyramid

26e–

O 0

hybridization of central atom sp3

d.

ClF3 molecular shape T-shape

28e–

e.

PO43– molecular shape Td hybridization of central atom sp3

Se

Cl

F 0

F

32e–

Cl

O

O

P O

0

0

F 0

hybridization of central atom dsp2

Cl

-

O

0

-

O

N

O

0

2 2.

(8 pts.) Rank in order of increasing bond strength (weakest on the left, strongest on the right). a.

N–H

P–H

As–H As–H < P–H < N–H

b.

N–NH2

P–NH2

As–NH2

As–NH2 < N–NH2 < P–NH2

3.

(15 pts.) Give the point groups for the following:

a.

XeF4

D4h N

N

b.

C3h ____________

N

c.

HCN

C∞v

H d.

C

C

C

H

Cl H

H

D2d ____________

Cl C

e.

H

C Cl

Cs ____________

3 4.

(27 pts.) Follow the steps below to derive the M.O. diagram for SH4. a.

(12 pts.) Show the symmetry adapted linear combinations (SALCs) of the hydrogen orbitals that would overlap with the sulfur orbitals. Use the axes and atom labels shown. Show both pictorial and mathematical forms. Give the correct symmetry labels for all SALCs. Key Hint: The four hydrogens are not all symmetry equivalent.

z H4

x

b.

H1

S

H3 H2

y H3 + H4 a1

H3 - H4 b2

H1 + H2 a1

H1 - H2 b1

(8 pts.) Draw the M.O. diagram. Show pictorial forms of all MOs. Use correct symmetry labels. Specify the bonding, antibonding, and nonbonding orbitals. 4a1

4a1 3a1 2b2

a1, b1, b2

3a1 antibonding

2b1

2b2

3p

2b1

3s (a1)

S

b1 b2 a1 a1

1b1

1b1

H 1b2 1b2

bonding 2a1 2a1 1a1 1a1

c.

(4 pts.) Show the electron occupancy in your M.O. diagram. Is the molecule paramagnetic or diamagnetic? diamagnetic

d.

(3 pts.) Briefly comment on what the MO diagram reveals about the average strength of the S-H bonds. What is the approximate S-H bond order? Four bonding and one antibonding orbitals are filled. Thus, the average bond order is 0.75 and the S–H bonds do not have full single-bond strength.

4 5.

(15 pts.) Indicate if the following species are chiral or achiral. H

CN

a.

b.

H3C

O

H

Cl

achiral

H

d.

Cl H3N Os CO H3N NH3

H2N Cl

e.

P

F

H

H

chiral

achiral

NH2 Cl Hf Cl Cl

achiral

chiral 6.

c. H

(10 pts.) The H-bonding between NH4+ and NH3, [H3N⋯H⋯NH3]+ can be readily explained by MO Theory. Consider the lone pairs on each ammonia to be sp3 hybrids. With these two orbitals and the hydrogen orbital, derive the MO energy level diagram for the N⋯H⋯N interaction. Show the electron occupation of this MO diagram. Draw pictorial representations of all molecular orbitals. •

antibonding

•

nonbonding

•

bonding

5 Data Pages

Periodic Table of the Elements 8A 18

1A 1 1 H 1.01 3 Li 6.94 11 Na 23.0 19 K 39.1 37 Rb 85.5 55 Cs 133 87 Fr (223)

2A 2

3A 13

4A 14

5A 15

6A 16

7A 17

2 He 4.00

4 Be 9.01 12 Mg 24.3 20 Ca 40.1 38 Sr 87.6 56 Ba 137 88 Ra 226

5 B 10.8

6 C 12.0

8 O 16.0 16 S 32.1 34 Se 79.0

9 F 19.0

14 Si 28.1 32 Ge 72.6 50 Sn 119 82 Pb 207

7 N 14.0 15 P 31.0 33 As 74.9 51 Sb 122 83 Bi 209

17 Cl 35.4 35 Br 79.9

10 Ne 20.2 18 Ar 39.9 36 Kr 83.8

52 Te 128 84 Po (209)

53 I 127 85 At (210)

54 Xe 131 86 Rn (222)

67 Ho 165 99 Es (252)

68 Er 167 100 Fm (257)

69 Tm 169 101 Md (258)

70 Yb 173 102 No (259)

71 Lu 175 103 Lr (260)

8B 3B 3

4B 4

5B 5

6B 6

7B 7

21 Sc 45.0 39 Y 88.9 57 La 139 89 Ac 227

22 Ti 47.9 40 Zr 91.2 72 Hf 178 104 Rf (261)

23 V 50.9 41 Nb 92.9 73 Ta 181 105 Ha (262)

24 Cr 52.0 42 Mo 95.9 74 W 184 106 Unh (263)

25 Mn 54.9 43 Tc (98) 75 Re 186 107 Uns (262)

58 Ce 140 90 Th 232

59 Pr 141 91 Pa 231

60 Nd 144 92 U 238

Lanthanides

Actinides

10

1B 11

2B 12

28 Ni 58.7

29 Cu 63.5

44 Ru 101 76 Os 190 108 Uno (265)

27 Co 58.9 45 Rh 103 77 Ir 192 109 Une (266)

46 Pd 106 78 Pt 195

47 Ag 108 79 Au 197

30 Zn 65.4 48 Cd 112 80 Hg 201

13 Al 27.0 31 Ga 69.7 49 In 115 81 Tl 204

61 Pm (145) 93 Np (237)

62 Sm 150 94 Pu (244)

63 Eu 152 95 Am (243)

64 Gd 157 96 Cm (247)

65 Tb 159 97 Bk (247)

66 Dy 162 98 Cf (251)

8 26 Fe 55.8

9

6 Start yes

yes

no

Linear?

yes Two or no more Cn, n>2?

i? yes

no yes

i?

yes

no

C∞v

C5? no

Ih

σ?

yes Are nC2 ⊥ to principle Cn?

Oh

i? yes

σh ?

no

yes

no yes σ ? no h

D∞h

no

Cn ?

no

yes

no

Td

yes

σd?

no

σv ?

yes

no S2n ?

yes Cnh

no Dnh

C2v A1 A2 B1 B2

E 1 1 1 1

C2 1 1 –1 –1

σv(xz) 1 –1 1 –1

Dnd

σv(yz) 1 –1 –1 1

Dn

z Rz x, Ry y, Rx

Cn

S2n

Cnv

x2, y2, z2 xy xz yz

Cs

Ci

C1