Name: 1: /33Grade: /100 2: /33

3: /33+1 point

Midterm Exam 2CHEM 181: Introduction to Chemical Principles

October 13, 2015

Directions:

Do all three problems.

Show all of your work neatly and clearly. Do not skip steps. Partial credit will beawarded for all problems. Correct answers will not receive credit if your work is notshown.

If you are not sure exactly what a question means, ask!

Not all problems are of equal difficulty, but all are worth the same fraction of theoverall grade.

1

1. Draw Lewis structures for the isomers with the molecular formula C3H3NO2 thatare shown on the following pages. For each isomer shown, do the following:

• Label non-zero formal charges.

• Include all resonance structures that follow the octet rule.

• Label each resonance structure as either major, minor, very minor, ornot valid (i.e., should not be considered).

• Rank the resonance structures in numerical order (1=best, 2=next best,etc.) If two structures are exactly equivalent, give them the same number.

(a) Methyl cyanoformate is an isomer of C3H3NO2, with the connectivityshown below:

CH

H

H

O C

O

C N

Provide a Lewis structure for methyl cyanoformate.

2

(b) Acetyl isocyanate is an isomer of C3H3NO2, with the connectivity shownbelow:

C

H

H

H

C

O

N C O

Provide a Lewis structure for acetyl isocyanate.

3

(c) Oxazolone is an isomer of C3H3NO2, with the connectivity shown below:

N C

OC

C

O

H

H

HProvide a Lewis structure for oxazolone.

4

2. Here is the best single electron-dot structure for acetyl azide (C2H3N3O):

H C

H

H

C

O

N N N

Consider the following proposed (but not necessarily correct) resonance struc-tures, labeled A through E. Categorize each one according to the following.(You will not receive credit for categorizing a structure if you do not provide anexplanation.)

H C

H

H

C

O

N N N

H C

H

H

C

O

N N N

H C

H

H

C

O

N N NH C

H

C

O

N N N

H C

H

H

C

O

N N N

H

A B C

D E

(a) Which structures (if any) must be included as resonance structures becausethey tell you something new about the molecule? Identify each structureby its label, and explain (one sentence each) what is important about its in-clusion (i.e., its effect on bond order, formal charge, and/or hybridization).

5

(b) Which structures (if any) could be included, but are very minor and theirinclusion would not change bond order, formal charge, or hybridization inany significant way. Identify each structure by its label, and explain (onesentence each) what makes the structure unimportant.

(c) Which structures (if any), are bad structures that can not be included.Identify each structure by its label, and explain (once sentence each) theproblem or problems with it.

6

3. Draw 4 isomers that have the molecular formula C2H3NO. Do not use rings.

(a) For each isomer, provide a Lewis structure with major and minor resonancestructures and non-zero formal charges (if present).

7

(b) Label which of the four isomers you expect to be the most stable compound(“most” or “most stable”) and explain (1 sentence):

(c) Label which of the four isomers you expect to be the second most stablecompound (“second most” or “second”) and explain (1 sentence):

(d) Label which of the four isomers you expect to be the third most stablecompound (“third most” or “third”) and explain (1 sentence):

(e) Label which of the four isomers you expect to be the least stable compound(“least” or “least stable”) and explain (1 sentence):

8

Electronegativity:

Atom χ

F 3.98O 3.44Cl 3.16N 3.04Br 2.96I 2.66S 2.58C 2.55H 2.20

Equations:Bond order = 1

2(# bonding electrons−# antibonding electrons).

Formal charge = # valence electrons−(# lone pair electrons+ 1

2# bonding electrons

)

9

The

Peri

odic

Tab

le o

f the

Ele

men

ts1 H

Hyd

roge

n1.

0079

4

2 He

Hel

ium

4.00

3

3 Li Lith

ium

6.94

1

4 BeBe

rylli

um9.

