3.3 MOLECULAR SHAPES AND DIPOLES
Career Connection: Biochemist
(Page 91)
Biochemistry is offered at most accredited universities. Other disciplines that are part of the
program include organic chemistry and biology.
Mini Investigation: Electrostatic Repulsion Model
(Page 92)
(a) Two balloons are at about 180° to each other (a linear arrangement).
(b) Three balloons are at about 120° to each other and all in one plane (a trigonal planar
arrangement).
(c) Four balloons are at about 109° to each other with each one pointing to one corner of a
tetrahedron (a tetrahedral arrangement).
(d) Balloons let you visualize electron orbitals and how they might be arranged. The electrostatic
repulsion is modelled by the balloons pushing equally against each other. A disadvantage of
the balloon model is that electron orbitals are not physical objects of a fixed size. A balloon
only contacts or “repels” where it touches another balloon. Electrostatic repulsion occurs to
varying extents between all electrons anywhere in the two nearby orbitals. In other words, the
repulsion is much more complicated than implied by the two touching balloons.
Practice
(Page 96)
1. VSEPR is an acronym for valence shell electron pair repulsion—a theory that predicts
molecular shape by assuming that repulsion between all pairs of electrons in the valence shell
of an atom controls the direction of those pairs formed by bonding, and thus determines the
shapes of molecules.
2. (a) [Note: While the electronegativities of beryllium and chlorine differ by only 1.6,
beryllium chloride is not, according to its physical properties, a molecular compound.
You may, therefore, choose to have students change the formula for beryllium chloride to
the formula for chlorine(I) oxide, Cl2O(g) in their textbooks. Both solutions are provided
here.]
linear (Be does not obey octet rule)
angular (V-shaped)
(b)
trigonal pyramidal
(c)
angular (V-shaped)
(d)
trigonal planar (B does not obey octet rule)
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(e)
tetrahedral
(f)
3. (a)
(b)
linear
PO43– will be tetrahedral in shape because it has four bond pairs around the P atom.
BrO3– will be pyramidal in shape, having three bond pairs and one lone pair around the
Br atom.
(c)
NH4+ will be tetrahedral in shape because it has four bond pairs around the N atom.
4. (a) According to VSEPR theory, the shape around each carbon atom in cubane should be
tetrahedral, since there are four bond pairs around each carbon atom.
(b) If we assume an ideal cubic shape, the bond angles around each carbon have to be 90°.
(c) The normal tetrahedral angle is about 109°. To make these bonding orbitals bend to about
90° would greatly increase the repulsion of the electron pairs. This stress likely makes
this molecule very unstable.
5. VSEPR theory was created to explain known molecular shapes. It provided a simpler way to
explain shapes and to predict molecular shapes.
Practice
(Page 98)
6. To make the rules of VSEPR theory work, multiple bonds must be treated just like single
bonds; that is, they are considered to be one bond, which involves 4 or 6 electrons
respectively, if the bond is a double or triple bond.
7. (a)
linear
(b)
linear
(c)
trigonal planar—first two carbons
tetrahedral—third carbon
(d)
linear—first two carbons
tetrahedral—third carbon
(e)
tetrahedral – end carbons
trigonal planar – centre carbon
(f)
linear
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8. VSEPR is a very successful scientific theory. It successfully predicts the shapes of most
molecular structures with a minimum of complexity—two criteria for a good theory.
Investigation 3.2: Evidence for Polar Molecules
(Pages 99, 131)
Purpose
The scientific purpose of this investigation is to test the empirical rules for predicting molecular
polarity.
Problem
Which of the liquids provided have polar molecules?
Prediction
According to the empirical rules in Table 8, water, ethanol, ethane-1,2-diol, and acetone all have
polar molecules because they fall into the categories of oxygen and other atoms, or carbon and
two other kinds of atoms. Pentane and hexane have nonpolar molecules because they fall into the
category of carbon and only one other kind of atom.
Design
At each station, charged plastic strips are held near a thin stream of liquid.
Evidence
Substance
acetone
ethane-1,2-diol
ethanol
hexane
pentane
water
Testing Liquids with Charged Strips
Chemical formula
Effect of charged strip
Acetate
(CH3)2(CO(l)
attracted
C2H4(OH)2(l)
attracted
C2H5OH(l)
attracted
C6H14(l)
no effect
C5H12(l)
no effect
H2O(l)
attracted
Vinyl
attracted
attracted
attracted
no effect
no effect
attracted
Analysis
According to the evidence collected, acetone, ethane-1,2-diol, ethanol, and water all have polar
molecules because they were all attracted to the charged strips. The hexane and pentane were not
attracted. Therefore, they do not have polar molecules.
