Hazardous Materials
do not easily ignite without first being kindled . . xygen, however, these materials are lik I . · ~hen they have been exposed to hqmd
o I The combustion of meta/,$. Certai: y to ignite spontaneously. gen The combination of mag • mhet~ls are likely to burn on contact with liquid oX}' · nesium s avmgs and I' 'd f J ·
chemically reactive that these elements explod iqm h oxygen, or examp e, 1s so parts also react with Jiqu · d e on _contact. T e components of aluminum
pf~~pn This phenomenon 1·s th I oxygefn, especially when the reaction is initiated by ncao • e reason or avoid " h f I · · ·
· ded for the storage and ha di " f . mg t e use O a ummum m eqmpment inten . n mg o cryogenic oxygen. The combustion of porous materials w; d
f orous materials. When Ji uid O • 00
' conc~e~e, and_ asphalt are examples 0 P d d di hq xygen contacts them, 1t 1s readily absorbed into their
Pores an pro uces me a t at can be sh k • • hi h I. "d h .11 d oc -sensitive. An example is an asphalt surface on w c 1qm oxygen as sp1 e . Thi · h . . .
when subjected to a mechanical impact.s oxygen-enr1c ed asphalt may igmte explosively
7.1-D CHEMICAL OXYGEN GENERATORS Chemical oxygen generators are portable devices that ca b h · II d
d d Th n e c em1ca y actuate to pro-duce oxygen on eman . ey are used in mines and oth 1 · h 1· · d "b"l . • · h h • er p aces wit 1m1te access1 1 -1ry m wh1c t e available oxygen supply may be limited Th d . . . ey were once use to escape from noXIous or po1so?ous atmospheres, but this practice is now obsolete.
One type of chemical oxygen gener t · f · . . . a or consists o an apparatus with two separate compartments contammg powdered iron and sodium chlorate, respectively. Oxygen is generated when these substances are mixed and heated.
Fe(s) + NaC I03(s) ---+ FeO(s) + NaCl(s) + 0 2 (g)
Iron Sodium chlorate Iro n(Il ) ox ide Sodiu m chl oride Oxygen
A percussion cap activated by a hammer provides the heat needed to initiate the reaction. The heat that is evolved by this exothermic reaction permits it to be self-sustaining until either of the reactants is depleted.
Another type of oxygen generator utilizes the thermal decomposition of an alkali metal perchlorate. Lithium perchlorate, for example, decomposes when heated as follows:
L iCI04(s) -- LiCl(s) + 20 2(g) Li thium pe rc hl orate Lith ium ch lo1i de Oxygen
The reaction is initiated through use of an igniter that is struck by a firing pin.
7.1-E WORKPLACE REGULATIONS INVOLVING BULK OXYGEN SYSTEMS
OSHA defines a bulk oxygen system as the assembly of equipment (storage tank, pres- sure regulators, safety devices, vaporizers, manifolds, and interconnecting piping) that Possesses an oxygen storage capacity at normal temperature and pressure of more than 13,000 cubic feet (368 m 3) when connected for service, or more than 25,000 cubic feet (?08 m3) when available as an unconnected reserve.
When bulk quantities of oxygen are stored at the premises of industrial or institutional co_n~umers, OSHA requires their owners to comply with certain regulations to prevent or nun1mize the risk of fire and explosion. These regulations pertain to the location of the system, its elevation, its accessibility to authorized personnel, measures associated with ~eakage from the system, diking, required distances betw~e n the syste~ and nearby expo- ures, and other safety issues. For exa mple, OSHA reqmres the locat1on of bulk oxygen
st0rage systems to be either outdoor s abovegro und or within a building of noncombustible ~ons~ruction that is adequately vented and use~ fo r tha t y urpose exclusively. The selecte_d ocat1on must be such that containers a nd a ssoci ated eqmpment are not exposed to electric Power lines, flamma ble gas lines, or fl amma ble liqu id lines.
