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Chapter 6

Chemical Bonding II

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Problem 1

Why do we use other bonding theories in addition to the Lewis model?

Joshua S.
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Problem 2

What is a chemical bond according to valence bond theory?

Joshua S.
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Problem 3

In valence bond theory, what determines the geometry of a molecule?

Joshua S.
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Problem 4

In valence bond theory, the interaction energy between the electrons and nucleus of one atom with the electrons and nucleus of another atom is usually negative (stabilizing) when ______________.

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Problem 5

What is hybridization? Why is hybridization necessary in valence bond theory?

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Problem 6

How does hybridization of the atomic orbitals in the central atom of a molecule help lower the overall energy of the molecule?

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Problem 7

How is the number of hybrid orbitals related to the number of standard atomic orbitals that are hybridized?

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Problem 8

Sketch each hybrid orbital.
a. sp
b. sp2
c. sp3
d. sp3d
e. sp3d2

Joshua S.
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Problem 9

In the Lewis model, the two bonds in a double bond look identical. However, valence bond theory shows that they are not. Describe a double bond according to valence bond theory. Explain why rotation is restricted about a double bond but not about a single bond.

Joshua S.
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Problem 10

Name the hybridization scheme that corresponds to each electron geometry.
a. linear
b. trigonal planar
c. tetrahedral
d. trigonal bipyramidal
e. octahedral

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Problem 11

What is a chemical bond according to molecular orbital theory?

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Problem 12

Explain the difference between hybrid atomic orbitals in valencebond theory and LCAO molecular orbitals in molecular orbital theory.

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Problem 13

What is a bonding molecular orbital?

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Problem 14

What is an antibonding molecular orbital?

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Problem 15

What is the role of wave interference in determining whether a molecular orbital is bonding or antibonding?

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Problem 16

In molecular orbital theory, what is bond order? Why is it important?

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Problem 17

How is the number of molecular orbitals approximated by a linear combination of atomic orbitals related to the number of atomic orbitals used in the approximation?

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Problem 18

Sketch each molecular orbital.
a. s2s
b. s*2s
c. s2p
d. s*2p
e. p2p
f. p*2p

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Problem 19

Draw an energy diagram for the molecular orbitals of period 2 diatomic molecules. Show the difference in ordering for B2, C2, and N2 compared to O2, F2, and Ne2.

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Problem 20

Why does the energy ordering of the molecular orbitals of the second-period diatomic molecules change in going from N2 to O2?

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Problem 21

Explain the difference between a paramagnetic species and a diamagnetic one.

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Problem 22

When applying molecular orbital theory to heteronuclear diatomic molecules, the atomic orbitals used may be of different energies. If two atomic orbitals of different energies make two molecular orbitals, how are the energies of the molecular orbitals related to the energies of the atomic orbitals? How is the shape of the resultant molecular orbitals related to the shapes of the atomic orbitals?

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Problem 23

In molecular orbital theory, what is a nonbonding orbital?

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Problem 24

Write a short paragraph describing chemical bonding according to the Lewis model, valence bond theory, and molecular orbital theory. Indicate how the theories differ in their description of a chemical bond and indicate the strengths and weaknesses of each theory. Which theory is correct?

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Problem 25

The valence electron configurations of several atoms are shown below. How many bonds can each atom make without hybridization?
a. Be 2s2
b. P 3s23p3
c. F 2s22p5

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Problem 26

The valence electron configurations of several atoms are shown below. How many bonds can each atom make without hybridization?
a. B 2s22p1
b. N 2s22p3
c. O 2s22p4

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Problem 27

Draw orbital diagrams (boxes with arrows in them) to represent the electron configurations-without hybridization-for all the atoms in PH3. Circle the electrons involved in bonding. Draw a three-dimensional sketch of the molecule and show orbital overlap. What bond angle do you expect from the unhybridized orbitals? How well does valence bond theory agree with the experimentally measured bond angle of 93.3?