Evaluation
The Design seems very simple with no obvious flaws. The materials were adequate for their
purpose. The procedure is judged to be adequate, but an improvement might be to specify clearly
where the charged strip should be placed. It is also not clear if the strips were charged the same
amount each time. Charging the strip could be somewhat controlled by rubbing the strip the same
number of times, each time it is charged. Neither of these improvements should substantially
affect the results. If quantitative results are desired, more sophisticated equipment and procedures
would be required. The skills required are quite simple and overall, I am reasonably confident in
the evidence. The question was clearly answered.
The prediction is clearly verified. The generalizations used appear to be acceptable.
The purpose of the Investigation was accomplished. Many more trials using more
substances would be required to increase confidence in the empirical rules used in prediction.
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Practice
(Pages 100–101)
9. (a) G+ G–
H—Cl
polar covalent
2.2 3.2
(b) G– G+
C—H
polar covalent
2.6 2.2
(c) G+ G–
N—O
polar covalent
3.0 3.4
(d) G+ G–
I—Br
polar covalent
2.7 3.0
(e) + –
Mg S
ionic
1.3 2.6
(f) P—H
nonpolar covalent
2.2 2.2
10. (a) polar covalent
(b) ionic
(c) nonpolar covalent
11. The list of the bonds in order of increasing bond polarity is assumed to be the same as the
order of increasing difference in electronegativity of the bonded atoms. Thus, the orders are
as follows:
(a) H—H, C—H, Be—H, N—H, Li—H, O—H, F—H (Note: Li—H and O—H have equal
dipoles.)
(b) I—I, I—Cl, P—Cl, Al—Cl, Li—I, Rb—F
(c) C—H, C—O, O—H
(d) C—H, C—Cl, C—F
Practice
(Pages 102–103)
12. (a)
tetrahedral
(b)
trigonal pyramidal
(c)
trigonal planar
(d)
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angular (V-shaped)
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13. (a) C { N
G+ĺG–
14. (a)
(b) N = O
G+ĺG–
(c) P — S
G+ĺG–
(d) C — C
no dipole
zero resultant
(b)
resultant dipole toward F side of molecule
(c)
no bond dipoles, no resultant
(d)
resultant dipole toward Cl side of molecule
15. An octane molecule, C8H18(l), should be nonpolar based on the generalization that any
molecule composed of carbon atoms and only one other kind of atom is a nonpolar molecule.
16. Hydrogen sulfide, H2S(g), has an angular molecule, with bond dipoles directed toward the
sulfur atom. Thus, the molecular dipole should be slightly negative on the sulfur side, and
slightly positive on the hydrogen side.
Cool the hydrogen sulfide to the liquid state. Pass a thin stream of the liquid past a
strong electric charge. Molecular polarity should cause deflection of the liquid stream.
Web Activity: Web Quest—Cleaning Up Dry Cleaning
(Page 104)
[The following information should be included in students’ presentations, along with evidence of
thorough research.]
How does dry cleaning differ from “normal” cleaning using water and detergent?
Ɣ “Dry” cleaning involves submerging the garment in a solvent, other than water, that
dislodges, dissolves, and removes the dirt and grease in the fabric. In this case, it is the
solvent itself that dissolves the oils. Washing in warm water and detergent involves softening
the grease with heat, then the detergent molecules surround the oil droplets and lift them into
suspension, away from the fabric. In this case, water is not a solvent and the grease is not
dissolved: it is in suspension.
What are the physical and chemical properties of PERC? Give theoretical explanations for
these properties.
Ɣ PERC is a liquid at room temperature, with a melting point of 19 qC and a boiling point of
121 qC. It does not dissolve in water and is more dense than water. Its structure (four chlorine
atoms) makes it highly nonpolar, so it is a very good solvent for other nonpolar substances,
including organic compounds such as oils. Because of the C=C bond it is somewhat reactive,
participating in reactions that change the double bond to a single bond (e.g., forming
trichloroacetic acid).
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What are the potential environmental and health risks associated with the use of PERC?
Ɣ PERC is a persistent pollutant in the environment, typically sinking through the soil to
contaminate the underlying water table. In surface water it is toxic to fish. When inhaled by
humans it irritates the respiratory system and depresses the central nervous system, causing
dizziness, loss of balance, nausea, and confusion. Repeated skin contact can result in irritation
and soreness because PERC dissolves the oils in the skin. Although there are some reports of
links with leukemia, Environment Canada concludes that PERC is “unlikely to be
carcinogenic to humans.” The same report does, however, acknowledge that exposure may
“result in neurological impairment, liver and kidney damage and cardiac ailments.”