0121

82

5 B Bor

on10

.811

6 C Car

bon

12.0

107

7 NN

itrog

en14

.006

74

8 OO

xyge

n15

.999

4

9 FFl

uorin

e18

.998

4032

10 Ne

Neo

n20

.179

7

11 Na

Sodi

um22

.989

770

12 Mg

Mag

nesiu

m24

.305

0

13 Al

Alu

min

um26

.981

538

14 Si Silic

on28

.085

5

15 PPh

osph

orus

30.9

7376

1

16 S Sulfu

r32

.066

17 Cl

Chlo

rine

35.4

527

18 Ar

Arg

on39

.948

19 KPo

tass

ium

39.0

983

20 Ca

Calc

ium

40.0

78

21 ScSc

andi

um44

.955

910

22 TiTi

tani

um47

.867

23 VV

anad

ium

50.9

415

24 Cr

Chr

omiu

m51

.996

1

25 Mn

Man

gane

se54

.938

049

26 Fe Iron

55.8

45

27 Co

Coba

lt58

.933

200

28 Ni

Nic

kel

58.6

934

29 Cu

Cop

per

63.5

46

30 Zn Zinc

65.3

9

31 Ga

Gal

lium

69.7

23

32 Ge

Ger

man

ium

72.6

1

33 As

Ars

enic

74.9

2160

34 SeSe

leni

um78

.96

35 Br Brom

ine

79.9

04

36 Kr

Kry

pton

83.8

037 R

bR

ubid

ium

85.4

678

38 SrSt

ront

ium

87.6

2

39 YY

ttriu

m88

.905

85

40 ZrZi

rcon

ium

91.2

24

41 Nb

Nio

bium

92.9

0638

42 Mo

Mol

ybde

num

95.9

4

43 TcTe

chne

tium

(98)

44 Ru

Rut

heni

um10

1.07

45 Rh

Rho

dium

102.

9055

0

46 Pd Palla

dium

106.

42

47 Ag

Silv

er10

7.86

82

48 Cd

Cad

miu

m11

2.41

1

49 In Indi

um11

4.81

8

50 Sn Tin

118.

710

51 SbA

ntim

ony

121.

760

52 TeTe

lluriu

m12

7.60

53 IIo

dine

126.

9044

7

54 Xe

Xen

on13

1.29

55 Cs

Ces

ium

132.

9054

5

56 Ba Bariu

m13

7.32

7

57 LaLa

ntha

num

138.

9055

72 Hf

Haf

nium

178.

49

73 Ta Tant

alum

180.

9479

74 WTu

ngste

n18

3.84

75 Re

Rhe

nium

186.

207

76 Os

Osm

ium

190.

23

77 Ir Iridi

um19

2.21

7

78 PtPl

atin

um19

5.07

8

79 Au

Gol

d19

6.96

655

80 Hg

Mer

cury

200.

59

81 TlTh

alliu

m20

4.38

33

82 Pb Lead

207.

2

83 Bi Bis

mut

h20

8.98

038

84 PoPo

loni

um(2

09)

85 At

Asta

tine

(210

)

86 Rn

Rad

on(2

22)

87 FrFr

anci

um(2

23)

88 Ra

Rad

ium

(226

)

89 Ac

Act

iniu

m(2

27)

104

Rf

Rut

herf

ordi

um(2

61)

105

Db

Dub

nium

(262

)

106

SgSe

abor

gium

(263

)

107

Bh Bohr

ium

(262

)

108

Hs

Has

sium

(265

)

109

Mt

Mei

tner

ium

(266

)

110

(269

)

111

(272

)

112

(277

)

113

114

58 Ce

Ceriu

m14

0.11

6

59 PrPr

aseo

dym

ium

140.

9076

5

60 Nd

Neo

dym

ium

144.

24

61 PmPr

omet

hium

(145

)

62 Sm Sam

ariu

m15

0.36

63 Eu Euro

pium

151.

964

64 Gd

Gad

olin

ium

157.

25

65 Tb Terb

ium

158.

9253

4

66 Dy

Dys

pros

ium

162.

50

67 Ho

Hol

miu

m16

4.93

032

68 Er Erbi

um16

7.26

69 Tm Thul

ium

168.

9342

1

70 Yb

Ytte

rbiu

m17

3.04

71 Lu Lute

tium

174.

967

90 Th Thor

ium

232.

0381

91 PaPr

otac

tiniu

m23

1.03

588

92 UU

rani

um23

8.02

89

93 Np

Nep

tuni

um(2

37)

94 PuPl

uton

ium

(244

)

95 Am

Am

eric

ium

(243

)

96 Cm

Cur

ium

(247

)

97 BkBe

rkel

ium

(247

)

98 Cf

Calif

orni

um(2

51)

99 EsEi

nste

iniu

m(2

52)

100

Fm Ferm

ium

(257

)

101

Md

Men

dele

vium

(258

)

102

No

Nob

eliu

m(2

59)

103

LrLa

wre

nciu

m(2

62)

1995

IUPA

C m

asse

s and

App

rove

d N

ames

from

http

://w

ww

.che

m.q

mw

.ac.

uk/iu

pac/

AtW

t/m

asse

s for

107

-111

from

C&

EN, M

arch

13,

199

5, p

. 35

112

from

http

://w

ww

.gsi.

de/z

112e

.htm

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