Kelter P., Mosher M., Scott A. Chemistry. The Practical Science

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558 Chapter 13 Modern Materials

Whereas metals and plastics can bend

and stretch, ceramic materials are less

ductile. In fact, their ability to withstand compression

makes them ideal candidates for supporting weight,

so ceramics are commonly used as building materials

such as plaster, mortar, and cement.

The formation of plaster and that of mortar are

similar in that a natural material is converted into a

different compound that can be molded before it re-

turns to its natural state. In plaster, the reactions that

take place are based on the dehydration and hydration

chemistry of calcium sulfate (CaSO

). To make plaster,

the mineral gypsum (CaSO

·2H

O) is mined and

ground into a powder. This powder, when heated at

150°C to remove some of the water, is converted into

calcium sulfate hemihydrate (CaSO

⁄

O). The hemi-

hydrate, also known as plaster of Paris (after a gypsum

deposit near Paris, France, that had been mined since

the seventeenth century), is then mixed with water to

make a paste. The hemihydrate reabsorbs the water to

re-form the gypsum, which is now in a shape that the

manufacturer has molded.

CaSO

·2H

O + heat n CaSO

⁄2H

O +

⁄2H

CaSO

⁄2H

O +

⁄2H

O n CaSO

·2H

O + heat

Construction workers use mortar to hold bricks

together in the shape of a wall like that shown in

Figure 13.20. Mortar is made from calcium carbonate

(CaCO

, limestone) by heating to a high temperature

in a kiln. The calcium oxide (CaO, lime) that results is

mixed with sand and added to water. This wet paste

is sandwiched between the bricks in a wall. Slowly, the

lime in the paste reacts to form calcium hydroxide

(Ca(OH)

, slaked lime). Over time, as the mortar is

“curing,” the reaction of carbon dioxide in the air

returns the slaked lime to limestone.

CaCO

+ heat n CaO + CO

CaO + H

O n Ca(OH)

Ca(OH)

+ CO

n CaCO

+ H

The mortar also reacts with the SiO

that is the

main chemical component of sand to form complex

calcium silicates via an acid–base reaction between

the lime (CaO; base) and the sand (SiO

; acid).

Cement is a much more complex material than

plaster or mortar, but the chemistry of its hardening is

very similar. Made by heating shale and limestone at

1500°C, the product (called clinker) is ground up in

large rotating chambers containing steel balls until it

has the consistency of ﬂour. Gypsum is often added to

the mixture to improve its hardening properties. The

result is dry cement. Speciﬁc amounts of water, sand,

and pebbles are combined with the dry cement to

make concrete. The reactions, shown below, that harden

the cement then begin, trapping the sand and pebbles

inside the mixture. Some of the grains of the mixture

begin absorbing water and react with it (hydration).

Although the rate of hydration is relatively slow for ce-

ment, a poured driveway is often hard enough to use

within a few days. Most cements continue to harden for

up to a year.

SiO

+ 3H

O n Ca

SiO

·2H

O + Ca(OH)

2Ca

SiO

+ 7H

O n Ca

·4H

O + 3Ca(OH)

+ 3CaSO

·2H

O + 24H

O n

(SO

)

(OH)

·24H

NanoWorld / MacroWorld

Big effects of the very small: The chemistry of cement

FIGURE 13.20

Mortar is used to

hold these bricks

together.

A ball mill in a cement plant grinds the material into clinker.

This is just one of the steps taken to produce cement.

13.3 Ceramics 559

Comparison of Ceramics with Other Materials

Hardness is a measure of the relative ability of a material to scratch another; it is

expressed using the Mohs hardness scale. The higher the hardness value, the harder

the material (talc  1, gypsum  2, diamond  10). The coefﬁcient of expansion

indicates the size change upon heating or cooling. A larger value indicates a greater

change in the size of the material as it is heated or cooled. Note that the ceramics

(alumina and zirconia) have relatively high melting points, are hard, and don’t

expand much when heated.