What are the potential environmental and health risks associated with the alternatives?
Ɣ Carbon dioxide is naturally present in the air, and is nontoxic as long as it is not inhaled in
high concentrations. It should not be released into the atmosphere, however, because it is a
greenhouse gas and is believed to contribute to global warming.
Ɣ There is little information about its effect on the environment or on human health.
GreenEarth, a company that is developing and promoting this process, claims that
“Regulatory compliance, disposal of waste, employee relations, landlord-friendly locations,
storage costs, Superfund charges, remediation expenses, and so forth, all get knocked down to
size with the GreenEarth Cleaning System.” The process is quite new, but testing has
indicated that there are “no significant adverse health or environmental effects.”
Ɣ Wet cleaning uses a considerable quantity of treated tapwater, along with liquid or powdered
detergents. Some detergents contain phosphates, but there are phosphate-free and
biodegradable detergents available. As long as the wastewater is properly treated before being
returned to the environment, there is no environmental or health risk.
Ɣ All cleaning processes involve “drying,” whether it is PERC, carbon dioxide, silicone liquid,
or water. For every process except water, the vaporized solvent must be collected and
recycled, rather than being released to the air.
Describe how each of the three suggested cleaning agents work. How effective are each of
the cleaners?
Ɣ Carbon dioxide is used under pressure, in liquid form. Because, like PERC molecules, carbon
dioxide molecules are nonpolar, other nonpolar molecules (such as the oil and grease on a
dirty garment) readily dissolve in the liquid.
Ɣ Silicone-based cleaners use a form of liquid silicone as a liquid medium to introduce the
detergent to the fabrics. The detergents then work in much the same way as they do in water:
surrounding the droplets of oil and holding them in suspension in the liquid. The silicone
solvent is promoted as being chemically inert, so it does not interact with the fabric or dyes in
any way. There is little information about its effect on the environment or on human health. It
breaks down to form silicon dioxide, carbon dioxide, and waterall nontoxic products.
Ɣ Wet cleaning (washing in warm water and detergent) involves softening the grease with heat,
then the dissolved detergent molecules surround the oil droplets and lift them into suspension,
and away from the fabric. Other dirt (hair, sand, etc.) is also suspended in the water. The
water is then spun away, taking the detergent, oil, and dirt away with it. A rinse completes the
process.
Ɣ According to Consumer Reports, cleaning with carbon dioxide gave the best results, judging
by appearance and degree of shrinkage. Silicone-based cleaning was the next best. Consumer
Reports did not, however, test how clean the clothes were.
Comparing the advantages and disadvantages of each alternative cleaner, which is the best
alternative to PERC? Why?
Ɣ Consumer Reports rated the appearance of the CO2-cleaned clothes the highest, as they did
not shrink and the fabric seemed to be unaffected by the cleaning process. Carbon dioxide is
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relatively environmentally benign, but requires very specialized cleaning machines to
withstand the high pressures required. Silicone-based cleaning seemed to be effective, and is
promoted as being environmentally friendly. However, it is so new that we may not yet be
aware of any long-term effects. Wet cleaning is possibly the most cost-effective and
environmentally friendly option, but may not be ideal for all garments, as wool garments tend
to shrink if handled roughly when wet, and some fabrics become weak when wet. In
conclusion, wet cleaning is the best alternative to PERC for garments that will not be
damaged by water. For more delicate fabrics, carbon dioxide cleaning is the best option.
[Students should select an alternative that they consider to be the best. A sample position
follows.]
Ɣ Wet cleaning is effective, inexpensive, widely available, environmentally benign (providing
the waste water is properly treated), and nontoxic, so is the best alternative to dry cleaning for
garments that will not be damaged by water (such as cotton, silk, polyester). For fabrics that
are affected by water, however, a non-water process would be better. The best of these is
probably liquid carbon dioxide, as it is probably relatively inexpensive to produce, nontoxic
(in moderate concentrations), and relatively environmentally benign (although it is a
greenhouse gas, so it should not be released in large quantities).
Section 3.3 Questions
(Page 104)
1. (a)
(b)
(c)
(d)
(e)
2. (a)
(b)
(c)
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(d)
(e)
3. (a)
(b)
(c)
4. (a) Beryllium bromide is nonpolar because it is symmetrical. Therefore, the bond dipoles add
up to a zero resultant (cancel) for the overall molecule.
(b) Nitrogen trifluoride is polar because it has polar bonds and is not symmetrical. The sum
of all of the bond dipoles produces a non-zero dipole for the whole molecule.
