11/22/2016
Homework 9 Chapter 09
Homework 9 Chapter 09
Due: 11:59pm on Wednesday, November 23, 2016
You will receive no credit for items you complete after the assignment is due. Grading Policy
Molecular Geometry II
The molecular geometry of a molecule describes the three­dimensional shape of just the atoms. This is in contrast to the electronic geometry, which describes the
shape of all regions of high electron density. The number of regions of high electron density, or steric number, determines the shape of the molecule.
Part A
What is the molecular geometry of a molecule with 2 outer atoms and 1 lone pair on the central atom?
Enter the molecular geometry of the molecule.
Hint 1. Determine the electronic geometry
What is the electronic geometry of this molecule?
ANSWER:
linear
trigonal bipyramidal
octahedral
trigonal planar
tetrahedral
ANSWER:
bent
Correct
Part B
What is the molecular geometry of a molecule with 4 outer atoms and 2 lone pairs on the central atom?
Enter the molecular geometry of the molecule.
Hint 1. Determine the electronic geometry
What is the electronic geometry of this molecule?
ANSWER:
linear
trigonal planar
octahedral
trigonal bipyramidal
tetrahedral
ANSWER:
square planar
Correct
Part C
What is the molecular geometry of a molecule with 2 outer atoms and 3 lone pairs on the central atom?
Enter the molecular geometry of the molecule.
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Hint 1. Determine the electronic geometry
What is the electronic geometry of this molecule?
ANSWER:
linear
trigonal planar
tetrahedral
octahedral
trigonal bipyramidal
ANSWER:
linear
Correct
Sample Exercise 9.1 Practice Exercise 1 with feedback
Part A ­ Using the VSEPR Model
Consider the following \rm AB_3 molecules and ions: \rm PCl_3, \rm SO_3, \rm AlCl_3, \rm SO_3{}^{2­} and \rm CH_3{}^+. How many of these molecules and ions
do you predict to have a trigonal­planar molecular geometry?
ANSWER:
1
2
3
4
5
Correct
For \rm AB_3 molecules that have an octet or fewer of electrons around \rm A, there are two possible molecular geometries. Those with three electron
domains will exhibit a trigonal­planar molecular geometry whereas those with four electron domains will exhibit a trigonal­pyramidal geometry. Of the
molecules and ions listed, \rm SO_3, \rm AlCl_3, and \rm CH_3{}^+ have three electron domains and will therefore exhibit a trigonal­planar geometry. The
other two will be trigonal pyramidal.
Sample Exercise 9.2 Practice Exercise 1 with feedback
Part A ­ Molecular Geometries of Molecules with Expanded Valence Shells
A certain \rm AB_4 molecule has a square­planar molecular geometry. Which of the following statements about the molecule is or are true?
I. The molecule has four electron domains about the central atom \rm A.
II. The \rm B\!­\!A\!­\!B angles between neighboring \rm B atoms is 90^\circ.
III. The molecule has two nonbonding pairs of electrons on atom \rm A.
ANSWER:
Only one of the statements is true
Statements I and II are true
Statements I and III are true
Statements II and III are true
All three statements are true
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Homework 9 Chapter 09
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To form the square shape for a square­planar molecular geometry, the bond angles between neighboring \rm B atoms is 90^\circ. A square­planar geometry
can result only from an octahedral electron­domain geometry in which there are two nonbonding domains. Hence the molecule must have six electron domains
and two of them must be nonbonding, so statement I is false and statements II and III is true.
Problem 9.42 with feedback
You may want to reference (
pages 356 ­ 357) Section 9.3 while completing this problem.
Part A
Predict whether each of the following molecules is polar or nonpolar.
Drag the appropriate items to their respective bins.
ANSWER:
Correct
Polar molecules require an asymmetrical geometry and polar bonds. The trigonal pyramidal geometry of \rm NH_3 and seesaw geometry of \rm SF_4 meet
these requirements. Although the tetrahedral geometry is typically symmetrical, it becomes asymmetrical when the atoms attached to the central atom are
not identical, which is why \rm CH_3Br is polar.