Coefﬁcient of

Melting Point Density Expansion

Material (°C) (g/cm

) Hardness (Mohs) (°C

–1

× 10

–6

)

Ceramics

Alumina (Al

) 2050 3.8 9 8.1

Zirconia (ZrO

) 2660 5.6 8 6.6

Alloys

Steel 1370 7.9 5 15

Brass 930 8.6 4 20

Metals

Zinc 420 7.1 2.5 35

Sodium 98 0.97 0.4 70

TABLE 13.5

Coating the hull with materials capable of withstanding the heat and resisting ex-

pansion protects the astronauts and the shuttle itself. Even though ceramics alone

can satisfactorily insulate the bottom of the shuttle, they are often glazed with sil-

ica glass. This improves the thermal insulation and the durability of the tiles.

Glass

The glass used to make test tubes for the hospital laboratory is an amorphous

solid ceramic material. Solids such as obsidian, a natural glass ejected from vol-

canoes, shown in Figure 13.21, lack order in the arrangement of the particles that

constitute them. Human-made glass has been known throughout history. For ex-

ample, the ancient Mesopotamians and Egyptians made a very successful busi-

ness out of the preparation of glass objects. Today, we use glass in many different

applications, as illustrated in Table 13.6. The windows on the doors into the hos-

pital, the screens on the computers at the nurses’ stations and the drinking glasses

in the cafeteria are all made of glass.

The simplest glass is made from quartz (crystalline silica). Figure 13.22 illus-

trates the veryorderedstructure of SiO

units in quartz.When heated tothe melting

point (1600°C), some of the silicon–oxygen bonds break and the ordered structure

is disturbed. Rapid cooling of this material results in a highly disordered struc-

ture. The resulting silica glass is composed of SiO

units connected in a very

disordered array. Because of the disorder, glass does not have a deﬁnite melting

point. Instead, silica glass has a transitional temperature where it begins to soften.

When it is in this softened state, glass can be molded into many different shapes.

Of particular use to the endoscopist at the hospital is the ﬁber optic line, a

hair-thin wire of plastic-coated glass that can be used to transport light along its

length (Figure 13.23). On the end of the line are a small camera and light. The In-

ternet that connects the emergency room computer to data sources across the

world also uses ﬁber optic lines to transmit information. How does light bend

along the path taken by the ﬁber optic wire? John Tyndall (1820–1893) noted that

light could be bent inside a bent stream of water by a property known as total in-

ternal reﬂectance. Total internal reﬂectance of the light ensures that any light

inside the glass pipe reaches the end of the ﬁber optic wire. Because a ﬁber optic

FIGURE 13.21

Obsidian, a natural glass.

Ancient peoples used obsidian

as a tool because of its

sharpness and durability.

Application

FIGURE 13.22

Silica glass (SiO

), also known as quartz glass, or

fused silica. While this type of glass is far superior

in many ways to other types of glass, it is much

more expensive and much more difﬁcult to mold

into useful apparatii.

wire can direct light around corners, the endoscopist can direct the light to

any part of a patient’s interior without the need for large incisions. The in-

credible ﬂexibility of ﬁber optic wire has increased its use as telephone wire

and for connections between computers and other electronic communica-

tion systems.

560 Chapter 13 Modern Materials

Quartz crystal Glass

Oxygen

Silicon

The Six Main Types of Glass

Resistance to Resistance to

Temperature Corrosive Sample Composition (in addition

Type of Glass Example of Use Shock Chemicals to silica, SiO

), Mass %

Soda-lime glass Most common Not good Fair 16% Na

O, 7% CaO, 3% MgO,

(90% of all glass) 1% Al

0.3% K

Lead crystal Good electrical Not good Fair 24+% PbO

insulator, brilliance

when cut

Borosilicate Light bulbs, bakeware Good Good 12% B

, 5% Na

O, 5% Al

Aluminosilicate Resistors Good Good 7.0% B

, 10.4% Al

, 21.0% CaO,

1.0% Na

Silica Chemical glassware Excellent Good 96% SiO

Fused silica Cuvettes, space Excellent Excellent 100% SiO

shuttle windows

TABLE 13.6

FIGURE 13.23

The endoscope in action. A ﬁber optic

cable can be used to explore inside a

body with no need for large incisions.

EXERCISE 13.5 Classifying Common Materials

Classify each of the following common household items as a metal, an alloy, a glass,

or a ceramic: A sidewalk, a 1-oz silver dollar, a bicycle frame, and a perfume bottle.

Solution

The sidewalk is made from concrete. Concrete is the mixture of cement and small

stones or sand. Cement is a ceramic material.