chemical oxygen generator Any portable device in which oxygen is produced upon demand by chemical reaction
bulk oxygen system For purposes of OSHA regulations, the assem- bly of equipment that has an oxygen storage capacity at normal temperature and pressure of more than 13,000 cubic feet (368 m3) when con- nected for service, or more than 25,000 cubic feet (708 m3) when available as an unconnected reserve
Chapter 7 Chemistry of Some Common Elements 227
I
I I
SOLVED EXERCISE 7.1 t ms to be eith er outdoo rs aboveground or with '
OSHA requires the location of bulk oxygen storage sys e d d used tor that purpose exclusive! T rn a build. ing of noncombustible construction that rs adeq_uately vente an not ex osed to electric ow Y: he selecte location must be such tha t containers and associated equipment are P . P er lines, flarnrn d ble or combustible liqu id li nes, or flammable gas lines. Despite
th e fa~ t::~n~:fl:~ :~ ~h~~~:~~,:nm;ble gas, 'Nh:;
is the most likely reason OSH A requires bulk oxygen storage syS t ems
O ion.
Solution: The most likely reason OSHA enacted this regulation is to minimi~e a pronounced risk of fire explosion . Although oxygen is a nonflammable gas, it supports th e com_buS
t ton of_ many common mater~~d
Because t hese materi als bu rn at increased rates in an oxygen-~nnche? envrronme~t, it rs p~udent to prevent th~ accumulation of oxygen near flammable or combustible materials. This is accomplished by installing bulk oxy e
. h . d' d f h' g n storage systems In t e In Icate as 10n .
OSHA advises the owners of bulk oxygen systems to l~c~te them on ground higher than flammable liquid storage tanks in nearby storage. When 1~ ts ?ecessary to locate a bulk oxygen system on ground lower than adjacent flammable hqwd storage tanks, OSHA requires the owner to provide a suitable means of diking, diversion curbing, or grading such that in the event of a release from these tanks, the liquids do not accumulate under the bulk oxygen system. Because flammable liquids burn at increased rates in an oxygen-enriched environment, it is prudent to prevent their accumulation near the components of a bulk oxygen system.
To warn workers of the presence of a bulk oxygen system within a workplace, OSHA also requires its owner at 29 C.ER. §1910.104(b)(8)(viii) to permanently post a placard that
reads as follows:
. -1 OXYGEN
01 NO SMOKING IQ\ NO OPEN \:::s, FLAMES
7.1-F TRANSPORTING OXYGEN When shippers offer GOX or LOX for transportation DOT · h 'd tify 1·ras h
· bl . , reg mres t em to 1 en , s own m Ta e 7.3 on the accompanymg shipping paper. DOT also requires them to affix NON-FLAMMABLE GAS and OXIDIZER labels to th 1· d h k ·ng Wh · e cy m ers or ot er pac agt •
. en earner~ transport 1001 pounds (454 kg) or more of GOX or LOX, pOT reqmres them to display NON-FLAMMABLE GAS I d h b lk k · used P acar s on t e u pac aging
TABLE 7.3 esrnptions of Oxygen and Oxygen Generators Shipping D · ·
OXYGEN
Compressed oxygen
Cryogenic oxygen
Oxygen generator, chemical
Spent oxygen generator, chemical
228 Chapter 7 Chemistry of Some Common Elements
SHIPPING DESCRIPTION
UN 10 72, Oxygen, compressed, 2.2, (5.1)
UN1073, Oxygen, refrigerated liquid, 2.2, (5- 1)
UN 3356, Oxygen generator, chemical, 5.1, PG 11
NA3356, Oxygen generator, chemical spent, 9, pG
111
the shipment; or, in lieu of posting NON-FLAMMABLE GAS placards DOT allows for · 0 YG ' carriers to display X EN placards that look like the following:
fhe motor van s_h~wn in Figure 7.3 has been placarded for the transportation of cylinders d tanks conta1mng both oxygen and nitrogen.
an When carriers transport oxygen within bulk packaging, DOT also requires them to di play the relevant identification number-1072 or 1073-on orange panels, across the
s ter area of the NON-FLAMMABLE GAS or OXYGEN placards, or on white square- ::point diamonds affixed on each side and each end of the packaging. For example, any of the following means may be used to display 1072, the identification number of oxygen:
1072
When oxygen is transported in bulk by highway or rail, DOT requires carriers to display the name OXYGEN on two opposing sides of the tankcar used for shipment.