A completely symmetrical molecule would be nonpolar whether or not it contains polar bonds. Molecules with planar geometries are therefore nonpolar: \rm
GaH_3 (trigonal planar) and \rm XeF_4 (square planar). The tetrahedral geometry is symmetrical when all attached atoms are identical, which is why \rm
CCl_4 is nonpolar.
All of these molecules are neutral, so you would identify their molecular geometries by determining how many bonding and lone­pair domains each central
atom had (given that the attached atoms also have no formal charge in each case). For example, \rm GaH_3 has a valence number of 3 (like other Group IIIA
elements), so you can conclude that it has three single bonds and zero lone pairs. The central atom in \rm SF_4 is a Group VIA element with a valence
number of 6, sp you can conclude it has four single bonds and one lone pair.
Formation of a Chemical Bond
Learning Goal:
To understand how a chemical bond is formed.
Chemical bonds hold atoms together in molecules. A chemical bond is a strong electrostatic attraction between two or more atoms. It is formed either by a transfer of
electrons or by a sharing of electrons such that each participating atom approaches a near­noble­gas configuration.
A bond formed by a transfer of electrons is called an ionic bond. A bond formed by a sharing of electrons is called a covalent bond. A coordinate covalent bond is
formed when an electron­rich atom donates a pair of electrons to an electron­deficient atom.
Part A
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Homework 9 Chapter 09
When two hydrogen atoms approach each other to form a chemical bond, different interactions occur between the atoms. Identify the correct statements with
respect to the interactions involved in the formation of chemical bond.
Check all that apply.
Hint 1. Interactions between the two hydrogen atoms
There are three sets of interactions in the formation of a chemical bond:
1. repulsion between the two electrons,
2. repulsion between the two nuclei, and
3. attraction between each electron and the other nucleus.
The formation of a chemical bond between two hydrogen atoms can be represented as follows:
ANSWER:
When two atoms of \rm H approach each other, each electron is attracted by the other nucleus.
When two atoms of \rm H approach each other, the electrons attract each other.
When two atoms of \rm H approach each other, the two nuclei repel each other.
When two atoms of \rm H approach each other, each electron repels the other nucleus.
When two atoms of \rm H are infinitely far apart, they show attractive interactions.
Correct
Imagine bringing together two atoms of \rm H that are initially very far apart. By convention, their potential energy is set to zero because there is no interaction
between them. As the two atoms approach each other, the following interactions occur:
1. Each electron is attracted to the other nucleus,
2. the two electrons repel each other, and
3. the two nuclei repel each other.
For the formation of a chemical bond between the two hydrogen atoms to occur, the attractive interactions must overcome the repulsive interactions. Since
there is equal attraction for both the electrons from both the nuclei, the electron pair will lie in between the two nuclei, resulting in the formation of a covalent
bond.
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Homework 9 Chapter 09
The potential energy curve for the formation of a hydrogen molecule
The changes in the potential energy associated with the formation of a hydrogen molecule from individual hydrogen atoms is represented by the potential energy curve.
Initially, when the two hydrogen atoms are far apart, there is no interaction between them. However, as the two atoms come closer together, attraction and repulsion
between the two atoms occur. For the formation of a chemical bond to occur, the attractive forces must overcome the repulsive forces. As the attractive force
increases, the internuclear distance decreases and the potential energy of the system decreases. At the point where the potential energy is at a minimum, the
attractive forces are balanced by the repulsive forces and a stable hydrogen molecule is formed. The internuclear distance corresponding to this point is called the
bond length. For hydrogen molecules, bond length is equal to 74 \rm pm.
If the internuclear distance between the hydrogen atoms decreases further, then this leads to repulsion between the atoms, thereby increasing the potential energy of
the system and creating instability within the molecule.
Part B
Two hydrogen atoms interact to form a hydrogen molecule. Classify the following statements that describe the stages of bond formation in a hydrogen molecule
according to the predominant force existing between the two hydrogen atoms.
Drag the appropriate items to their respective bins.