Some coins, such as the 1-oz U.S. silver dollar, are made of nearly pure silver

(>99% pure silver metal). However, most coins (such as the U.S. Golden Dollar), are

made of alloys to help reduce the expense of their manufacture.

The bicycle frame, in order to be light yet durable, is made out of a metal alloy.

The top bicyclists use only aluminum, magnesium, or titanium alloys.

The perfume bottle is typically made from glass. The transparency, esthetic

beauty, and low cost of glass make it an excellent material for bottles.

PRACTICE 13.5

Try some on your own. What class of materials makes up the rims on your car? . . .

a casserole dish? . . . the toilet in your bathroom? ...a wedding band?

See Problems 51 and 52.

Superconductors

Some patients in the hospital enter through the emergency room as cases of

trauma. Often, the patient presents symptoms that are not easily identiﬁable by

external examination of the body. At this point, physicians call upon devices that

can “see” inside a patient, such as an X-ray machine, which can take images of

bones. Another instrument that can investigate the structure of the organs in the

body is the magnetic resonance imager (MRI) shown in Figure 13.24.

The MRI works by measuring the absorption of energy by a particular type of

nucleus in the radio frequency range of the electromagnetic spectrum. However,

in order for the absorption to be noticed, the nucleus must be aligned with a

strong magnetic ﬁeld. The size of the magnetic ﬁelds required is much greater

that what can be obtained with a bar magnet. In fact, the bulk of the

MRI is occupied by a superconducting magnet that can generate

these large magnetic ﬁelds. What is a superconductor and how does

it ﬁnd use as a magnet?

superconductor is a type of ceramic material that acts quite

differently than a metal. The conduction of electrons in metals is

met with resistance. We use this property to toast bread when the

NiChrome (an alloy of nickel, chromium, and, often, iron and other

metals) coils of a toaster get quite hot and exhibit their characteris-

tic orange glow, as a consequence of their high electrical resistance.

We might imagine that copper wires could be used to toast bread,

too. However, copper melts when you put enough voltage through

it to get it hot enough to toast bread. NiChrome wire, on the other

hand, has a high melting point and won’t melt inside your toaster.

Why do ceramics and metals such as NiChrome get hot when

electrons ﬂow through them? The resistance is due to a combina-

tion of impurities in the alloy and to the existence of induced quan-

tized vibrations (called

phonons) in the metal lattice. Resistance

13.3 Ceramics 561

Application

HEMICAL ENCOUNTERS:

Magnetic Resonance

Imaging

KELTER/MOSHER 1/e

329670_ula_13_07.eps

11/22/05

size: 17p0 x 9p3

External

magnetic field

+ Energy

Nucleus

External

magnetic field

A nucleus acts as a tiny bar magnet and aligns with an

external magnetic ﬁeld. If the nucleus then absorbs en-

ergy in the radio frequency range, it can ﬂip over and

align against the external magnetic ﬁeld. This higher

energy state is unstable, and as the nucleus realigns

with the external magnetic ﬁeld, it gives off the energy

it absorbed.

FIGURE 13.24

The magnetic resonance imager at work.

The MRI generates powerful magnetic ﬁelds

using a superconducting ceramic.

Video Lesson: CIA

Demonstration:

Superconductivity

Visualization: Magnetic

Levitation by a Superconductor

(a)

(b)

562 Chapter 13 Modern Materials

A nuclear magnetic resonance spec-

trophotometer can be used to obtain

information on the structure of a

molecule. A hydrogen atom spectrum

of ethylbenzene illustrates the infor-

mation that is obtained. This informa-

tion can be used to build the struc-

ture of a molecule.

increases as the vibrations of the metal lat-

tice increase (the metal gets hotter), and

eventually the metal can reach its melting

point or combust. For instance, if too

much current is placed into the NiChrome

wires in your toaster, they will catch ﬁre.

When metals are cooled, the resistance

to the ﬂow of electrons drops until it

reaches a constant positive value. A super-

conductor acts differently. As the tempera-

ture of the superconducting ceramic is

lowered, the resistance to the ﬂow of elec-

trons decreases, as in metals. However, at

the

transition temperature, the resistance to

the ﬂow of electrons becomes negligible.

What does “negligible” mean to us and how

does that make a superconductor useful?

No resistance implies that no energy is lost

in the movement of electrons along the su-

perconductor’s lattice. If the coil in your

toaster were made from a superconducting

wire, the electrons could pass through it

without resistance, and it wouldn’t get hot.