(c) Methanol is polar because it has polar bonds and is not symmetrical. The sum of all the
bond dipoles produces a non-zero dipole for the whole molecule.
(d) Hydrogen peroxide is polar because it has polar bonds and the OH bonds must not be in
exactly opposite directions.
(e) Ethylene glycol is polar because it has polar bonds and is not symmetrical. The bond
dipoles do not cancel.
5. (a) According to the empirical rules, CHFCl2 is polar because it contains carbon and two
other kinds of atoms. The theoretical explanation is shown below.
(b) According to the empirical rules, C2H4 is nonpolar because it contains carbon and only
one other kind of atom. The theoretical explanation is shown below.
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(c) According to the empirical rules, C2H5Cl is polar because it contains carbon and two
other kinds of atoms. The theoretical explanation is shown below.
(d) According to the empirical rules, CH3NH2 is polar because it contains carbon and two
other kinds of atoms or it is compound containing nitrogen and other atoms. The
theoretical explanation is shown below.
(e) According to the empirical rules, C2H5OH is polar because it contains carbon and two
other kinds of atoms or it is compound containing oxygen and other atoms. The
theoretical explanation is shown below.
6. Pentane is nonpolar, whereas water is polar. Therefore, water is a better candidate because it
can attract charged particles better.
7. Dr. Ronald Gillespie, the co-creator of VSEPR theory, is currently Professor Emeritus at
McMaster University. He holds B.Sc. and Ph.D. degrees in science, and a D.Sc. from London
University. He has won many significant awards, including the Chemical Institute of Canada
Medal, the Henry Marshall Tory Medal of the Royal Society of Canada, and the Izaak Walter
Killam Memorial Prize of the Canada Council for Pure Science, to name just a few. He is a
visiting professor at nine international universities in Europe, Australia, and Asia, and has
been awarded four honorary doctorates. His major topic of research, in co-operation with Dr.
Richard Bader, is using calculated electron probability distributions to better understand the
VSEPR model.
8. The sense of taste seems to be built around the ability of taste receptors to form hydrogen
bonds at specific locations with certain molecules. Thus, artificial sweeteners, such as
saccharin, cyclamate, and acesulfame-K, are non-nutritive molecules with structural
similarities to natural sugars. The molecular structure for sweetness includes a small pentagon
or hexagon of atoms with the ability to hydrogen bond at a specific spot on the ring.
Similarly, artificial compounds, such as Bitrex“—added as a safety precaution to many
medications to make the taste extremely bitter—are structured to a shape that triggers human
taste receptors for bitterness to react strongly.
9. The entire field of cleaning and stain removal is based on knowledge of polar and nonpolar
substances. Stains may be caused by polar or nonpolar substances. Nonpolar substances pose
particular problems because the common liquid for washing is water, which is very polar.
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Soaps and detergents are molecules selected or designed to have both polar and nonpolar
regions so that they can dissolve oily or greasy dirt from a stain, and also dissolve in water to
carry the material away.
10. (a) Cl—Cl
nonpolar covalent
3.2
(b) +
K
3.2
–
I
ionic
0.8 2.7
(c) G+ G–
P—Cl
polar covalent
2.2 3.2
(d) G– G+
O—H
polar covalent
3.4 2.2
(e) + –
Mg O
1.3
(f) G+ G–
Xe—F
2.6
ionic
3.4
polar covalent
4.0
Extension
11. (a) H–C { C–C { C–C { N, linear shape
(b) Astronomers detect molecules in space by spectroscopic analysis or electromagnetic
radiation (light) absorbed and emitted by regions of space.
12. Empirical (measured, observed) values change as new and better methods of determining
values are found. As well, the electronegativity of an atom is not an exact concept. It also
varies somewhat, depending on the bonded atoms. The new values could be described as
more accurate or better-defined values, but the word “true” cannot properly be applied to
relative numerical values of this sort.
3.4 INTERMOLECULAR FORCES
Web Activity: Canadian Achiever—Gerhard Herzberg
(Page 109)
1. The technological device central to Herzberg’s research was the spectrograph.
2. A free radical is a molecule with an unpaired valence electron. Because it has this unpaired
electron, a free radical is highly reactive.
3. Although he was a physicist, Herzberg was awarded the Nobel Prize in chemistry because his
discoveries of the internal geometry and energy states of molecules are extremely important
to chemists.
Practice
(Page 109)
1. (a)
(b)
(c)
(d)
(e)
(f)
dipole–dipole forces and London forces
London forces
London forces
dipole–dipole forces and London forces
dipole–dipole forces and London forces
London forces
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