Hint 1. How to approach the problem
To determine the predominant force existing between the two hydrogen atoms, you need to determine the change in potential energy of the system at the
particular point in the given potential energy curve. When there is no interaction between the two atoms as they are far apart, the potential energy of the
system is zero. The decrease in potential energy of the system corresponds to a net attractive force between the hydrogen atoms, whereas the increase in
potential energy of the system corresponds to a net repulsive force between the hydrogen atoms.
Hint 2. Identify the change in potential energy as the hydrogen atoms interact
Observe the potential energy curve for the formation of hydrogen molecule.
ANSWER:
remains constant
As the two hydrogen atoms approach each other to form a hydrogen molecule, the potential energy of the system decreases
.
increases
Hint 3. Identify the change in potential energy after the bond is formed
Observe the potential energy curve for the formation of a hydrogen molecule. When a bond is formed, the internuclear distance between the two hydrogen
atoms equals the bond length.
ANSWER:
suddenly increases
If the two atomic nuclei of the hydrogen atoms are further compressed, the potential energy of the system suddenly decreases
.
becomes zero
ANSWER:
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Homework 9 Chapter 09
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By looking at the plot of the energy of interaction for two atoms of \rm H versus the internuclear distance from zero to infinity, you can observe the following:
1. The potential energy is zero when the two atoms of \rm H are infinitely separated and no force exists between the atoms.
2. Where the curve slopes downward from right to left, the attractive interaction is predominant, as indicated by the decrease in potential energy.
3. Where the curve reaches a minimum, the molecule is most stable at this internuclear distance (74 \rm pm).
4. Where the curve slopes upward from right to left, the repulsive interaction is predominant, as indicated by the increase in potential energy.
Part C
Bond length is the distance between the centers of two bonded atoms. On the potential energy curve, the bond length is the internuclear distance between the two
atoms when the potential energy of the system reaches its lowest value.
Given that the atomic radii of \rm H and \rm Br are 25.0 \rm pm and 115 {\rm pm} , respectively, predict the bond length of the \rm HBr molecule.
Express your answer to three significant figures and include the appropriate units.
Hint 1. How to approach the problem
Bond length is the distance between the centers of two atoms joined by a covalent bond. To estimate the bond length of the given molecule, you need to
calculate the total distance between the centers of the two atoms given their atomic radii.
For example, consider the following molecule, \rm AB. The atomic radii of \rm A is \rm {\it r}(A) and \rm B is \rm {\it r}(B). The predicted bond length of the
molecule will be equal to the sum of \rm {\it r}(A) and \rm {\it r}(B).
Hint 2. Identify the formula to determine the bond length
Bond length of the \rm HBr molecule can be determined from the atomic radii of \rm H and \rm Br. Consider that the atomic radius of \rm H is \rm {\it r}(H)
and the atomic radius of \rm Br is \rm {\it r}(\rm Br). Identify the correct formula to determine the bond length of the \rm HBr molecule.
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Homework 9 Chapter 09
ANSWER:
\rm Bond~length = {\it r}(\rm Br)~­~{\it r}(H)
\rm Bond~length ={\it r}(H)~­~{\it r}(\rm Br)
\large{\rm Bond~length =\frac{\rm {\it r}(H)~+~{\it r}(\rm Br )}{2}}
\rm Bond~length = {\it r}(H)~+~{\it r}(\rm Br)
ANSWER:
Bond length = 140 {\rm pm}
Correct
The actual bond length for \rm HBr might be different from the estimated bond length. As the bonded atoms experience some vibrations while moving toward
and away from each other, the distance between bonded atoms will vary slightly over a period of time.
The term bond length specifically refers to the average positions of the two atoms during the harmonic vibrations that they undergo.
Problem 9.6
The orbital diagram that follows presents the final step in the formation of hybrid orbitals by a silicon atom.
Part A
Which of the following best describes what took place before the step pictured in the diagram?
ANSWER:
An electron was promoted from the 3s orbital to the 3p orbital.
Two 3p electrons became unpaired.
An electron was promoted from the 2p orbital to the 3s orbital.
Correct
Part B
What type of hybrid orbital is produced in this hybridization?
