Chemistry: The Central Science

Theodore E. Brown, Eugene H LeMay, Bruce E. Bursten

Chapter 12 Modern Materials - all with Video Answers

Educators

Chapter Questions

02:33

Problem 1

Based on the following band structures, which material is a metal? [Section 12.2]

Susan Hallstrom

Numerade Educator

03:05

Problem 2

The band structures below both belong to semiconductors. One is AlAs and the other GaAs. Which is which? [Section 12.3]

Susan Hallstrom

Numerade Educator

02:42

Problem 3

Shown below are cartoons of two different polymers. Based on these cartoons, which polymer would you expect to be denser? Which one would have the higher melting point? [Section 12.6]

Susan Hallstrom

Numerade Educator

05:49

Problem 4

Which picture best represents molecules that are in a liquid crystalline phase? [Section 12.8]

Susan Hallstrom

Numerade Educator

12:21

Consider the molecules shown below. (a) Which of these would be most likely to form an addition polymer? (b) Which would be most likely to form a condensation polymer? (c) Which would be most likely to form a liquid crystal? [Sections $12.6$ and 12.8]

Susan Hallstrom

Numerade Educator

09:41

Problem 6

The three plots to the right show the conductivity as a function of temperature for three different types of materials: a semiconductor with a small band gap, a metallic conductor, and a superconductor. Which plot corresponds to which material? (Hint: Remember in a semiconductor thermal energy is needed to excite electrons from the valence band to the conduction band.) [Sections $12.1,12.3$, and $12.5]$

Susan Hallstrom

Numerade Educator

00:47

Problem 7

Classify each of the following materials as metal, semiconductor, or insulator: (a) GaN (b) B (c) $\mathrm{ZnO}$ (d) $\mathrm{Pb}$

Bhumika Jayee

Numerade Educator

00:38

Problem 8

Classify each of the following materials as metal, semiconductor, or insulator: (a) InAs (b) $\mathrm{MgO}$ (c) $\mathrm{HgS}$ (d) Sn

Bhumika Jayee

Numerade Educator

10:38

Problem 9

The molecular orbital diagrams for linear chains of hydrogen atoms with two and four atoms in the chain are shown in Figure 12.1. Construct a molecular orbital diagram for a chain containing six hydrogen atoms and use it to answer the following questions. (a) How many molecular orbitals are there in the diagram? (b) How many nodes are there in the lowest energy molecular orbital? (c) How many nodes are there in the highest energy molecular orbital? (d) How many nodes are there in the highest occupied molecular orbital (HOMO)? (e) How many nodes are there in the lowest unoccupied molecular orbital (LUMO)?

Susan Hallstrom

Numerade Educator

08:52

Problem 10

Repeat exercise $12.9$ for a linear chain of eight hydrogen atoms.

Susan Hallstrom

Numerade Educator

00:57

Problem 11

State whether each statement is true or false, and why.
(a) Semiconductors have a larger band gap than insulators.
(b) Doping a semiconductor makes it more conductive.
(c) Metals have delocalized electrons.
(d) Most metal oxides are insulators.

Bhumika Jayee

Numerade Educator

01:01

Problem 12

State whether each statement is true or false, and why.
(a) A typical band gap energy for an insulator is $400 \mathrm{~kJ} / \mathrm{mol}$
(b) The conduction band is higher in energy than the valence band.
(c) Electrons can conduct well if they are in a filled valence band.
(d) Holes refer to empty atomic sites in a solid crystal.

Bhumika Jayee

Numerade Educator

01:30

Problem 13

For each of the following pairs of semiconductors, which one will have the larger band gap: (a) $\mathrm{CdS}$ or $\mathrm{CdTe}$ (b) GaN or InP (c) GaAs or InAs?

Ly Tran

Numerade Educator

01:18

Problem 14

For each of the following pairs of semiconductors, which one will have the larger band gap: (a) InP or InAs (b) Ge or AlP (c) AgI or CdTe?

Aadit Sharma

Numerade Educator

00:49

Problem 15

If you want to dope GaAs to make an n-type semiconductor with an element to replace Ga, which element(s) would you pick?