Joshua S.
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Problem 28

Draw orbital diagrams (boxes with arrows in them) to represent the electron configurations-without hybridization-for all the atoms in SF2. Circle the electrons involved in bonding. Draw a three-dimensional sketch of the molecule and show orbital overlap. What bond angle do you expect from the unhybridized orbitals? How well does valence bond theory agree with the experimentally measured bond angle of 98.2?

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Problem 29

Draw orbital diagrams (boxes with arrows in them) to represent the electron configuration of carbon before and after sp3 hybridization.

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Problem 30

Draw orbital diagrams (boxes with arrows in them) to represent the electron configurations of carbon before and after sp hybridization.

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Problem 31

Which hybridization scheme allows the formation of at least one pi bond?

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Problem 32

Which hybridization scheme allows the central atom to form more than four bonds?

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Problem 33

Write a hybridization and bonding scheme for each molecule. Sketch the molecule, including overlapping orbitals, and label all bonds using the notation shown in Examples 6.1 and 6.2.
a. CCl4
b. NH3
c. OF2
d. CO2

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Problem 34

Write a hybridization and bonding scheme for each molecule. Sketch the molecule, including overlapping orbitals, and label all bonds using the notation shown in Examples 6.1 and 6.2.
a. CH2Br2
b. SO2
c. NF3
d. BF3

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Problem 35

Write a hybridization and bonding scheme for each molecule or ion. Sketch the structure, including overlapping orbitals, and label all bonds using the notation shown in Examples 6.1 and 6.2.
a. COCl2 (carbon is the central atom)
b. BrF5
c. XeF2
d. I3

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Problem 36

Write a hybridization and bonding scheme for each molecule or ion. Sketch the structure, including overlapping orbitals, and label all bonds using the notation shown in Examples 6.1 and 6.2.
a. SO3 2-
b. PF6-
c. BrF3
d. HCN

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Problem 37

Write a hybridization and bonding scheme for each molecule that contains more than one interior atom. Indicate the hybridization about each interior atom. Sketch the structure, including overlapping orbitals, and label all bonds using the notation shown in Examples 6.1 and 6.2.
a. N2H2 (skeletal structure HNNH)
b. N2H4 (skeletal structure H2NNH2)
c. CH3NH2 (skeletal structure H3CNH2)

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Problem 38

Write a hybridization and bonding scheme for each molecule that contains more than one interior atom. Indicate the hybridization about each interior atom. Sketch the structure, including overlapping orbitals, and label all bonds using the notation shown in Examples 6.1 and 6.2.
a. C2H2 (skeletal structure HCCH)
b. C2H4 (skeletal structure H2CCH2)
c. C2H6 (skeletal structure H3CCH3)

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Problem 39

Consider the structure of the amino acid alanine. Indicate the hybridization about each interior atom.

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Problem 40

Consider the structure of the amino acid aspartic acid. Indicate the hybridization about each interior atom.

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Problem 41

Sketch the bonding molecular orbital that results from the linear combination of two 1s orbitals. Indicate the region where interference occurs and state the kind of interference (constructive or destructive).

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Problem 42

Sketch the antibonding molecular orbital that results from the linear combination of two 1s orbitals. Indicate the region where interference occurs and state the kind of interference (constructive or destructive).

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Problem 43

Draw an MO energy diagram and predict the bond order of Be2+ and Be2-. Do you expect these molecules to exist in the gas phase?

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Problem 44

Draw an MO energy diagram and predict the bond order of Li2+and Li2-. Do you expect these molecules to exist in the gas phase?

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Problem 45

Sketch the bonding and antibonding molecular orbitals that result from linear combinations of the 2px atomic orbitals in a homonuclear diatomic molecule. (The 2px orbitals are those whose lobes are oriented along the bonding axis.)

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Problem 46

Sketch the bonding and antibonding molecular orbitals that result from linear combinations of the 2pz atomic orbitals in a homonuclear diatomic molecule. (The 2pz orbitals are those whose lobes are oriented perpendicular to the bonding axis.) How do these molecular orbitals differ from those obtained from linear combinations of the 2py atomic orbitals? (The 2py orbitals are also oriented perpendicular not only to the bonding axis, but also to the 2pz orbitals.)