ANSWER:
sp
sp^2
sp^3
sp^3d
sp^3d^2
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Homework 9 Chapter 09
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Orbital Overlap: Sigma and Pi Bonding
When two atoms form a single covalent bond, two orbitals (one from each atom) overlap such that the electron pair can be in both orbitals simultaneously. Double and
triple bonds involve the sharing of multiple electron pairs. Each additional electron pair requires the overlap of another set of orbitals. A set of overlapping orbitals is
called a \sigma bond (sigma bond) if the overlap is head­on and a \pi bond (pi bond) if the overlap is sideways.
Part A
How many \sigma and \pi bonds are present in a molecule of cumulene?
Enter the number of \sigma bonds followed by the number of \pi bonds separated by a comma.
Hint 1. Identify the makeup of single, double, and triple bonds
Which of the following best describes the makeup of each type of bond?
Match the words in the left­hand column to the appropriate blanks in the sentences on the right. Make certain each sentence is complete before
submitting your answer.
Hint 1. The orientation of sigma bonds and pi bonds
Sigma bonds result from head­on overlap between two atoms. A pi bond results from the sideways overlap between two p orbitals.
ANSWER:
Reset
Help
1. A single bond consists of one sigma bond .
one pi bond
2. A double bond consists of one sigma bond and one pi bond .
two sigma bonds and one
pi bond
3. A triple bond consists of one sigma bond and two pi bonds .
two pi bonds
ANSWER:
7,3
Correct
Each \pi bond is rotated 90 degrees from any adjacent \pi bonds, which makes this molecule planar.
Part B
What types of orbital overlap occur in cumulene?
Check all that apply.
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Hint 1. The difference between orbital overlap in \sigma bonds and \pi bonds
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Homework 9 Chapter 09
When orbitals overlap in a head­to­head fashion, a \sigma bond results. When p orbitals overlap in a sideways fashion a \pi bond results. A \sigma bond
may form between s or p orbitals or sp, sp^2, or sp^3 hybrid orbitals. A \pi bond forms between unhybridized p orbitals. Consider that cumulene forms both
\sigma bonds and \pi bonds.
Hint 2. Determine the type of orbital used by hydrogen atoms for bonding
In any hydrogen­containing molecule, what type of orbitals are used by the hydrogen atoms for bonding?
ANSWER:
s
p
d
sp
sp^2
sp^3
Hint 3. Determine the type of orbitals used by the outer carbon atoms for bonding
In cumulene, what type of hybrid orbitals are used by the first and fourth carbon atoms for bonding?
Hint 1. How to determine hybridization
The type of hybrid orbitals used by an atom is directly related to the number of "things" that atom is bonded to, also known as the steric number,
number of electron domains, or the number of electron clouds.
Steric number Hybridization
2
sp
3
sp^2
4
sp^3
5
sp^3d
6
sp^3d^2
Determine the steric number of each atom in question by counting the number of "things" it is bonded to, then relate that to the hybridization.
ANSWER:
sp
sp^2
sp^3
sp^3d
sp^3d^2
Hint 4. Determine the type of orbitals used by the inner carbon atoms for bonding
In cumulene, what type of orbitals are used by the second and third carbon atoms for bonding?
Hint 1. How to determine hybridization
The type of hybrid orbitals used by an atom is directly related to the number of "things" that atom is bonded to, also known as the steric number,
number of electron domains, or the number of electron clouds.
Steric number Hybridization
2
sp
3
sp^2
4
sp^3
5
sp^3d
6
sp^3d^2
Determine the steric number of each atom in question by counting the number of "things" it is bonded to, then relate that to the hybridization.
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ANSWER:
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Homework 9 Chapter 09
sp
sp^2
sp^3
sp^3d
sp^3d^2
Hint 5. Determine the role of the p orbitals
Consider an sp^2 hybridized carbon atom. One s orbital and two p orbitals combine to form three sp^2 hybrid orbitals, leaving one atomic p orbital. Will that
p orbital be involved in bonding between carbon and another atom?
ANSWER:
No, because it does not contain any electrons.
Yes, because it contains one electron.