Ly Tran

Numerade Educator

00:39

Problem 16

If you want to dope GaAs to make a p-type semiconductor with an element to replace As, which element(s) would you pick?

Eugene Schneider

University of Minnesota - Twin Cities

01:20

Problem 17

What advantages does silicon have over other semiconductors for use in integrated circuits?

Bhumika Jayee

Numerade Educator

01:00

Problem 18

Why is it important for Si crystals to be $99.999999999 \%$ pure, as opposed to $99 \%$ pure, for silicon chips?

Bhumika Jayee

Numerade Educator

00:40

Problem 19

What material is traditionally used to make the gate in a MOSFET transistor? What material is used in the next generation transistors?

Bhumika Jayee

Numerade Educator

01:49

Problem 20

Look up the diameter of a silicon atom, in $\AA$. The channel length in a Pentium 4 processor chip is $65 \mathrm{~nm}$ long. How many silicon atoms does this correspond to?

Aadit Sharma

Numerade Educator

07:53

Problem 21

Silicon has a band gap of $1.1 \mathrm{eV}$ at room temperature. (a) What wavelength of light would a photon of this energy correspond to? (b) Draw a vertical line at this wavelength in the figure shown, which shows the light output of the sun as a function of wavelength. Does silicon absorb all, none or a portion of the visible light that comes from the sun?

Susan Hallstrom

Numerade Educator

01:29

Problem 22

Cadmium telluride is an important material for solar cells. (a) What is the band gap of CdTe? (b) What wavelength of light would a photon of this energy correspond to? (c) Draw a vertical line at this wavelength in the figure shown with exercise $12.21$, which shows the light output of the Sun as a function of wavelength. (d) With respect to silicon, does CdTe absorb a larger or smaller portion of the solar spectrum?

Eugene Schneider

University of Minnesota - Twin Cities

01:20

Problem 23

The semiconductor GaP has a band gap of $2.2 \mathrm{eV}$. Green LEDs are made from pure GaP. What wavelength of light would be emitted from an LED made from GaP?

Narayan Hari

Numerade Educator

01:15

Problem 24

The first LEDs were made from GaAs, which has a band gap of $1.43 \mathrm{eV}$. What wavelength of light would be emitted from an LED made from GaAs? What region of the electromagnetic spectrum does this light correspond to:
UV, Visible, or IR?

Narayan Hari

Numerade Educator

08:47

Problem 25

Ga As and GaP make solid solutions that have the same crystal structure as the parent materials, with As and $P$ randomly distributed through the crystal. $\mathrm{GaP}_{x}$ As $_{1-x}$ exists for any value of $x .$ If we assume that the band gap varies linearly with composition between $x=0$ and $x=1$, estimate the band gap for $\mathrm{GaP}_{0.5} \mathrm{As}_{0.5} .$ What wavelength of light does this correspond to?

Susan Hallstrom

Numerade Educator

08:44

Problem 26

Red light-emitting diodes are made from GaAs and GaP solid solutions, $\mathrm{GaP}_{x} \mathrm{As}_{1-x}$ (see exercise 12.25). The original red LEDs emitted light with a wavelength of $660 \mathrm{~nm}$. If we assume that the band gap varies linearly with composition between $x=0$ and $x=1$, estimate the composition (the value of $x$ ) that is used in these LEDs.

Susan Hallstrom

Numerade Educator

03:42

Problem 27

Metals, such as Alor Fe, and many plastics are recyclable. With the exception of many glasses, such as bottle glass, ceramic materials in general are not recyclable. What characteristics of ceramics make them less readily recyclable?

Bhumika Jayee

Numerade Educator

02:37

Problem 28

It is desirable to construct automobiles out of lightweight materials to maximize fuel economy. All of the ceramics listed in Table $12.3$ are less dense than steel. Why do you think ceramic materials are not more widely used in the construction of automobiles?

Susan Hallstrom

Numerade Educator

03:14

Problem 29

Why is the formation of very small, uniformly sized and shaped particles important for many applications of ceramic materials?

