Chemistry part 36, Julia Burdge,2e (2009) ppsx

In Sections 23.5 through 23.7 we will study the chemistry of selected metals from Group lA the alkali metals, Group 2A the alkaline earth metals, and Group 3A aluminum.. Because of their

890 CHAPTER 23 Metallurgy and the Chemistry of Metals

Figure 23.11 (a) Silicon crystal

doped with phosphorus (b) Silicon

crystal doped with boron Note the

formation of a negative center in (a)

and a positive center in (b)

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Figure 23.12 Main group metal s

(gree n ) and Group 2B metal s (b lue )

according to their po s itions in the

periodic table

for example, when a trace amount of boron or phosphorus is added to solid silicon (Only about five out of every million Si atoms are replaced by B or P atoms.) The structure of solid silicon is similar to that of diamond; that is, each Si atom is covalently bonded to four other Si atoms Phos-phorus ([Ne]3/3p 3 ) has one more valence electron than silicon ([Ne ]3 s 2 3 p 2), so there is a valence

electron left over after four of them are used to form covalent bonds with silicon (Figure 23.11)

This extra electron can be removed from the phosphorus atom by applying a voltage across the solid The free electron can move through the structure and function as a conduction electron Impurities of this type are known as donor impurities, because they provide conduction electrons Solids containing donor impurities are called n-type semiconductors, where n stands for negative (the charge of the "extra" electron)

The opposite effect occurs if boron is added to silicon A boron atom has three valence electrons Oi2/2pl), one less than silicon Thus, for every boron atom in the silicon crystal, there

is a single vacancy in a bonding orbital It is possible, though, to excite a valence electron from a nearby Si into this vacant orbital A vacancy created at that Si atom can then be filled by an elec-tron from a neighboring Si atom, and so on In this manner, electrons can move through the crystal

in one direction while the vacancies, or "positive holes," move in the opposite direction, and the

solid becomes an electrical conductor Impurities that are electron deficient are called acceptor impurities Semiconductors that contain acceptor impuritie ~ are called p-type semiconductors,

where p stands for positive

In both the p-type and n-type semiconductors, the energy gap between the valence band and the conduction band is effectively reduced, so only a small amount of energy is needed to excite the electrons Typically, the conductivity of a semiconductor is increased by a factor of 100,000 or

so by the presence of impurity atoms

The growth of the semiconductor industry since the early 1960s has been truly remarkable Today semiconductors are essential components of nearly all electronic equipment, ranging from radios and television sets to pocket calculators and computers One of the main advantages of

solid-state devices over vacuum-tube electronics is that the former can be made on a single "chip"

of silicon no larger than the cross section of a pencil eraser Consequently, much more equipment can be packed into a small volume a point of particular importance in space travel, as well as in handheld calculators and microprocessors (computers-on-a-chip)

Periodic Trends in Metallic Properties

Metals are lustrous in appearance, solid at room temperature (with the exception of mercury), good conductors of heat and electricity, malleable (can be hammered fiat), and ductile (can be drawn into wires) Figure 23.12 shows the positions of the representative metals and the Group

SECTION 23.5 The Alkali M etals 891

2B metals in the periodic table (The transition metals are discussed in Chapter 22.) As we saw in

Chapter 8, the electronegativity of elements increases from left to right across a period and from

bottom to top in a group [ ~ ~ Section 8.4, Figure 8.6] The metallic character of metals increases

in just the opposite directions-that is, from right to left across a period and from top to bottom

in a group Because metals generally have low electronegativities, they tend to form cations and

almost always have positive oxidation numbers in their compounds However, beryllium and mag

-nesium in Group 2A and the metals in Group 3A and beyond also form covalent compounds

In Sections 23.5 through 23.7 we will study the chemistry of selected metals from Group lA (the alkali metals), Group 2A (the alkaline earth metals), and Group 3A (aluminum)

As a group, the alkali metals (the Group 1A elements) are the most electropositive (or the least

electronegative) elements known They exhibit many similar properties, some of which are listed

in Table 23.4 Based on their electron configurations, we expect the oxidation number of these

ele-ments in their compounds to be + 1 because the cations would be isoelectronic with the preceding

noble gases This is indeed the case

The alkali metals have low melting points and are soft enough to be sliced with a knife These metals all possess a body-centered crystal structure with low packing efficiency This accounts

for their low densities among metals In fact, lithium is the lightest metal known Because of

their great chemical reactivity, the alkali metals never occur naturally in elemental form; instead,

they are found combined with halide, sulfate, carbonate, and silicate ions In this section we will

describe the chemistry of two members of Group lA sodium and potassium The chemistry of

lithium, rubidium, and cesium is less important; all isotopes of francium, the last member of the

group, are radioactive

Sodium and potassium are about equally abundant in nature They occur in silicate minerals such as albite (NaAISi30g) and orthoclase (KAISi30g) Over long periods of time (on a geologic

scale), silicate minerals are slowly decomposed by wind and rain, and their sodium and potassium

ions are converted to more soluble compounds Eventually rain leaches these compounds out of

the soil and carries them to the sea Yet when we look at the composition of seawater, we find

that the concentration ratio of sodium to potassium is about 28 to 1 The reason for this uneven

distribution is that potassium is essential to plant growth, while sodium is not Thus, plants take

up many of the potassium ions along the way, while sodium ions are free to move on to the sea