No, because it contains two electrons.
ANSWER:
sp^2/sp^2 overlap
sp/sp^2 overlap
s/sp overlap
s/sp^2 overlap
s/s overlap
sp/sp overlap
p/p overlap
Correct
Each \pi bond results from the sideways overlap of two p orbitals. All other overlap is head­on, resulting in \sigma bonds.
Part C
In cumulene, what are the \rm C\!\!=\!\!C\!\!=\!\!C and \rm H\!\!­\!\!C\!\!­\!\!H bond angles, respectively?
Enter the \rm C\!\!=\!\!C\!\!=\!\!C bond angle followed by the \rm H\!\!­\!\!C\!\!­\!\!H bond angle separated by a comma.
Hint 1. How to approach the problem
We saw in Part B that the inner carbon atoms in cumulene use sp hybrid orbitals for bonding. Those inner carbon atoms are at the centers of the \rm
C\!\!=\!\!C\!\!=\!\!C bonds. Identify the geometry associated with sp hybrid orbitals to determine the \rm C\!\!=\!\!C\!\!=\!\!C bond angles in cumulene.
We also saw in Part B that the outer carbon atoms in cumulene use sp^2 hybrid orbitals for bonding. Those outer carbon atoms are at the apex of the \rm
H\!\!­\!\!C\!\!­\!\!H bond angles. Identify the geometry associated with sp^2 hybrid orbitals to determine the \rm H\!\!­\!\!C\!\!­\!\!H bond angles in cumulene.
Hint 2. Identify the geometry associated with sp hybrid orbitals
What shape is associated with sp hybridized orbitals, like those associated with the inner carbon atoms in cumulene?
ANSWER:
linear
trigonal planar
tetrahedral
trigonal bipyramidal
octahedral
Hint 3. Identify the geometry associated with sp^2 hybrid orbitals
What shape is associated with sp^2 hybridized orbitals, like those associated with the outer carbon atoms in cumulene?
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Homework 9 Chapter 09
linear
trigonal planar
tetrahedral
trigonal bipyramidal
octahedral
ANSWER:
180,119 degrees Correct
Problem 9.64
Part A
Write a single Lewis structure for {\rm SO}_{3}.
Draw the Lewis dot structure for {\rm SO}_{3}. Include all lone pairs of electrons.
ANSWER:
Correct
Part B
Determine the hybridization at the {\rm S} atom.
ANSWER:
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Homework 9 Chapter 09
sp
sp^2
sp^3
sp^3d
sp^3d^2
Correct
Part C
Are there other equivalent Lewis structures for the molecule?
ANSWER:
There are other equivalent Lewis structures for {\rm SO_3}.
There are no other equivalent Lewis structures for {\rm SO_3}.
Correct
Part D
Would you expect {\rm SO}_{3} to exhibit delocalized \pi bonding?
ANSWER:
sp^2 hybridization indicates delocalized \pi bonding.
The multiple resonance structures indicate no delocalized \pi bonding.
sp^2 hybridization indicates no delocalized \pi bonding.
The multiple resonance structures indicate delocalized \pi bonding.
Correct
Problem 9.65
In the formate ion, {\rm HCO_2}^ ­ , the carbon atom is the central atom with the other three atoms attached to it.
Part A
Draw a Lewis structure for the formate ion.
Draw the Lewis dot structure for {\rm HCO_2}^ ­. Include all lone pairs of electrons. Show the formal charges of all atoms in the correct structure.
ANSWER:
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Part B
What hybridization is exhibited by the {\rm C} atom?
ANSWER:
sp
sp^2
sp^3
sp^3d
sp^3d^2
Correct
Part C
Are there multiple equivalent resonance structures for the ion?
ANSWER:
Yes, there is one other resonance structure.
Yes, there are two other resonance structures.
Yes, there are three other resonance structures.
No, there are no other resonance structures.
Correct
Part D
Which of the atoms in the ion have p_\pi orbitals?
Choose all that apply.
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Homework 9 Chapter 09
ANSWER:
{\rm C} atom
both {\rm O} atoms
{\rm H} atom
Correct
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