It wouldn’t toast either. With a superconducting wire, electricity could be deliv-

ered to your home efﬁciently and very inexpensively (gone would be the need for

the step-down transformer on the telephone pole near your home). This same

lack of resistance also implies that a superconductor can carry extremely large

electric currents. This is exactly what is needed in the MRI.

To generate the sizable magnetic ﬁeld needed in the MRI, a large electric cur-

rent is placed in a coil of superconducting ceramic wire. As the electrons travel

around the coil, they generate the magnetic ﬁeld perpendicular to the ﬂow of

the electrons. In order to be superconducting, the ceramic must be kept cold,

typically at the temperature of boiling liquid helium (4 K). However, ongoing

research on “high-temperature”superconductors may one day eliminate the need

for liquid helium. This would be quite beneﬁcial because helium is a nonrenew-

able resource. Table 13.7 lists some superconductors and the

temperature at which each is superconducting.

The MRI is actually based on a key instrument in chemical

research: the nuclear magnetic resonance spectrophotome-

ter (NMR). Within nuclei of the same type (here, all hydro-

gen nuclei) a particular range of frequencies is commonly

absorbed. Moreover, speciﬁc electromagnetic environments

Comparison of Superconducting Materials

The record for the highest superconducting temperature for a ceramic

material is held by a complex mixture of atoms (Hg

0.8

0.2

8.33

The noninteger subscripts are used in these formulas to show a simple

ratio of the atoms. In order for us to observe the superconductive proper-

ties of the commonly used superconductors, they must be immersed in

liquid helium (boiling point = –268.9°C, or 4.2 K).

Highest Superconducting

Material Temperature (K)

0.8

0.2

8.33

138 (record)

HgBa

133–135

YPd

C23

LuNi

C 16.6

Tm Ni

C11

ErNi

C 10.5

0.6

0.4

9.8

Lead (fcc) 7.196

Tantalum (bcc) 4.47

Aluminum (fcc) 1.175

Platinum (fcc) 0.0019

TABLE 13.7

within a molecule cause individual nuclei to absorb slightly different frequencies

of radio waves. When a compound is placed in the NMR, these speciﬁc frequen-

cies are recorded. Then they can be used to identify the environments within a

molecule. With a very powerful magnetic ﬁeld, chemists can use this information

from the NMR to determine the structural formula for a molecule.

13.4 Plastics

Today’s hospitals are completely different from those of 50 years ago. Advances in

the ﬁeld of medicine from open-heart surgery to the use of more powerful and

useful medicines are among the most dramatic changes. Walk into a hospital

and what do you see? Plastics. Everywhere you look,

plastics have found a use. The

nurse, the physician, and the staff work with plastics in the form of gloves,

smocks, masks, and surgical booties. During the time in the hospital, the patient

is exposed to plastic in the form of syringes, tubing, sterile packaging, bandages,

and even the chairs in the waiting room.

What is plastic? All plastics are polymers (see Chapter 12), but not all poly-

mers are plastics. The DNA and protein within us, for example, are naturally

occurring polymers that are not plastics. Plastics are polymers that can be

molded into a shape and then hardened in that form. In 1907, the U.S. chemist

Leo Baekeland prepared the ﬁrst completely synthetic plastic. Named after its in-

ventor, bakelite could be formed easily into almost any shape. After it hardened,

bakelite was tough, durable, and resistant to heat. Bakelite was used in the early

half of the 1900s as a lightweight counterpart to steel. Like most plastics, it also

works as a good insulator, which increases its utility as handles for frying pans,

spatulas, electrical plate covers, and other household items.

The successes with Bakelite prompted the preparation of many more types of

plastics, including polyethylene, saran, Teﬂon, nylon, neoprene, and a host of oth-

ers. Today, our world is inundated with plastics, which are used wherever possible

because they are light in weight and low in cost (see Table 13.8).

Why are there so many different types of plastics? The short answer

is that different types of plastics have different properties. Differing

properties suit different uses. A rigid plastic would not make a good

climbing rope because it does not bend. A stretchable plastic wouldn’t

make a good ruler or calculator housing. Therefore, we make different

plastics for different speciﬁc applications. The properties of a plastic

are related to the way it is manufactured and to its composition. By

controlling the polymerization reactions, we can make short polymer

chains (with lower melting points) or long polymer chains.