Bhumika Jayee

Numerade Educator

03:03

Problem 30

Describe the general chemical steps in a sol-gel process, beginning with $\mathrm{Zr}(s)$ and $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}(l)$. Indicate whether each step is an oxidation-reduction reaction (refer to Section 4.4), condensation reaction, or other process.

Bhumika Jayee

Numerade Educator

02:55

Problem 31

The hardnesses of several substances according to a scale known as the Knoop value are as follows:
$$
\begin{array}{ll}
\hline \text { Substance } & \text { Knoop Value } \\
\hline \mathrm{Ag} & 60 \\
\mathrm{CaCO}_{3} & 135 \\
\mathrm{MgO} & 370 \\
\text { Soda-lime glass } & 530 \\
\mathrm{Cr} & 935 \\
\mathrm{ZrB}_{2} & 1550 \\
\mathrm{Al}_{2} \mathrm{O}_{3} & 2100 \\
\mathrm{~T}_{2} \mathrm{C} & 2000 \\
\hline
\end{array}
$$
Which of the materials in this list would you classify as. a ceramic? What were your criteria for making this classification? Does classification as a ceramic correlate with Knoop hardness? If you think it does, is hardness alone a sufficient criterion to determine whether a substance is a ceramic? Explain.

Bhumika Jayee

Numerade Educator

02:33

Problem 32

Silicon carbide, $\mathrm{SiC}$, has the three-dimensional structure shown in the figure.
(a) Name another compound that has the same structure.
(b) Would you expect the bonding in $\mathrm{SiC}$ to be predominantly ionic, metallic, or covalent?
(c) How do the bonding and structure of SiC lead to its high thermal stability (to $\left.2700^{\circ} \mathrm{C}\right)$ and exceptional hardness?

Bhumika Jayee

Numerade Educator

02:20

Problem 33

To what does the term superconductivity refer? Why might superconductive materials be of value?

Bhumika Jayee

Numerade Educator

02:16

Problem 34

What are the differences in the electrical and magnetic properties of an excellent metallic conductor of electricity (such as silver) and a superconducting substance (such as $\mathrm{Nb}_{3} \mathrm{Sn}$ ) below its superconducting transition temperature?

Bhumika Jayee

Numerade Educator

01:28

Problem 35

The following graph shows the resistivity (a measure of electrical resistance) of $\mathrm{MgB}_{2}$ as a function of temperature in the region from about $4 \mathrm{~K}$ to $100 \mathrm{~K}$. What is the significance of the sharp drop in resistivity below $40 \mathrm{~K}$ ?

Bhumika Jayee

Numerade Educator

02:48

Problem 36

(a) What is the superconducting transition temperature, $T_{c} ?(\mathrm{~b})$ The discovery by Müller and Bednorz of superconductivity in a copper oxide ceramic at $35 \mathrm{~K}$ set off a frantic scramble among physicists and chemists to find materials that exhibit superconductivity at higher temperatures. What is the significance of achieving $T_{c}$ values above $77 \mathrm{~K}$ ?

Bhumika Jayee

Numerade Educator

06:00

Problem 37

Explain how the Meissner effect can be used to levitate trains. What can the tracks and train wheels be made of, and which one would be more likely to be cooled?

Susan Hallstrom

Numerade Educator

18:01

Problem 38

The group $4 \mathrm{~B}$ metal nitrides (TiN, ZrN, and HfN) as well as the group $5 \mathrm{~B}$ metal nitrides (VN, $\mathrm{NbN}$, and TaN) are all superconductors at low temperature. Niobium(III) nitride, which has the highest $T_{c}$, superconducts below $16.1 \mathrm{~K}$. All of these compounds have crystal structures that are analogous to the sodium chloride structure. Scandium nitride also adopts the sodium chloride structure, but it is not a superconductor. (a) At room temperature will NbN be a metallic conductor or an insulator?
(b) At room temperature will ScN be a metallic conductor or an insulator? (c) Why do you think the properties of $\mathrm{ScN}$ are so different than the group $4 \mathrm{~B}$ and $5 \mathrm{~B}$ metal nitrides? (Hint: Consider the electron configuration of the metal cation.)

Susan Hallstrom

Numerade Educator

01:06

Problem 39

What is a monomer? Give three examples of monomers, taken from the examples given in this chapter.