Other minerals that contain sodium or potassium are halite (NaCl), shown in Figure 23.13, Chile

saltpeter (NaN03), and sylvite (KCI) Sodium chloride is also obtained from rock salt

Metallic sodium is most conveniently obtained from molten sodium chloride by electroly-' sis in the Downs cell (review Figure 19.10) The melting point of sodium chloride is rather high

(801 °C), and much energy is needed to keep large amounts of the substance molten Adding a

suitable substance, such as CaCl2, lowers the melting point to about 600°C a more convenient

temperature for the electrolysis process

Standard reduction potential (V)t -3.05 -2.71 -2.93 -2.93 -2.92

* Refers to the cation M + , where M denotes an alkali metal atom

t The ha l f-reaction i s M +(aq) + e - • M(s)

Figure 23.13 Halite (NaCt)

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892 CHAPTER 23 Metallurg y and the Chemistr y of Metals

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Multimedia Periodic Table properties of the alkal i and

alkaline earth metals

Figure 23.14 Self-contained

breathing apparatus

Metallic potassium cannot be easily prepared by the electrolysis of molten KCl because it is too soluble in the molten KCl to float to the top of the cell for collection Moreover, it vaporizes readily at the operating temperatures, creating hazardous conditions Instead, it is usually obtained

by the distillation of molten KCl in the presence of sodium vapor at 892°C The reaction that takes place at this temperature is

Na (g) + KCl(l) + NaCl(l) + K(g)

This reaction may seem strange given that potassium is a stronger reducing agent than sodium

(see Table 23.4) Potassium has a lower boiling point (770°C) than sodium (892°C), however, so

it is more volatile at 892°C and distills off more easily According to Le Chfltelier's principle,

con-stantly removing the potassium vapor drives the reaction to the right, ensuring metallic potassium

is recovered

Sodium and potassium are both extremely reactive, but potassium is the more reactive ofthe

two Both react with water to form the corresponding hydroxides In a limited supply of oxygen,

sodium bums to form sodium oxide (Na?O) In the presence of excess oxygen, however, sodium forms the pale-yellow peroxide:

Sodium peroxide reacts with water to give an alkaline solution and hydrogen peroxide:

Like sodium, potassium forms the peroxide In addition, potassium also forms the superoxide when it bums in air:

K(s) + 0 2 (g) + K02(s) When potassium superoxide reacts with water, oxygen gas is evolved:

This reaction is utilized in breathing equipment (Figure 23.14) Exhaled air contains both ture and carbon dioxide The moisture reacts with K02 in the apparatus to generate oxygen gas as shown in the preceding reaction Furthermore, K02 also reacts with exhaled CO2, which produces more oxygen gas:

mois-Thus, a person using the apparatus can continue to breathe oxygen without being exposed to toxic fumes outside

Sodium and potassium metals dissolve in liquid ammonia to produce beautiful blue solutions:

Na N H 3 Na + +

e-

Both the cation and the electron exist in the solvated form, and the solvated electrons are sible for the characteristic blue color of such solutions Metal-ammonia solutions are powerful reducing agents (because they contain free electrons); they are useful in synthesizing both organic and inorganic compounds It was discovered that the hitherto unknown alkali metal anions, M-, are also formed in such solutions This means that an ammonia solution of an alkali metal contains ion pairs such as Na +Na- and K+K- ! (In each case, the metal cation exists as a complex ion with

respon-crown ether, an organic compound with a high affinity for cations.) In fact, these "salts" are so

stable that they can be isolated in crystalline form This finding is of considerable theoretical est, because it shows clearly that the alkali metals can have an oxidation number of -1 , although

inter 1 is not found in ordinary compounds

Sodium and potassium are essential elements of living matter Sodium ions and potassium ions are present in intracellular and extracellular fluids, and they are essential for osmotic balance and enzyme functions We now describe the preparations and uses of several of the important com-pounds of sodium and potassium

Sodium Chloride

The source, properties, and uses of sodium chloride were discussed in Chapter 7

SECTION 23.6 The Alkaline Earth Metals 893

Sodium Carbonate

Sodium carbonate (called soda ash) is used in all kinds of industrial processes, including water

treatment and the manufacture of soaps, detergents, medicines, and food additives Today about

half of all Na2C03 produced is used in the glass industry Sodium carbonate ranks eleventh among

the chemicals produced in the United States For many years, Na2C03 was produced by the

Solvay2 process, in which ammonia is first dissolved in a saturated solution of sodium chloride

Bubbling carbon dioxide into the solution precipitates sodium bicarbonate as follows:

Sodium bicarbonate is then separated from the solution and heated to give sodium carbonate:

However, the rising cost of ammonia and the pollution problem resulting from the by-products

have prompted chemists to look for other sources of sodium carbonate One is the mineral trona

