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Chemistry Assignment #3 (Ch. 8,9)
8.24 Explain the following trends in lattice energy:
(a) MgO > MgCI2; (b) NaCI > RbBr > CsBr;
(c) BaO > KF.
8.64 (a) Describe the molecule chlorine dioxide, CI02, using
three possible resonance structures. (b) Do any of these
resonance structures satisfy the octet rule for every atom
in the molecule? Why or why not? (c) Using formal
charges, select the resonance structure(s) that is (are)
most important.
8.68 Use bond enthalpies (Table 8.4) to estimate the enthalpy
change for each of the following reactions:
(a) 3 H2C=CH2(g) ~ C6H
1
2(g) (the six carbon atoms
from a six-membered ring with two hydrogen atoms
on each carbon atom)
(b) SiClH3(g) + 3 CI2(g) ~ SiCI4(g) + 3 HCI(g)
(c) 8 H2S(g) ~ 8 H2(g) + S8(s)
(See Figure 7.28. Strictly speaking, the average bond enthalpy
values apply to species in the gas phase. The heat
of formation of S8(g) is 102.3 kJ/mo!. Apply the needed
correction in order to estimate the enthalpy change for
the reaction as shown.)
8.99 The compound chloral hydrate, known in detective stories
as knockout drops, is composed of 14.52% C, 1.83%
H, 64.30% CI, and 19.35% 0 by mass and has a molar
mass of 165.4 g/mol. (a) What is the empirical formula
of this substance? (b) What is the molecular formula of
this substance? (c) Draw the Lewis structure of the molecule,
assuming that the CI atoms bond to a single C
atom and that there is a C-C bond and two C-O
bonds in the compound.
8.105 Consider benzene (C6H6) in the gas phase. (a) Write the
reaction for breaking all the bonds in C6H6(g), and use
data in Appendix C to determine the enthalpy change
for this reaction. (b) Write a reaction that corresponds to
breaking all the carbon-carbon bonds in C6H6(g). (c) By
combining your answers to parts (a) and (b) and using
the average bond enthalpy for C- H from Table 8.4, calculate
the average bond enthalpy for the carbon-carbon
bonds in C6H6(g). (d) Comment on your answer from
part (c) as compared to the values for C-C single
bonds and C=C double bonds in Table 8.4 (p.3).
9.26 Give approximate values for the indicated bond angles
in the following molecules:
9.38 Dichlorobenzene, C6H4Cl2, exists in three forms (isomers),
called ortho, meta, and para:
JvCI ortho meta para
Which of these would have a nonzero dipole moment?
Explain.
9.42 (a) Draw a Lewis structure for silane (SiH4), and predict
its molecular geometry. (b) Is it necessary to promote an
electron before forming hybrid orbitals for the Si atom?
(c) What type of hybridization exists in SiH4? (d) In one
diagram, sketch two of the two-electron bonds formed
between a hybrid orbital on Si and an H Is orbital. How
would the other Si- H bonds be oriented relative to
your sketch?
9.72 (a) The nitric oxide molecule, NO, readily loses one electron
to form the NO+ ion. Why is this consistent with
the electronic structure of NO? (b) Predict the order of
the N -0 bond strengths in NO, NO+, and NO-, and
describe the magnetic properties of each. (c) With what
neutral homonucIear diatomic molecules are the NO+
and NO- ions isoelectronic (same number of electrons)?
9.88 Butadiene, C4H6, is a planar molecule that has the following
carbon-carbon bond lengths:
(a) Predict the bond angles around each of the carbon
atoms, and sketch the molecule. (b) Compare the bond
lengths to the average bond lengths listed in Table 8.5.
Can you explain any differences?
1
Chemistry Assignment #4 (Ch. 10-13)
11.42
The critical temperatures (K) and pressures (atm) of a series of halogenated methanes are as follows:
11.62 Aluminum metal crystallizes in a cubic close-packed structure (face-centered cubic cell, Figure 11.34). (a) How many aluminum atoms are in a unit cell? (b) What is the coordination number of each aluminum atom? (c) Assume that the aluminum atoms can be represented as spheres, as shown in the drawing for Exercise 11.61. If each AI atom has a radius of 1.43 A, what is the length of a side of the unit cell? (d) Calculate the density of aluminum metaL
12.24 Write the chemical equation that represents the formation of (a) polychloroprene from chloroprene
(Polychloroprene is used in highway-pavement seals, expansion joints, conveyor belts, and wire and cable jackets.); (b) polyacrylonitrile from acrylonitrile.
12.79 Hydrogen bonding between polyamide chains plays an
important role in determining the properties of a nylon
such as nylon 6,6 (Table 12.2). Draw the structural formulas
for two adjacent chains of nylon 6,6, and show
where hydrogen-bonding interactions could occur between
them.