Narayan Hari

Numerade Educator

00:58

Problem 40

The structure of decane is shown in Practice Exercise 12.5. Decane is not considered a polymer, whereas polyethylene is. What is the distinction?

Bhumika Jayee

Numerade Educator

02:11

Problem 41

An ester is a compound formed by a condensation reaction between a carboxylic acid and an alcohol. Use the index to find the discussion of esters in Chapter 25, and give an example of a reaction forming an ester. How might this kind of reaction be extended to form a polymer (a polyester)?

Bhumika Jayee

Numerade Educator

01:21

Problem 42

Write a chemical equation for formation of a polymer via a condensation reaction from the monomers succinic acid $\left(\mathrm{HOOCCH}_{2} \mathrm{CH}_{2} \mathrm{COOH}\right)$ and ethylenediamine $\left(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\right)$.

Aadit Sharma

Numerade Educator

01:54

Problem 43

Draw the structure of the monomer(s) employed to form each of the following polymers shown in Table $12.5$ (a) polyvinyl chloride, (b) nylon 6,6, (c) polyethylene terephthalate.

Aadit Sharma

Numerade Educator

01:47

Problem 44

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

(Polyacrylonitrile is used in home furnishings, craft yarns, clothing, and many other items.)

Aadit Sharma

Numerade Educator

01:16

Problem 45

The nylon Nomex $^{@}$, a condensation polymer, has the following structure:
Draw the structures of the two monomers that yield Nomex $^{@}$.

Aadit Sharma

Numerade Educator

00:51

Problem 46

Proteins are polymers formed by condensation reactions of amino acids, which have the general structure

In this structure, $R$ represents $-\mathrm{H},-\mathrm{CH}_{3}$, or another group of atoms. Draw the general structure for a polyamino acid polymer formed by condensation polymerization of the molecule shown here.

Aadit Sharma

Numerade Educator

01:27

Problem 47

What molecular features make a polymer flexible? Explain how cross-linking affects the chemical and physical properties of the polymer.

Aadit Sharma

Numerade Educator

00:36

Problem 48

What molecular structural features cause high-density polyethylene to be denser than low-density polyethylene?

Eugene Schneider

University of Minnesota - Twin Cities

02:05

Problem 49

Are high molecular masses and a high degree of crystallinity always desirable properties of a polymer? Explain.

Bhumika Jayee

Numerade Educator

04:09

Problem 50

Briefly describe each of the following: (a) elastomer,
(b) thermoplastic, (c) thermosetting plastic,
(d) plasticizer.

Bhumika Jayee

Numerade Educator

02:41

Problem 51

Neoprene is a polymer of chlorobu tadiene.
The polymer can be used to form flexible tubing that is resistant to chemical attack from a variety of chemical reagents. Suppose it is proposed to use neoprene tubing as a coating for the wires running to the heart from an implanted pacemaker. What questions would you ask to determine whether it might be suitable for such an application?

Bhumika Jayee

Numerade Educator

03:47

Problem 52

On the basis of the structure shown in Table $12.5$ for polystyrene and polyurethane, which of these two classes of polymer would you expect to form the most effective interface with biological systems? Explain.

Bhumika Jayee

Numerade Educator

01:33

Problem 53

Patients who receive vascular grafts formed from polymer material such as Dacron $^{\otimes}$ are required to take anticoagulation drugs on a continuing basis to prevent blood clots. Why? What advances in such vascular implants are needed to make this precaution unnecessary?

Bhumika Jayee

Numerade Educator

01:02

Problem 54

Several years ago a biomedical company produced and marketed a new, efficient heart valve implant. It was later withdrawn from the market, however, because patients using it suffered from severe loss of red blood cells. Describe what properties of the valve could have been responsible for this result.

Bhumika Jayee

Numerade Educator

01:08

Problem 55

Skin cells from the body do not differentiate when they are simply placed in a tissue culture medium; that is, they do not organize into the structure of skin, with different layers and different cell types. What is needed to cause such differentiation to occur? Indicate the most important requirements on any material used.