[N as(C0 3 MHC0 3) • 2H20], large deposits of which have been found in Wyoming When trona is

crushed and h.,ated, it decomposes as follows:

The sodium carbonate obtained this way is dissolved in water, the solution is filtered to remove

the insoluble impurities, and the sodium carbonate is crystallized as Na2C03 lOH20 Finally, the

hydrate is heated to give pure, anhydrous sodium carbonate

Sodium Hydroxide and Potassium Hydroxide

The properties of sodium hydroxide and potassium hydroxide are very similar These hydroxides

are prepared by the electrolysis of aqueous NaCI and KCI solutions; both hydroxides are strong

bases and very soluble in water Sodium hydroxide is used in the manufacture of soap and many

organic and inorganic compounds Potassium hydroxide is used as an electrolyte in some storage

batteries, and aqueous potassium hydroxide is used to remove carbon dioxide and sulfur dioxide

from air

Sodium Nitrate and Potassium Nitrate

Large deposits of sodium nitrate (Chile saltpeter) are found in Chile It decomposes with the

evo-lution of oxygen at about 500°C:

Potassium nitrate (saltpeter) is prepared beginning with the "reaction"

KCI(aq) + NaN0 3 (aq) - - + KN0 3 (aq) + NaCI(aq)

This process is carried out just below 100°C Because KN03 is the least soluble salt at room

remperature, it is separated from the solution by fractional crystallization Like NaN03, KN03

decomposes when heated

Gunpowder consists of potassium nitrate, wood charcoal, and sulfur in the approximate

pro-portions of 6: 1: 1 by mass When gunpowder is heated, the reaction is

The sudden formation of hot nitrogen and carbon dioxide gases causes an explosion

The Alkaline Earth Metals

The alkaline earth metals are somewhat less electropositive and less reactive than the alkali

met-als Except for the first member of the family, beryllium, which resembles aluminum (a Group 3A

metal) in some respects, the alkaline earth metals have similar chemical properties Because their

~

~.1 ions attain the stable electron configuration ofthe preceding noble gas, the oxidation number

0.- alkaline earth metals in the combined form is almost always +2 Table 23.5 lists some common

:: F::nest So l v ay (1838-1922) Belgian chemist Solvay's main contribution to indu s trial chemi s tr y wa s the de v elopment of

!he proc e s s for the production of sodium carbonate that now bear s his name

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894 CHAPTER 23 Metal l urgy and the Chemistry of Metals

Second ionization energy (k J/mol ) 1757 1450 1145 1058 958

Standard reduction potential (V)t -1.85 -2.37 -2.87 -2.89 -2.90

* Refers to the cation M2+ , w here M denotes an alkali earth metal atom

t The half-reaction is M 2 + (aq) + 2e - • M(s)

properties of these metals Radium is not included in the table because all radium isotopes are radioactive and it is difficult and expensive to study the chemistry of this Group 2A element

Magnesium

Magnesium is the sixth most plentiful element in Earth's crust (about 2.5 percent by mass) Among the principal magnesium ores are brucite [Mg(OH)z], dolomite (CaC03 • MgC03) (Figure 23.l5), and epsomite (MgS04 7HzO) Seawater is a good source of magnesium there are about 1.3 g of magnesium in each kilogram of seawater As is the case with most alkali and alkaline earth met-als, metallic magnesium is obtained by electrolysis, in this case from its molten chloride, MgCl2 (obtained from seawater)

The chemistry of magnesium is intermediate between that of beryllium and the heavier Group 2A elements Magnesium does not react with cold water but does react slowly with steam:

MgO (s) + H z O(I) - -+ Mg(OHh(s)

Magnesium is a typical alkaline earth metal in that its hydroxide is a strong base (The only line earth hydroxide that is not a strong base is Be(OHh which is amphoteric.)

alka-The major uses of magnesium are in lightweight structural alloys, for cathodic protection; in organic synthesis; and in batteries Magnesium is essential to plant and animal life, and Mg2+ ions are not toxic It is estimated that the average adult ingests about 0.3 g of magnesium ions daily

Magnesium plays several important biological roles It is present, for instance, in intracellular and extracellular fluids, and magnesium ions are essential for the proper functioning of a number

of enzymes Magnesium is also present in the green plant pigment chlorophyll, which plays an important part in photosynthesis

Calcium

Earth's crust contains about 3.4 percent calcium by mass Calcium occurs in limestone, calcite, chalk, and marble as CaC03 ; in dolomite as CaC03 • MgC03 (see Figure 23.l5); in gypsum as

CaS04 2H20; and in fluorite as CaF2 (Figure 23.16) Metallic calcium is best prepared by the

electrolysis of molten calcium chloride (CaCI2)

As we read down Group 2A from beryllium to barium, metallic properties increase Unlike

beryllium and magnesium, calcium (like strontium and barium) reacts with cold water to yield the

corresponding hydroxide, although the rate of reaction is much slower than those involving the

alkali metals:

Ca(s) + 2H 2 0(l) - Ca(OHMaq) + H 2 (g)