13.46 Calculate the number of moles of solute present in each of
the following solutions: (a) 245 mL of 1.50M HN03(aq),
(b) 50.0 mg of an aqueous solution that is 1.25 m NaCI,
(c) 75.0 g of an aqueous solution that is 1.50% sucrose
(C12H22O11) by mass.
13.74 A dilute aqueous solution of an organic compound soluble
in water is formed by dissolving 2.35 g of the compound
in water to form 0.250 L solution. The resulting
solution has an osmotic pressure of 0.605 atm at 25°C.
Assuming that the organic compound is a nonelectrolyte,
what is its molar mass?
10.32 Calculate each of the following quantities for an ideal gas:
(a) the volume of the gas, in liters, if 1.75mol has a pressure of 0.985 atm at a temperature of -6°C; (b) the absolute temperature of the gas at which 3.33 X 10-3 mol occupies 255 mL at 720 torr; (c) the pressure, in atmospheres, if
0.0467 mol occupies 413 mL at 122°C; (d) the quantity of gas, in moles, if 67.5 L at 54°Chas a pressure of 11.25kPa.
10.95 Consider the arrangement of bulbs shown in the drawing. Each of the bulbs contains a gas at the pressure shown. What is the pressure of the system when all the stopcocks are opened, assuming that the temperature
remains constant? (We can neglect the volume of the
capillary tubing connecting the bulbs.)
12.1 Classes of Materials 491
Insulators and Ceramics
Semiconductors have band gap energies that range from ~50 kJjmol to
~300 kJjmol (Table 12.1).Insulators have a band structure similar to that found
in semiconductors, except that in insulators the band gap is larger-more than
~350 kJjmol (Table 12.1 and Figure 12.2). Because the energy required to pro-
mote an electron from the valence band to the conduction band is about the
same as the energy required to break chemical bonds in the material, insulators
are not electrically conductive. Many ionic solids and complex network solids
and most organic compounds are insulators; elements that are solid insulators
include carbon in its diamond form and sulfur.
Ceramics are inorganic ionic solids that are normally hard and brittle and
are stable at high temperatures. They are generally electrical insulators. Ceram-
ic materials include such familiar objects as pottery, china, cement, roof tiles,
and spark-plug insulators.
Ceramic materials (Table 12.2T) come in a variety of chemical forms, in-
cluding oxides (oxygen and metals), carbides (carbon and metals), nitrides (nitro-
gen and metals), silicates (silica, Si02, mixed with metal oxides), and aluminates
(alumina, A1203, mixed with metal oxides).
Ceramics are highly resistant to heat, corrosion, and wear; do not readily
deform under stress; and are less dense than the metals used for high-
temperature applications. Some ceramics used in aircraft, missiles, and space-
craft weigh only 40% as much as the metal components they replace. In spite of
these many advantages, the use of ceramics as engineering materials has been
limited because they are extremely brittle. Whereas a metal component might
suffer a dent when struck, a ceramic part typically shatters because the greater
extent of ionic bonding in a ceramic prevents the atoms from sliding over one
another.
TABLE 12.2 Properties of Some Ceramic and Nonceramic Materials
Coefficient
Melting Density Hardness Modulus of of Thermal
Material Point (0C) (g/cm3) (Mohs)” Elasticity” Expansion”
Ceramic materials
Alumina, AI203 2050 3.8 9 34 8.1 ~ ]-DMODELSilicon carbide, SiC 2800 3.2 9 65 4.3 Silicon Carbide
Zirconia, Zr02 2660 5.6 8 24 6.6
Beryllia, BeO 2550 3.0 9 40 10.4
Nonceramic materials
Mild steel 1370 7.9 5 17 15
Aluminum 660 2.7 3 7 24
aThe Mohs scale is a logarithmic scale based on the relative ability of a material to scratch another softer material. Diamond, the
hardest material. is assigned a value of 10.
b A measure of the stiffness of a material when subjected to a load (MPa x 104). The larger the number, the stiffer the material.
eIn units of (K-1 x 10-6). The larger the number, the greater the size change upon heating or cooling.
Superconductors
Even metals are not infinitely conductive; there is some resistance to electron
flow. However, in 1911the Dutch physicist H. Kamerlingh Onnes discovered
that when mercury is cooled below 4.2 K, it loses all resistance to the flow of an
electrical current. Since that discovery, scientists have found that many sub-
stances exhibit this “frictionless” flow of electrons, known as superconductivity.
Substances that exhibit superconductivity do so only when cooled below a
particular temperature called the superconducting transition temperature, Te.
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Titles
Insulators and Ceramics
491
Superconductors
Tables
Table 1