Bhumika Jayee

Numerade Educator

02:52

Problem 56

If you were going to attempt to grow skin cells in a medium that affords an appropriate scaffolding for the cells and you had only two fabrics available, one made from polystyrene and the other from polyethyleneterephthalate (Table 12.5), which would you choose for your experiments? Explain.

Bhumika Jayee

Numerade Educator

01:56

Problem 57

In what ways are a nematic liquid crystalline phase and an ordinary liquid the same, and in what ways do they differ?

Bhumika Jayee

Numerade Educator

01:38

Problem 58

In contrast to ordinary liquids, liquid crystals are said to possess "order." What does this mean?

Bhumika Jayee

Numerade Educator

03:10

Problem 59

Describe what is occurring at the molecular level as a substance passes from the solid to the nematic liquid crystalline to the isotropic (normal) liquid phase upon heating.

Bhumika Jayee

Numerade Educator

02:01

Problem 60

What observations made by Reinitzer on cholesteryl benzoate suggested that this substance possesses a liquid crystalline phase?

Bhumika Jayee

Numerade Educator

01:36

Problem 61

The molecules shown in Figure $12.39$ possess polar groups (that is, groupings of atoms that give rise to sizable dipole moments within the molecules). How might the presence of polar groups enhance the tendency toward liquid crystal formation?

Shahina -

Numerade Educator

01:25

Problem 62

Liquid crystalline phases tend to be more viscous than the isotropic, or normal, liquid phase of the same substance. Why?

Bhumika Jayee

Numerade Educator

00:27

Problem 63

The smectic liquid crystalline phase can be said to be more highly ordered than the nematic. In what sense is this true?

Amy Jiang

Numerade Educator

03:26

Problem 64

One of the more effective liquid crystalline substances employed in LCDs is this molecule.
By comparing this structure with the structural formulas and models shown in Figure 12.39, describe the features of the molecule that promote its liquid crystalline behavior.

Bhumika Jayee

Numerade Educator

00:28

Problem 65

Describe how a cholesteric liquid crystal phase differs from a nematic phase.

Amy Jiang

Numerade Educator

03:26

Problem 66

It often happens that a substance possessing a smectic liquid crystalline phase just above the melting point passes into a nematic liquid crystalline phase at a higher temperature. Account for this type of behavior in terms of the ideas developed in Chapter 11 relating molecular energies to temperature.

Bhumika Jayee

Numerade Educator

03:43

Problem 67

Explain why "bands" may not be the most accurate description of bonding in a solid when the solid has nanoscale dimensions.

Arun Bana

Numerade Educator

00:43

Problem 68

CdS has a band gap of $2.4 \mathrm{eV}$. If large crystals of $\mathrm{CdS}$ are illuminated with ultraviolet light they emit light equal to the band gap energy.
(a) What color is the emitted light?
(b) Would appropriately sized CdS quantum dots be able to emit blue light? (c) What about red light?

Eugene Schneider

University of Minnesota - Twin Cities

00:25

Problem 69

True or false:
(a) The band gap of a semiconductor decreases as the particle size decreases, in the $1-10-\mathrm{nm}$ range.
(b) The light that is emitted from a semiconductor, upon external stimulation, is longer and longer in wavelength as the particle size of the semiconductor decreases.

Bhumika Jayee

Numerade Educator

00:14

Problem 70

True or false:
If you want a semiconductor that emits blue light, you could either use a material that has a band gap corresponding to the energy of a blue photon or you could use a material that has a smaller band gap, but make a nanoparticle out of it of the right size.

Bhumika Jayee

Numerade Educator

08:27

Problem 71

Gold is a face-centered cubic structure that has a unit cell edge length of $4.08 \AA$ (Figure 11.34). How many gold atoms are there in a sphere that is $20 \mathrm{~nm}$ in diameter? Recall that the volume of a sphere is $\frac{4}{3} \pi r^{3}$.

Susan Hallstrom

Numerade Educator

06:30

Problem 72

Cadmium telluride, CdTe, takes the zinc blende structure (Section 11.8) with a unit cell edge length of $6.49 \AA$. There are four cadmium atoms and four tellurium atoms per unit cell. How many of each type of atom are there in a cubic crystal with an edge length of $5 \mathrm{~nm}$ ?