Calcium hydroxide [Ca(OH)zJ is commonly known as slaked lime or hydrated lime Lime (CaO),

which is also referred to as quicklime, is one of the oldest materials known to humankind

Quick-lime is produced by the thermal decomposition of calcium carbonate:

whereas slaked lime is produced by the reaction between quicklime and water:

CaO(s) + H 2 0 ( I ) - Ca ( OH Maq)

Quicklime is used in metallurgy (see Section 23.2) and in the removal of S02 when fossil fuel is

burned Slaked lime is used in water treatment For many years, farmers have used lime to lower

the acidity of the soil for their crops (a process called limin g) Nowadays lime is also applied to

lakes affected by acid rain

Metallic calcium has rather limited uses It serves mainly as an alloying agent for metals

like aluminum and copper and in the preparation of beryllium metal from its compounds It is also

used as a dehydrating agent for organic solvents

Calcium is an essential element in living matter It is the major component of bones and

teeth; the calcium ion is present in a complex phosphate salt called hydroxyapatite [Ca5(P04)30HJ

A characteristic function of Ca2+ ions in living systems is the activation of a variety of metabolic

processes, including a vital role in heart action, blood clotting, muscle contraction, and nerve

impulse transmission

Aluminum

Aluminum is the most abundant metal and the third most plentiful element in Earth's crust

(7.5 percent by mass) The elemental fOlln does not occur in nature; instead, its principal ore is

bauxite (A120 3 2H20) Other minerals containing aluminum are orthoclase (KA1Si30 8), beryl

(Be3AI2Si6018), cryolite (Na3AlF6), and corundum (A120 3) (Figure 23.17)

Aluminum is usually prepared from bauxite, which is frequently contaminated with silica

(Si02), iron oxides, and titanium(IV) oxide The ore is first heated in sodium hydroxide solution to

convelt the silica into soluble silicates:

Si02(s) + 20H - (aq) - SiO j - (aq) + H 2 0 (l)

At the same time, aluminum oxide is converted to the aluminate ion (AI0 2):

Iron oxide and titanium oxide are unaffected by this treatment and are filtered off Next, the so

lu-tion is treated with acid to precipitate the insoluble aluminum hydroxide:

After filtration, the aluminum hydroxide is heated to obtain aluminum oxide:

Anhydrous aluminum oxide, or co rundum, is reduced to aluminum by the Ha1l 3 process Figure

23.18 shows a Hall electrolytic cell, which contains a series of carbon anodes The cathode is also

made of carbon and constitutes the lining inside the cell The key to the Hall process is the use of

3 Charles Martin H a ll (1863-1914) American invent or While H a ll was an und e r gra duate at Oberlin Co ll ege he b ecame

inter este d in findin g a n in ex pen s ive way t o extract aluminum Shortly after graduation, when h e was only 22 years o ld,

Hall s ucceeded in o bt ain in g aluminum from a luminum oxi d e in a backyard woodshed A ma zi n g l y, the sa m e discovery was

mad e at almost the sa m e moment in France by Paul H ero ult , a n othe r 22-year-old inve ntor wo r ki n g in a s imilar make s h ift

lab o ratory

896 CHAPTER 23 Metallurgy and the Chemistry of Metals

Figure 23.18 Electrol y tic

Anode ( o x ida t ion ) : 3[202- - - 0 2 (g) + 4e - ]

Ca t hod e (re duc t ion ) : 4[AI3+ + 3e - • AI(l)]

O v erall: 2Al20 3 • 4AI(l) + 30 2 (g)

Oxygen gas reacts with the carbon anodes (at elevated temperatures) to form carbon monoxide, which escapes as a gas The liquid aluminum metal (m.p 660.2°C) sinks to the bottom of the ves-

sel, from which it can be drained from time to time during the procedure

Aluminum is one of the most versatile metals known It has a low density (2.7 g/cm 3 ) and high tensile strength (i.e., it can be stretched or drawn out) Aluminum is malleable, it can be rolled into thin foils, and it is an excellent electrical conductor Its conductivity is about 65 percent that of copper However, because aluminum is cheaper and lighter than copper, it is widely used in high-voltage transmission lines Although aluminum's chief use is in aircraft construction, the pure metal itself is too soft and weak to withstand much strain Its mechanical properties are greatly improved

by alloying it with small amounts of metals such as copper, magnesium, and manganese, as well as silicon Aluminum is not used by living systems and is generally considered to be nontoxic

As we read across the periodic table from left to right in a given period, metallic properties ually decrease Thus, although aluminum is considered an active metal, it does not react with water as

grad-do sodium and calcium Aluminum reacts with hydrochloric acid and with strong bases as follows:

2AI(s) + 6HCI ( aq) -+ 2AICI 3 (aq) + 3Hig) •

2AI(s) + 2NaOH(aq) + 2H20(/) • 2NaAI02(aq) + 3H2(g) Aluminum readily forms the oxide Al20 3 when exposed to air:

A tenacious film of this oxide protects metallic aluminum from further corrosion and accounts for some of the unexpected inertness of aluminum Aluminum oxide has a very large exothermic enthalpy of formation (!1H 'f = -1670 kJ / mol) This property makes aluminum suitable for use

in solid propellants for rockets such as those used for some space shuttles When a mixture of aluminum and ammonium perchlorate (NH4CI04) is ignited, aluminum is oxidized to A120 3, and the heat liberated in the reaction causes the gases that are formed to expand with great force This action lifts the rocket

The great affinity of aluminum for oxygen is illustrated nicely by the reaction of aluminum powder with a variety of metal oxides, particularly the transition metal oxides, to produce the cor-responding metals A typical reaction is

!1H o = -852 kJ/mol

which can result in temperatures approaching 3000°C This transformation, which is used in the

welding of steel and iron, is called the thermite reaction (Figure 23.19)

Aluminum chloride exists as a dimer:

Each of the bridging chlorine atoms forms a normal covalent bond and a coordinate covalent bond

(each indicated by an arrow) with two aluminum atoms Each aluminum atom is assumed to be

Sp 3 -hybridized, so the vacant si hybrid orbital can accept a lone pair from the chlorine atom

(Fig-ure 23.20) Aluminum chloride undergoes hydrolysis as follows:

AICI3(s) + 3H 2 0(l) - - + AI(OHMs) + 3HCI(aq)

Aluminum hydroxide, like Be(OHh, is amphoteric:

AI(OHMs) + 3H + (aq) • AI 3+ (aq) + 3H 2 0(I)

AI(OHMs) + OH - (aq) • AI(OH)4 (aq)

In contrast to the boron hydrides, which are a well-defined series of compounds, aluminum hydride

is a polymer in which each aluminum atom is surrounded octahedrally by bridging hydrogen

atoms (Figure 23.21)

When an aqueous mixture of aluminum sulfate and potassium sulfate is evaporated slowly,

crystals of KAI(S04h 12H20 are formed Similar crystals can be formed by substituting Na + or

NHt for K+, and Cr3+ or Fe3+ for AI3+ These compounds are called alums, and they have the

general formula

M +: K+, Na+, NH4 +

M3+: AI3+, Cr3+ , Fe3+ Alums are examples of double salts that is, salts that contain two different cations

Figure 23.21 Structure of aluminum hydride Note that this

compound i s a polymer Each Al

atom is s urrounded in an octahedral arrangement by six bridging H atoms

898 CHAPTER 23 Metallurgy and the Chemistry of Metals

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Applying What You've Learned

Most health problems related to copper are the result of errors in copper metabolism

However, although it is rare, copper deficiency can result from a diet that is poor in per Symptoms of dietary copper deficiency include anemia (a deficiency of red blood cells) and neutropenia (a deficiency of a particular type of white blood cell)

cop-The fact that copper is essential to human health was first demonstrated with a group of children in Peru One patient's ordeal was detailed by Cordano and Graham in

the journal Pediatrics in 1966 During her first few years of life, the patient was

hospital-ized several times with anemia, neutropenia, osteoporosis, and multiple fractures At age

6, over a period of 3 months, she received 20 blood transfusions for her severe anemia,

which had not responded to treatment When Dr Cordano became aware of the patient's history, he initiated treatment with copper supplementation The patient never required another transfusion and after 6 months on copper supplements, at age 7, she walked for the first time in her life

CHAPTER SUMMARY

Section 23.1

• Depending on their reactivities, metals exist in nature in either the free

or combined state (More reactive metals are found combined with

other elements.) Most metals are found in minerals Minerals with

high metal content are called ores

Section 23.2

• Metallurgy involves recovering metal from ores The three stages of

metal recovery are preparation, separation, and purification An alloy

is a solid mixture of one or more metals, sometimes also containing

one or more nonmetals An amalgam is a mixture of mercury and one

or more other metals

• The methods ' commonly used for purifying metals are distillation,

electrolysis, and zone refining Pyrometallurgy refers to metallurgical

processes carried out at high temperatures

Section 23.3

• Metallic bonds can be thought of as the force between positive ions

immersed in a sea of electrons In terms of band theory, the atomic

orbitals merge to form energy bands

• A substance is a conductor when electrons can be readily promoted

to the conduction band, where they are free to move through the

substance In an insulator, the energy gap between the valence band

and the conduction band is so large that electrons cannot be promoted

into the conduction band

• Semiconductors are substances that normally are not conductors but

will conduct electricity at elevated temperatures or when combined

Section 23.1: Occurrence of Metals

Review Questions

23 1

23 2

23 3

Define the terms mineral and ore

List three metals that are usually found in an uncombined s tate in nature and three metals that are always found in a combined state

in nature

Write chemical formulas for the following mineral s : (a) calcite,

(b) dolomite, (c) fluorite, (d) halite, (e) corundum, (f) magnetite, (g) beryl, (h) galena, (i) epsomite, (j) anhydrite

