Chemistry 1A Spring 1999
Exam #4 KEY Chapters 9 & 10
Electronegativities
H
2.2
Li
0.98
Be
1.57
B
2.04
C
2.55
N
3.04
O
3.44
F
3.98
Na
0.93
Mg
1.31
Al
1.61
Si
1.9
P
2.19
S
2.58
Cl
3.16
K
0.82
Ca
1.0
Ga
1.81
Ge
2.01
As
2.18
Se
2.55
Br
2.96
Rb
0.82
Sr
0.95
In
1.78
Sn
1.96
Sb
2.05
Te
2.1
I
2.66
Cs
0.79
Ba
0.89
Tl
2.33
Pb
2.02
Bi
2.0
Po
2.2
Xe
2.6
1
For each of the following, write the word, words, or number in each blank that best
completes each sentence. (2 points each)
1. An atom’s highest energy “s” and “p” electrons are called valence electrons.
2. Isomers are molecules that have the same molecular formula but a different
structural formula. They have the same atoms but a different order of linking the
atoms.
3. Draw a reasonable Lewis structure and a geometric sketch for each of the following
molecules. Determine whether each is polar or nonpolar. (6 points each)
a. C2H6
nonpolar (no polar bonds)
b. BBr3
nonpolar (symmetrical distribution of polar bonds)
c. CF4
nonpolar (symmetrical distribution of polar bonds)
d. SCl2
polar (asymmetrical distribution of polar bonds)
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e. CH3OH
polar (asymmetrical distribution of polar bonds – all alcohols have polar
molecules)
4. Draw a reasonable Lewis structure for a nitric acid, HNO3. Use the skeleton below.
The structure is best described in terms of resonance, so draw all of its reasonable
resonance structures and the resonance hybrid that summarizes these structures.
(4 points)
5. Draw two reasonable Lewis structures for phosphoric acid, H3PO4. (Your structures
should not contain any O-O bonds.) One of the Lewis structures should emphasize the
octet rule, and the other structure should minimize formal charges. If either Lewis
structure is best described with resonance, draw all of its reasonable resonance
structures and the resonance hybrid. (4 points)
Obeys octet rule
No formal charges
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6. Methyl cyanoacrylate is the main ingredient in Super Glue. Using its Lewis structure
below, do each of the following. (20 points)
•
On the structure below, indicate the hybridization for each atom that has an
arrow pointing toward it.
•
Describe each of the bonds with an arrow pointing at it as either a sigma
(σ
σ) or a pi (π
π) bond, and list the atomic orbitals that overlap to form the
bond.
•
Write the name of the electron group geometry around each atom that
has an arrow pointing to it.
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•
Draw the geometric sketch of the molecule, including bond angles. Write
the name of the molecular group geometry around each atom that has an
arrow pointing to it.
There are many ways to sketch the molecule and still show the correct
geometry. The following is just one example.
7. For each of the following, write the name of the type of attraction holding these
particles in the solid and liquid form. Indicate the formula in each pair that represents
the substance that you would expect to have the stronger attractions that hold the
particles in the liquid or solid form. (1 ½ points each blank)
a. Potassium phosphate, K3PO4 (used to make liquid soaps)
type of attraction ionic bonds
or HF (additive to liquid rocket propellants)
type of attraction Hydrogen bonds
stronger attractions K3PO4
Ionic bonds are generally stronger than hydrogen bonds, so the
ionic bonds in K3PO4 are expected to be stronger.
b. Butanoic acid, CH3(CH2)2CO2H (used to make perfumes and
pharmaceuticals)
type of attraction hydrogen bonds and London forces
or stearic acid, CH3(CH2)16CO2H (used to makes soap, pharmaceuticals, and
cosmetics)
type of attraction hydrogen bonds and London forces
stronger attractions stearic acid
The larger stearic acid molecules have stronger London forces.
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c. Titanium, Ti (structural material in aircraft, jet engines, and surgical
instruments)
type of attraction metallic bonds
or hydrogen cyanide, HCN (used to manufacture rodenticides and pesticides)
type of attraction dipole-dipole attractions and London
forces
stronger attractions Ti
Metallic bonds are generally stronger than dipole-dipole attractions and
London forces, so the metallic bonds in Ti are expected to be stronger.
d. Boron tribromide, BBr3 (a catalyst for the formation of diborane)
type of attraction London forces
or boron trifluoride, BF3 (a catalyst)
type of attraction London forces
stronger attractions BBr3
Both have nonpolar molecules held together by London forces. The larger
BBr3 molecules have stronger London forces.
e. hydrogen peroxide, H2O2 (used in rocket fuels)
type of attraction hydrogen bonds
or fluorine, F2 (used to produce fluorocarbons)
type of attraction London forces
stronger attractions H2O2
For molecules that are about the same size, hydrogen bonds are stronger
than London forces, so H2O2 has the stronger attractions.
6
Answer the following in short answer form. (6 points each)
8. The Lewis structure for phosphorus pentafluoride, PF5, is below. With reference to
the assumptions of the valence bond model, write an explanation for how phosphorus
atoms are able to form five equivalent covalent bonds in molecules like PF5.
Only the highest energy electrons participate in bonding. The phosphorus atom is
surrounded by five electron groups (five bonds), so we predict that it is sp3d
hybridized. To form these bonds, it is as if an electron is promoted from a 3s orbital
to an empty 3d orbital, and then the 3s, the three 3p, and the one 3d orbital blend to
form sp3d hybrid orbitals.
Covalent bonds form to pair unpaired electrons, so we get five equivalent bonds.
9. At room temperature and pressure, fluorine, F2, and chlorine, Cl2, are gases, bromine,
Br2, is a liquid, and iodine, I2, is a solid. In each case, the attractions that could hold
the particles together are London forces. Write an explanation for why the London
forces between halogen molecules get stronger as one moves down the periodic table.
A larger molecule has more electrons and a greater chance of having its electron
cloud distorted from its idealized shape. Thus, instantaneous dipoles are more
likely to form in larger molecules. The electron clouds in larger molecules are
also larger, so the average distance between the nuclei and the electrons is
greater; as a result, the electrons are less tightly held and shift more easily to
create a dipole.
The increased strength of London forces with increased size of molecules explains
a trend in the properties of halogens. Fluorine, F2, and chlorine, Cl2, which
contain relatively small molecules, are gases at room temperature and pressure.
Bromine, Br2, with larger molecules and therefore stronger London forces
between them, is a liquid. Iodine, I2, with the largest molecules of all the
halogens, is a solid.
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