Susan Hallstrom

Numerade Educator

02:20

Problem 73

One major difference in the behavior of semiconductors and metals is that semiconductors increase their conductivity as you heat them (up to a point), but the conductivity of a metal decreases as you heat it. Suggest an explanation.

Bhumika Jayee

Numerade Educator

01:22

Problem 74

What properties of the typical nematic liquid crystalline molecule are likely to cause it to reorient when it is placed in an electrical field that is perpendicular to the direction of orientation of the molecules?

Bhumika Jayee

Numerade Educator

01:23

Problem 75

Teflon $^{\otimes}$ is a polymer formed by the polymerization of $\mathrm{F}_{2} \mathrm{C}=\mathrm{CF}_{2}$. Draw the structure of a section of this polymer. What type of polymerization reaction is required to form it?

Bhumika Jayee

Numerade Educator

00:39

Problem 76

Classify each of the following as a ceramic, polymer, or liquid crystal.
a.
b.
c.
d.

Bhumika Jayee

Numerade Educator

02:54

Problem 77

Ceramics are generally brittle, subject to crack failure, and stable to high temperatures. In contrast, plastics are generally deformable under stress and have limited thermal stability. Discuss these differences in terms of the structures and bonding in the two classes of materials.

Bhumika Jayee

Numerade Educator

02:21

Problem 78

A watch with a liquid crystal display (LCD) does not function properly when it is exposed to low temperatures during a trip to Antarctica. Explain why the LCD might not function well at low temperature.

Jennifer Landry

Numerade Educator

02:36

Problem 79

The temperature range over which a liquid possesses liquid crystalline behavior is rather narrow (for examples, see Figure 12.39). Why?

Bhumika Jayee

Numerade Educator

08:12

Problem 80

Suppose that a liquid crystalline material such as cholesteryl benzoate is warmed to well above its liquid crystalline range and then cooled. On cooling, the sample unexpectedly remains clear until it reaches a temperature just below the melting point, at which time it solidifies. What explanation can you give for this behavior?

Susan Hallstrom

Numerade Educator

02:16

Problem 81

Hydrogen bonding between polyamide chains plays an important role in determining the properties of a nylon such as nylon 6,6 (Table 12.5). Draw the structural formulas for two adjacent chains of nylon 6,6, and show where hydrogen-bonding interactions could occur between them.

Bhumika Jayee

Numerade Educator

00:46

Problem 82

A particular liquid crystalline substance has the phase diagram shown in the figure. By analogy with the phase diagram for a non-liquid crystalline substance (Section 11.6), identify the phase present in each area.

Bhumika Jayee

Numerade Educator

01:50

Problem 83

In fabricating microelectronics circuits, a ceramic conductor such as TiSi_{2} is employed to connect various regions of a transistor with the outside world, notably aluminum wires. The TiSi is deposited as a thin film via chemical vapor deposition, in which $\mathrm{TiCl}_{4}(g)$ and $\mathrm{SiH}_{4}(g)$ are reacted at the Si surface. Write a balanced equation for the reaction, assuming that the other products are $\mathrm{H}_{2}$ and $\mathrm{HCl}$. Why might TiSi $_{2}$ behave better as a conducting interconnect on Si than on a metal such as Cu?

Bhumika Jayee

Numerade Educator

04:03

Problem 84

Employing the bond enthalpy values listed in Table $8.4$, estimate the molar enthalpy change occurring upon
(a) polymerization of ethylene, (b) formation of nylon 6,6 ,
(c) formation of polyethylene terephthalate (PET).