QUESTIONS AND PROBLEMS 899

with a small amount of certain other elements Semiconductors in which an electron-rich impurity is added to enhance conduction

are known as n-type semiconductors Semiconductors in which an

electron-poor impurity is added to enhance conduction are known as

p-type semiconductors

Section 23.4

• Metals typically are good conductors and are malleable and ductile

Metallic character increases from top to bottom in a group and decreases from left to right across a period

Section 23.5

• The alkali metals are the most reactive of all the metallic elements

They have an oxidation state of + I in their compounds Under special conditions, some of them can form anions with an oxidation state of -1

Section 23.6

• The alkaline earth metals are somewhat less reactive than the alkali

metals They almo s t always have an oxidation number of +2 in their

compounds The properties of the alkaline earth elements become increasingly metallic from top to bottom in their group

Section 23.7

• Aluminum ordinarily does not react with water due to a protective

coating of aluminum oxide; its hydroxide is amphoteric The Hall

process is used to reduce aluminum oxide to aluminum

Section 23.2: Metallurgical Processes

Review Questions

23.5 23.6 23.7

Define the terms metallurgy, alloy, and amalgam

Describe the main steps invol ve d in the preparation of an ore

What does roasting mean in metallurgy ? Why is roasting a major

so urce of air pollution and acid rain ?

900 CHAPTER 23 Metallurgy and the Chemistr y of Metals

23.8 Describe with example s the chemical and electr o l y ti c redu c tion

proces s e s u s ed in the production of m e t a l s

23.9 Describe the main s tep s u s ed to purif y m e tal s

23.10 Describe the extraction o f ir o n in a bla s t furnace

23.11 Briefl y di s cu s s the s te e lmakin g pro c e ss

23.12 Briefly de s cribe the zone refining pro cess

23.13 In the Mond proce ss for the purific a ti o n of ni c kel , CO i s p ass ed

o v er metallic nickel to gi v e N i ( CO )4 :

Ni (s) + 4CO (g) + ==' N i ( CO Mg)

Given that the standard free ener g ie s of form a tion of CO (g)

and Ni(CO Mg) are - 13 7 3 a nd - 58 7 4 kJ / m o l , r es p ec ti v el y, calculate the equilibrium con s tant of the reaction at 8 0 ° C

(As s ume i1 G f to be indep e ndent of t e mperature )

23.14 Copper i s purified b y electrol ys is (s ee Figur e 2 3 6 ) A 5.00- k g

anode i s used in a cell w here the current i s 37 8 A Ho w long ( in hours) mu s t the current run t o di ss ol v e thi s anode a nd ele c troplate

it onto the cathode ?

23.15 Consider the electrol y tic procedure for purif y ing copper

described in Figure 23.6 Suppo s e that a s ample of copper contain s the follo w ing impuritie s : Fe , Ag , Zn , Au , Co , Pt , and Pb

Which of the metal s will be oxidi z ed and di ss ol v ed in so luti o n and which will be unaffected and s impl y form the s ludge that accumulate s at the bottom of th e cell ?

23.16 How would you obtain z inc from s phalerite ( ZnS )?

23.17 A certain mine produce s 2.0 X 108 kg of copper from

chalcopyrite ( CuFeS 2 ) each y ear The ore c ontain s onl y 0.80 percent Cu b y ma ss ( a ) If the den s it y o f the or e i s 2 8 g / cm 3 ,

calculate the v olume ( in cm3) of ore remo v ed each y ear ( b ) Calculate the ma ss (in kg ) of SO l produ ce d b y ro as ting ( a ss ume chalcopyrite to be the onl y s ource of s ulfur )

23.18 Starting with rutile (TiOl ), explain ho w y ou w ould obtain pur e

titanium metal (Hint: First con v ert TiOz to TiCI4 N e x t , reduce TiCl4 with Mg Look up ph y sical propertie s of TiCI4, Mg , and MgCIl in a chemistr y handbook.)

23.19 Which of the following compound s would require electrol y sis to

yield the free metal s : Ag l S , CaCI2, NaCI, F ez 0 3, AI 1 0 3, TiCI4?

23.20 Although iron i s only about two-third s a s abund a nt a s aluminum

in Earth's cru s t, mas s for mas s it co s t s onl y about one-quart e r a s much to produce Why ?

Section 23.3: Band Theory of Conductivity

Review Questions

23.21 Define the following term s : c o nduc t or, insul a tor, se mi cond u c t ing

element s , donor impuritie s , a c ceptor imp u ri t i e s , n - ty p e

semicondu c tors , p- ty pe s emico n duc t or s

23.22 Briefly discus s the nature of bonding in metal s , in s ulator s, and

semiconducting element s

23.23 Describe the general characteristic s of n-t y pe and p-t y pe

semiconductor s

23.24 State w hether s ilicon would form n-t y pe or p-type

se miconductor s w ith the following elements: Ga , Sb , AI, As

Section 23.4: Periodic Trends in Metallic Properties

Review Questions

23 2 5 Di sc u ss the gener a l propertie s of metals

23 2 6 Us e periodic tre n d s in ioni z ation energ y and electronegativity to

s h ow ho w the metalli c c haracter change s within a group

23.27 U se p e riodi c tr e nd s in ioniz a tion energ y and electronegativity to

s h ow ho w th e metallic chara c ter change s across a period

Section 23.5: The Alkali Metals

Review Questions

23 28 Ho w i s so dium prepared commerciall y?