Crosslinking,

or linking the chains of adjacent polymer strands together, bestows

strength on the overall plastic. Orienting the chains in parallel makes

stretchable ﬁbers.

Fibers

Some airplanes are made largely of plastic—not the same type of plastic that is in

your soda bottle, but a plastic that is made in roughly the same manner. The type

of plastic that makes up the wings of some planes is a polymeric ﬁber. A

ﬁber,

some examples of which are shown in Table 13.9 on page 566, is a polymer whose

chains are aligned in one direction. Fibers have a high

tensile strength (they

stretch without breaking when you pull them), a property that is good for, among

other products, airplane wings. Airplane wings need to be able to stretch when

the plane rides through turbulence. If the plastic weren’t stretchable, the air-

plane’s wings could snap off.

13.4 Plastics 563

Parallel chains improve

the strength of a polymer.

Crosslinking the chains makes

the polymer even stronger.

564 Chapter 13 Modern Materials

Selected Plastics and Their Chemical Composition

Plastic (and

representative uses) Structure Monomer

Polyethylene (plastic

bags, sheeting)

Polypropylene

(car parts, food

packaging)

Styrofoam

(coffee cups)

Polyethylene

terephthalate

(soda bottles)

CH CH

TABLE 13.8

OO HH

CCCOH

CH CH

CHH

CH CH

CHHC

C CH

O CH

13.4 Plastics 565

Other plastic ﬁbers can be spun into thread and then woven into textiles. Put

a price tag and a label on them, and you could sell them in the clothing store.

Nylon and polyester, when made into clothing like that shown in Figure 13.25, are

two very common plastic ﬁbers. Fibers do have drawbacks, however. They stretch

only in the direction in which they are aligned, so if you pull on a ﬁber in the

wrong direction, it tends to come apart quite easily. To alleviate this problem,

the ﬁber can be manufactured with the addition of a different material. The re-

sult is a

composite material that possesses the strengths of both components.

Composite materials don’t need to be made from only plastics. In fact, a brick

wall is an example of a composite material (cement and ceramic bricks). Another

example is the 50/50 cotton/polyester blend used to make shirts.

(Continued)

Plastic (and

representative uses) Structure and Monomer

TABLE 13.8

Polyurethane

(foam, gaskets,

bearings)

NNCOO

Nylon [6,

Polyester [3

,16]

OOCCH

(CH

)

FIGURE 13.25

Nylon and polyester.

566 Chapter 13 Modern Materials

Common Plastic Fibers

Fiber Structure

Kevlar

(note interactions between strands)

Nylon

(note interactions

between strands)

Polyester

TABLE 13.9

Carbon ﬁbers

(note graphite-like structure)

N N N N N N N

O CH

O (CH

)

13.4 Plastics 567

Aramid (short for aromatic amide)

plastics such as Kevlar®, shown in Fig-

ure 13.26, are an example of a composite

ﬁber that is particularly strong. What makes

this material so strong? The adjacent poly-

mer chains in the material are held together

by hydrogen bonds. If the ﬁber is mixed

with an epoxy resin when it is manufac-

tured, it becomes even stronger. The ﬁbers

are then woven together to make fabric that

is light, ﬂexible, and so strong that it can

even stop bullets, enabling police depart-

ments to use them as bulletproof vests.

High tensile strength and low density make

Aramid plastics better than steel.

Carbon ﬁber is another of the compos-

ite materials. A polymerization reaction of

acrylonitrile gives the polymer polyacry-

lonitrile (PAN) via the reactions illustrated

in Figure 13.27. Heating PAN in air to

300°C oxidizes the polymer. To ﬁnish the

preparation of a carbon ﬁber, the oxidized

PAN is heated to 3000°C, resulting in a structure that is very similar to graphite

and is extremely strong. It is three times stronger than high-tensile steel, but only

one-sixth as dense. This makes carbon ﬁber composites the perfect choice for air-

planes, boats, and other structural components (light, yet extremely strong).

FIGURE 13.26

Aramid polymers and

the bulletproof vest.

N N N N N N N

C C C C C C C C

Polyacrylonitrile

Heat

700°C

400–600°C

FIGURE 13.27

Reactions that make carbon ﬁber.

Application