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Problem 47

Using the molecular orbital energy ordering for second-period homonuclear diatomic molecules in which the p2p orbitals lie at lower energy than the s2p, draw MO energy diagrams and predict the bond order in a molecule or ion with each number of total valence electrons. Will the molecule or ion be diamagnetic or
paramagnetic?
a. 4
b. 6
c. 8
d. 9

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Problem 48

Using the molecular orbital energy ordering for second-period homonuclear diatomic molecules in which the p2p orbitals lie at higher energy than the s2p, draw MO energy diagrams and predict the bond order in a molecule or ion with each number of total valence electrons. Will the molecule or ion be diamagnetic
or paramagnetic?
a. 10
b. 12
c. 13
d. 14

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Problem 49

Apply molecular orbital theory to predict if each molecule or ion exists in a relatively stable form.
a. H2 2-
b. Ne2
c. He2 2+
d. F2 2-

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Problem 50

Apply molecular orbital theory to predict if each molecule or ion exists in a relatively stable form.
a. C2 2+
b. Li2
c. Be2 2+
d. Li2 2-

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Problem 51

According to MO theory, which molecule or ion has the highest bond order? Highest bond energy? Shortest bond length?

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Problem 52

According to MO theory, which molecule or ion has the highest bond order? Highest bond energy? Shortest bond length?

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Problem 53

Draw an MO energy diagram for CO. (Use the energy ordering of O2.) Predict the bond order and make a sketch of the lowest energy bonding molecular orbital.

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Problem 54

Draw an MO energy diagram for HCl. Predict the bond order and make a sketch of the lowest energy bonding molecular orbital.

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Problem 55

For each compound, draw the Lewis structure, determine the geometry using VSEPR theory, determine whether the molecule is polar, identify the hybridization of all interior atoms, and make a sketch of the molecule, according to valence bond theory, showing orbital overlap.
a. COF2 (carbon is the central atom)
b. S2Cl2 (ClSSCl)
c. SF4

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Problem 56

For each compound, draw the Lewis structure, determine the geometry using VSEPR theory, determine whether the molecule is polar, identify the hybridization of all interior atoms, and make a sketch of the molecule, according to valence bond theory, showing orbital overlap.
a. IF5
b. CH2CHCH3
c. CH3SH

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Problem 57

Amino acids are biological compounds that link together to form proteins, the workhorse molecules in living organisms. The skeletal structures of several simple amino acids are shown here. For each skeletal structure, determine the hybridization about each interior atom.

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Problem 58

The genetic code is based on four different bases with the structures shown here. Determine the hybridization in each interior atom in these four bases.
a. cytosine
b. adenine
c. thymine
d. guanine

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Problem 59

The structure of caffeine, present in coffee and many soft drinks, is shown here. How many pi bonds are present in caffeine? How many sigma bonds? Insert the lone pairs in the molecule. Which kinds of orbitals do the lone pairs occupy?

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Problem 60

The structure of acetylsalicylic acid (aspirin) is shown here. How many pi bonds are present in acetylsalicylic acid? How many sigma bonds? Which parts of the molecule are free to rotate? Which parts are rigid?

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Problem 61

Draw a molecular orbital energy diagram for ClF. (Assume that the sp orbitals are lower in energy than the  orbitals.) What is the bond order in ClF?

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Problem 62

Draw Lewis structures and MO diagrams for CN+, CN, and CN-. According to the Lewis model, which species is most stable? According to MO theory, which species is most stable? Do the two theories agree?

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Problem 63

Bromine can form compounds or ions with any number of fluorine atoms from one to five. Write the formulas of all five of these species, assign a hybridization, and describe their electron and molecular geometry.

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Problem 64

The compound C3H4 has two double bonds. Describe its bonding and geometry, using a valence bond approach.