2 3.29 Wh y i s p ot a ss ium u s uall y not prepared electrolytically from one

23.33 Write a balanced equation for each of the following reactions:

( a ) s odium react s w ith water, ( b) an aqueous solution of NaOH react s w ith CO lo (c ) s olid Nal C03 reacts with an HCl solution , ( d ) s olid NaHC03 react s with an HCl solution, (e) solid NaHC03

i s heated , ( f) s olid N al C03 i s heated

23 34 Sodium h y dride ( NaH ) can be u s ed as a drying agent for many

organic s ol v ent s E xplain how it works

23.35 Calculate t he v olume of CO l at 1O.0 ° C and 746 mmHg pressure

obtained by treating 25.0 g of Na 2 C0 3 with an excess of

h y drochloric acid

Section 23.6: The Alkaline Earth Metals

Review Questions

23 36 Li s t the common ore s of magnesium and calcium

2 3 3 7 Ho w are magne s ium and calcium obtained commercially?

Problems

23 3 8 From the thennodynamic data in Appendix 2, calculate the i1Ho

v alue s for the following decompositions:

(a ) MgC03( s ) - _ MgO( s ) + CO 2 (g)

( b) CaC03( s) • CaO( s ) + CO i g)

Which of the two compound s i s more ea s ily decomposed by heat ?

23.39 Starting with magnesium and concentrated nitric acid, describe

how you would prepare magnesium oxide [Hint: First convert

Mg to Mg(N03) 2 ' Next, MgO can be obtained by heating

Mg(N03) 2 ']

23.40 Describe two ways of preparing magnesium chloride

23.41 The second ionization energy of magnesium is only about twice

as great as the first, but the third ionization energy i s 10 times as great Why does it take so much more energy to remove the third electron?

23.42 List the sulfates of the Group 2A metals in order of increa s ing

s olubility in water Explain the trend (Hint: You need to consult a chemistry handbook.)

23.43 Helium contains the same number of electron s in it s outer shell as

do the alkaline earth metals Explain why helium i s inert whereas

the Gn : >up 2A metals are not

23.44 When exposed to air, calcium first forms calcium oxide, which

is then converted to calcium hydroxide, and finally to calcium carbonate Write a balanced equation for each s tep

23.45 Write chemical formulas for (a) quicklime and (b) slaked lime

Section 23 7: Aluminum

Review Questions

23.46 Describe the Hall process for preparing aluminum

23.47 What action renders aluminum inert?

Problems

23.48 Before Hall invented his electrolytic process , aluminum was

produced by the reduction of its chloride with an active metal

Which metals would you use for the production of al uminum in

Consider the relative sizes of AI 3+ , F-, and Cl - ions.)

The overall reaction for the electrolytic production of aluminum

by means of the Hall process may be represented as

AI 2 0 3 (s) + 3C(s) - _ 2AI(l) + 3CO(g)

At 1000 ° C , the s tandard free-energy change for this proce s s

is 594 kl/mol (a) Calculate the minimum voltage required to

produce 1 mole of aluminum at this temperature ( b ) If the actual voltage applied is exactly three times the ideal value, calculate the energy required to produce 1.00 kg of the metal

7 3.52 In basic solution, aluminum metal is a strong reducing agent and

is oxidized to Al02 Give balanced equation s for the reaction

of Al in basic solution with the following: ( a ) NaN03, to give ammonia; (b) water, to give hydrogen; (c ) Na 2 Sn0 3 , to give

metallic tin

23.53 Write a balanced equation for the thermal decompo s ition of

aluminum nitrate to form aluminum oxide, nitrogen dioxide , and oxygen gas

23.54 Describe some of the properties of aluminum that make it one of

the mo s t versatile metals known

23.55 The pre s sure of gaseous Al2Ci6 increases more rapidly with

temperature than predicted by the ideal gas equation even though

Al2Cl6 behaves like an " ideal gas Explain

23.56 Starting with aluminum, describe with balanced equations how

you would prepare (a) A12C16, (b) A120 3, (c) AI2(S04) 3 , (d)

NH4AI(S0 4h 12H20

23.57 Explain the change in bonding when Al2Ci6 di ss ociates to form

AlCl3 in the gas phase

Additional Problems

23.58 In steelmaking, nonmetallic impurities such a s P, S, and Si are

removed a s the corre s ponding oxide s The inside of the furnace is usually lined with CaC03 and MgC03, which decompose at high

temperature s to yield CaO and MgO How do CaO and MgO

help in the removal of the nonmetallic oxides?

23.59 When 1.164 g of a certain metal sulfide was roasted in air,

0.972 g of the metal oxide wa s formed If the oxidation number

of the metal is + 2 , calculate the molar mass of the metal

23.60 An early view of metallic bonding assumed that bonding in

metal s con s i s ted of localized, shar e d electron-pair bonds between

metal atoms What evidence would help you to argue against this

viewpoint ?