OXYGEN
DOT requires shippers who transport chemical oxygen generators or spent chemical oxygen generators to identify them as shown in Table 7.3 on the accompanying shipping
Airgas Puri:tan Medical
www.airgas.com
FIGURE 7 .3 DOT auth- orizes carriers at 49 C.F.R. § 172.504(f)(7) to display OXYGEN placards in lieu of NON-FLAMMABLE GAS placards on vehicles used for the sole shipment of oxygen in nonbulk cylin- ders or other containers. To emphasize that oxygen is an oxidizer, the back- ground color of the OXY- GEN placard is yellow, and the color of the symbol, text, hazard class number, and border is black. When carriers transport oxygen and other nonflammable gases in nonbulk cylinders or other containers within the same vehicle, DOT per- mits them to display NON- FLAMMABLE GAS placards or both OXYGEN and NON-FLAMMABLE GAS placards on the vehicle. (Courtesy of Airgas Inc., Radnor, Pennsylvania.)
Chapter 7 Chemistry of Some Common Elements 229
I' I ,11 I I
l I I
II
allotrope A form of the same element having its own unique set of physical and chemical properties
· d with an attached means of ini · . paper. When the generators are eqmppe . b d . tlation l) . . . . l for shipment y emonstratmg th , (). reqmres their earners to obtam approva . . . . at the h 1 . . . • f reventmg their unintentional a t . Y a ' been outfitted with two positive means O P c Uation. 11e
7 .1-G RESPONDING TO INCIDENTS INVOLVING A RELEASE OF OXYGEN
When oxygen is first released from a storage or transpo~tation ~evice, i~s concentra . becomes elevated within the immediate area compared to its level m the air. Then s tion ary fires are likely to occur, especially when the oxygen contacts fuels. An attem;t :~~:d- be made to combat secondary fires only when they are located far from the area wh Id
f . • ereth
oxygen is released or when the flow of oxygen rom its contamment vessel can be e . stopped without exposing personnel to unnecessary nsk.
In the event of a transportation mishap involving the release of oxygen to the en • . d . f h . V1ton. ment DOT recommends an initial downwm evacuatwn ° unaut onzed person , f h . h f . s toa distance of at least 0.33 miles (500 m) f~om the sce?e O t e _mis ap. I a ra~ tankcar or tank truck is involved in a fire, DOT advises evacuat10n to a diSt ance of 0.5 nules (800 m).
7 .1 -H OZONE, THE ALLOTROPE OF OXYGEN Chemists refer to ozone as an allotrope of oxygen. An allotrope is a unique variation of an element possessing physical and chemical properties that substantially differ from those of any other form of the element. Allotropic forms are exhibited by certain ele- ments that occupy Groups 4A through 6A on the periodic table. The allotropes of four elements are noted in this chapter: oxygen, phosphorus, sulfur, and carbon.
Ozone is a form of elemental oxygen having three atoms of oxygen per molecule instead of the usual two; thus, its chemical formula is represented as 03 and its Lewis structure is represented as follows:
:Q: Q::Q: or 0 -0=0
Ozone is produced when either an electrical discharge or ultraviolet radiation is passed through oxygen. Although it is reasonably stable at relatively low temperatures, ozone decomposes rapidly at room temperature; that is, it spontaneously reverts into "ordinary" oxygen. It is this relative instability that accounts for the fact that ozone is generally encountered at very low concentrations.
At ordinarily encountered temperatures and pressures, ozone exists as a pale blue gas. With a vapor density of 1.7 relative to air, ozone is denser than normal oxygen. It P?5~ sesses a pleasing smell in low concentrations, but has an irritating, pungent, "meta~ic odor at moderate to high concentrations. The fresh, clean, spring-rain smell sometunes detected around operating electric motors or following a lightning storm usually can be attributed to the presence of ozone.
Two characteristics are responsible for ozone's reputation as one of the moSt _h~z- ardous materials known: a pronounced chemical reactivity and a distinct to~icitl} 0 · f 1 'd' · d b en 1tse · zone is a power u oxi izmg agent-consi era ly more reactive than oxyg en For example, ozone converts lead sulfide rapidly into lead sulfate, whereas 0 "yg reacts so slowly with lead sulfide that the reaction is virtually imperceptible.