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Problem 65

How many hybrid orbitals do we use to describe each molecule?
a. N2O5
b. C2H5NO (4 C-H bonds and one O-H bond)
c. BrCN (no formal charges)

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Problem 66

Indicate which orbitals overlap to form the s bonds in each compound.
a. BeBr2
b. HgCl2
c. ICN

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Problem 67

In VSEPR theory, which uses the Lewis model to determine molecular geometry, the trend of decreasing bond angle in CH4, NH3, and H2O is accounted for by the greater repulsion of lone pair electrons compared to bonding pair electrons. How is this trend accounted for in valence bond theory?

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Problem 68

The results of a molecular orbital calculation for H2O are shown here. Examine each of the orbitals and classify them as bonding, antibonding, or nonbonding. Assign the correct number of electrons to the energy diagram. According to this energy diagram, is H2O stable? Explain.

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Problem 69

The results of a molecular orbital calculation for NH3 are shown here. Examine each of the orbitals and classify them as bonding, antibonding, or nonbonding. Assign the correct number of electrons to the energy diagram. According to this energy diagram, is NH3 stable? Explain.

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Problem 70

cis-2-Butene isomerizes (changes its structure) to trans-2-butene via the reaction:

a. If isomerization requires breaking the pi bond, what minimum energy is required for isomerization in J>mol? In J>molecule?
b. If the energy for isomerization comes from light, what minimum frequency of light is required? In what portion of the electromagnetic spectrum does this frequency lie?

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Problem 71

The ion CH5+ can form under very special high-energy conditions in the vapor phase in a mass spectrometer. Propose a hybridization for the carbon atom and predict the geometry

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Problem 72

Neither the VSEPR model nor the hybridization model is able to account for the experimental observation that the F-Ba-F bond angle in gaseous BaF2 is 108 rather than the predicted 180. Suggest some possible explanations for this observation.

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Problem 73

How does each of the three major bonding theories (the Lewis model, valence bond theory, and molecular orbital theory) define a single chemical bond? A double bond? A triple bond? How are these definitions similar? How are they different?

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Problem 74

The most stable forms of the nonmetals in groups 4A, 5A, and 6A of the first period are molecules with multiple bonds. Beginning with the second period, the most stable forms of the nonmetals of these groups are molecules without multiple bonds. Propose an explanation for this observation based on
valence bond theory.

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Problem 75

Consider the bond energies of three iodine halides

How might you use valence bond theory to help explain this trend?

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Problem 76

How many atomic orbitals form a set of sp3 hybrid orbitals? A set of sp2 hybrid orbitals? A set of sp hybrid orbitals? What is the relationship between these numbers and the number of electron groups around the central atom?

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Problem 77

Have each group member pick one of these hybridization schemes (sp, sp2, and sp3) and describe it to the other group members. In the description, include a drawing of the orbitals in the hybridization scheme and indicate how to determine if that hybridization scheme is the correct one for a particular atom in a molecule.

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Problem 78

Divide your group into two subgroups. Have one subgroup use molecular orbital theory to explain bonding in N2+ and the other in N2-. Then have the entire group describe how the bond strengths in these ions are different from those of the bonds in neutral N2.

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Problem 79

A molecular orbital calculation for H2 results in the molecular orbital diagram shown here. The energy listed next to each orbital in the diagram corresponds to the energy of an electron in that orbital according to the calculation. Study the diagram and answer the questions that follow.
a. What is the ionization energy of H in kJ>mol?
b. Based on the calculation, what is the ionization energy of H2 in kJ>mol?
c. Based on the calculation, what is the bond energy for H2 in kJ>mol? (Hint: The bond energy is the difference between the energies of the two electrons when they are in H atoms and the energies of the two electrons when they are in H2.)
d. Look up the bond energy for H2 in Table 5.3. How does the calculated bond energy compare to the experimentally measured bond energy? Calculate the percent difference between the calculated bond energy and the experimentally measured bond energy.

Joshua S.
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