23.61 Referring to Figure 23.6 , would you expect H20 and H to be

reduced at the cathode and H20 oxidized at the anode?

23.62 A 0.450-g s ample of steel contains manganese as an impurity

23.63

23.64

The sample is dis s olved in acidic solution and the manganese

i s oxidi z ed to the permanganate ion MnO 4 The MnO 4 ion is

reduced to Mn 2+ by reacting with 50.0 mL of 0.0800 M FeS0 4

solution The exces s Fe2+ ion s are then oxidized to Fe3+ by

22.4 mL of 0.0100 M K 2 Cr 2 07' Calculate the percent by mass of manganese in the s ample

Given that LlGf?(Fe 2 0 3 ) = -741.0 kl/mol and that LlG f?( AI20 3) = - 1576.4 kl / mol, calculate LlG o for the

23.65 When an inert atmosphere i s needed for a metallurgical process,

nitrogen is frequently u s ed However , in the reduction of TiCl4 by

magnesium , helium is used Explain why nitrogen is not suitable for thi s process

23.66 It ha s been s hown that Na 2 specie s form in the vapor phase

De s cribe the formation of the "Eiisodium molecule" in terms of

a molecular orbital energy level diagram Would you expect the

alkaline earth metals to exhibit a similar property?

23.67 Explain each of the following s tatements: (a) An aqueous

s olution of AlCl3 i s acidic ( b ) Al ( OH ) 3 i s soluble in NaOH

s olution but not in NH 3 s olution

902 CHAPTER 23 Metallurgy and the Chemistry of Metals

23.68

23.69

23.70

Write balanced equations for the following re act ion s : ( a)

the heatin g of aluminum carbonate, ( b ) the reaction between

AICI3 and K, (c) th e reaction between so lution s of Na 2 C0 3 and Ca(OH)2'

Write a balanced equation for the rea ctio n bet wee n calcium oxide and dilute HCI solution

What i s wrong with the following pr oced ure for o btainin g

magne s ium ?

• MgC03(s) - _ MgO(s ) + CO 2 (g)

MgO( s) + CO (g) • Mg(s) + CO z(g)

23.71 Explain why most metal s have a flickering appeara nce

23.72 Predict the chem i cal propertie s of francium, the last member of

Group lA

23.73 Describe a medicinal or health-related application for each of the

following compound s : NaP, Li2C03, Mg(OH)2 , CaC03, BaS0 4 '

23.74 The following are two reaction s cheme s invol v ing magne si um

Sch eme I: When magne s ium burns in oxygen, a white so lid (A)

is formed A dissolves in 1 M HCI to give a colorless so lution (B) Upon addition of Na 2 C0 3 to B , a w hite precipitate i s formed

(C) On heating , C decompo ses to D and a colorless gas is

generated (E) When E is pa sse d through lime wa ter [an aqueou s suspension of Ca(OH)2], a w hite precipitate appears ( F ) Scheme

II: Magnesium react s with 1 M H2S04 to produc e a colorless solution (G) Treating G with an exce ss of NaOH produces

a white precipitate ( H ) H di sso lve s in 1 M HN03 to form a colorless solution When the sol ution i s s lowl y evaporated, a white solid (I) appear s On heating I, a brown gas is given off

Identify A - I , and write equations representing the rea ctions

it s reaction with oxygen and nitrogen ? Con s ult a handbook of

c hemistry and compare the so lubilitie s of carbonates, fluorides ,

T o prevent the formation of oxides, peroxides , and superoxides, alkali m etals are some tim es sto red in an inert atmosphere Which

of the foll owi ng gases should not be used for lithium: Ne, Al", Nz, Kr?Why?

23.77 Which of the following metals is not found in the free state in

n a ture: Ag, Cu, Zn, Au, Pt ?

23.78 After heating, a metal surface (s u ch as that of a cooking pan

or s killet) develop s a color pattern like an oi l slick on water

Explain

23.79 A s ample of 10.00 g of sodium reacts with oxygen to form

13.83 g of so dium oxi de (Na2 0) and sodium peroxide (Na 2 02) '

Calculate th e perc e nt composition of the mixture

23.80 The electrical conductance of copper and metal decrea s es

with temperature, but that of a CUS04 s olution increases with

temperature Explain

23.81 As stated in the chapter, pota ssi um s uperoxide (KOz) is a useful

so urce of oxygen emp lo yed in breathing equipment Calcu l ate the pre ss ure at which oxygen gas s tored at 20 ° C would have the

same density as the oxygen gas provided by KOz The density of

KOz at 20 ° C is 2.15 g/cm 3

23.82 Chemica l tests of four metal s A, B, C, and D s how the following

re s ults: (a) Only Band C react with 0.5 M HCI to give H2 gas

( b ) When B is added to a so lution containing the ion s of the other

metals, metallic A, C, and D are formed (c) A reacts with 6 M

HN03, but D does not Arrange the metals in the increasing order

as reducing agents Sugge s t four metals that fit these descriptions