Bhumika Jayee

Numerade Educator

03:37

Problem 85

Although polyethylene can twist and turn in random ways, as illustrated in Figure $12.24$, the most stable form is a linear one with the carbon backbone oriented as shown in the figure below:
The solid wedges in the figure indicate bonds from carbon that come out of the plane of the page; the dotted wedges indicate bonds that lie behind the plane of the page.
(a) What is the hybridization of orbitals at each carbon atom? What angles do you expect between the bonds?
(b) Now imagine that the polymer is polypropylene rather than polyethylene. Draw structures for polypropylene in which (i) the $\mathrm{CH}_{3}$ groups all lie on the same side of the plane of the paper (this form is called isotactic polypropylene); (ii) the $\mathrm{CH}_{3}$ groups lie on alternating sides of the plane (syndiotactic polypropylene); or (iii) the $\mathrm{CH}_{3}$ groups are randomly distributed on either side (atactic polypropylene). Which of these forms would you expect to have the highest crystallinity and melting point, and which the lowest? Explain in terms of intermolecular interactions and molecular shapes.
(c) Polypropylene fibers have been employed in athletic wear. The product is said to be superior to cotton or polyester clothing in wicking moisture away from the body through the fabricto the outside. Explain the difference between polypropylene and polyester or cotton (which has many -OH groups along the molecular chain), in terms of intermolecular interactions with water.

Bhumika Jayee

Numerade Educator

02:30

Problem 86

In the superconducting ceramic $\mathrm{YBa}_{2} \mathrm{Cu}_{3} \mathrm{O}_{7}$, what is the average oxidation state of copper, assuming that $Y$ and Ba are in their expected oxidation states? Yttrium can be replaced with a rare-earth element such as La, and Ba can be replaced with other similar elements without fundamentally changing the superconducting properties of the material. However, general replacement of copper by any other element leads to a loss of superconductivity. In what respects is the electron configuration of copper different from that of the other two metallic elements in this compound?

Bhumika Jayee

Numerade Educator

03:46

Problem 87

A sample of the superconducting oxide $\mathrm{HgBa}_{2} \mathrm{Ca}_{2} \mathrm{Cu}_{3} \mathrm{O}_{8+x}$ is found to contain $14.99 \%$ oxygen by mass. (a) Assuming that all other elements are present in the ratios represented by the formula, what is the value of $x$ in the formula? (b) Which of the metallicelementsin the compound is (are) most likely to have noninteger average charges? Explain your answer. (c) Which of the metallic ions in the substance is likely to have the largest ionic radius? Which will have the smallest?

Bhumika Jayee

Numerade Educator

02:43

Problem 88

(a) In polyvinyl chloride shown on Table $12.5$, which bonds have the lowest average bond enthalpy? (b) In the thermal conversion of polyvinyl chloride to diamond, at high pressure, which bonds are most likely to rupture first on heating the material? (c) Employing the values of average bond enthalpy in Table $8.4$, estimate the overall enthalpy change for converting PVC to diamond.

Bhumika Jayee

Numerade Educator

05:53

Problem 89

Consider para-azoxyanisole, which is a nematic liquid crystal in the temperature range of $21{ }^{\circ} \mathrm{C}$ to $47^{\circ} \mathrm{C}$ :
(a) Write out the Lewis structure for this molecule, showing all lone-pair electrons as well as bonds. (b) Describe the hybrid orbitals employed by each of the two nitrogens. What bond angles do you anticipate about the nitrogen atom that is bonded to oxygen? (c) Replacing one of the $-\mathrm{OCH}_{3}$ groups in para-azoxyanisole by $\mathrm{a}$ $-\mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}$ group causes the melting point of
the substance to drop; the liquid crystal range changes to $19{ }^{\circ} \mathrm{C}$ to $76^{\circ} \mathrm{C}$. Explain why this substitution produces the observed changes in properties. (d) How would you expect the density of para-azoxyanisole to change upon melting at $117^{\circ} \mathrm{C}$ ? Upon passing from the nematic to the isotropic liquid state at $137{ }^{\circ} \mathrm{C}$ ? Explain.

Bhumika Jayee

Numerade Educator

10:09

Problem 90

Silicon has diamond structure (Figure 12.3) with unit cell edge length of $5.43 \AA$ and eight atoms per unit cell.
(a) How many silicon atoms are there in $1 \mathrm{~cm}^{3}$ of material? (b) Suppose you dope that $1-\mathrm{cm}^{3}$ sample of silicon with 1 ppm of phosphorus that will increase the conductivity by a factor of a million. How many milligrams of phosphorus are required?

Susan Hallstrom

Numerade Educator