3PbS(s) + 403(g) 3PbS04(s) Lead sul fid e Ozone Lead sul fate
. . id inha· Because ozone 1s a toxic substance, precaution should be undertaken to avo_ une
lation exposure. When it is inhaled, ozone damages the scavenger cells of the iJllill hell system. These cells, called macrophages, customarily destroy foreign bacteria, but ~ 0se they are damaged, they are unable to effectively accomplish this task. Individuals W macrophages have been damaged are more susceptible to contracting diseases,
230 Chapter 7 Chemistry of Some Common Elements
water
7.1-1 COMMERCIAL USES OF OZONE Ozone is used commercially for several purposes. It is used to bleach undesirable colors from oils, fats, textiles, and sugar solutions. It is also used as a microbicide at drinking water and wastewater treatment plants. A microbicide is a substance that kills disease- causing microorgani~ms. Ozone is an especially effective microbicide. Most importantly, it is capable of ~est~oymg the parasite Cryptosporidium parvum that is sometimes found in chlorinated drmkmg water. Chlorine alone does not destroy this parasite (Section 7.3-B).
When water contaminated with Cryptosporidium parvum is consumed, the parasite causes gastrointestinal diseases that can be fatal. In 1993, for example, consumption of water in the greater Milwaukee area caused 403 ,000 people to experience stomach cramps, fever, diarrhea, and dehydration. 1 Moreover, the deaths of 104 people were attributed to the outbreak of diseases caused by the presence of Cryptosporidium parvum in their drinking water. Fortunately, such incidents are now rare in the United States and other developed countries that choose to ozonize their drinking water supplies.
When ozone is used for water treatment, the processes illustrated in Figure 7.4 are imple- mented. Air is first compressed to separate oxygen and nitrogen. Then, the oxygen is zapped with electricity to produce ozone, which is passed into less-than-pristine water under pressure. The ozone reacts with the impurities in the water, thereby killing its constituent microorgan- isms. The water is then filtered and pumped into the municipal drinking water supply.
The use of ozone is also advantageous for treating wastewater. Ozone converts the constituent hydrogen sulfide (sewer gas, Section 10.13) into sulfuric acid. At the concen- tration produced, the sulfuric acid is benign.
3 H2S (s) + 40 3(g) --> 3H2S04(a q) Hyd rogen sulfid e Ozone Sul furi c ac id
When ozone is used for treating drinking water or wastewater, the process is called ozonation. Because ozone is unstable and extremely poisonous, it is always synthesized at its
intended point of use in minutely low concentrations. This synthesis is accomplished "'.ithin an ozone generator, an apparatus that supplies an ~l_ectrica~ c~rent to oxygen ?r a1r. The ozone generator shown in Figure 7.5 may be_ fam1h~r to f1re~1g~ters, because 1ts use often is encouraged within buildings damaged by fire. While the bmldmgs are enclosed and vacated, ozone produced by the generator o~dizes the com~onents of smoke. 1:his removes the offensive odors from furni ture, clothmg, and other items damaged by fire. Ozone generators are also used to produce ozone for removal of musty odors that persist Within walls and ca rpeting damaged by mold or mildew.
~ , "A mas sive o utbreak in Milwauk ee of Cryptosporidium infection transmitted through he public wa ter su pply," N. Engl. ]. Med ., Vol. 33 1 (199 4) pp. 161-167.
FIGURE 7 .4 A simplified schematic route that illustrates the production of ozone from air and its subsequent use for producing pure water.
microbicide A chemical substance capable of destroying microorganisms
ozonation The chem- ical reaction involving the addition of ozone to a substance; the treatment of contami- nated drinking water to kill the microorganisms that cause waterborne diseases
Chapter 7 Chemistry of Some Common Elements 231
llllllii.......
FIGURE 7 .5 Following fires and floods, ozone may be used to decon- taminate and restore buildings and their fur- nishings to their original or improved condition . Because ozone is an unstable gas, it must be generated for use on demand. (Courtesy of Medallion Clean Indoor Air Solutions of Las Vegas, Las Vegas, Nevada.)
ground-level ozone (troposp heric ozone) The ozone that forms in the troposphere (the lower atmosphere) by photochemically catalyzed reactions between voes and the nitrogen oxides
volati le organic compounds (VOCs) Certain vapors that undergo photochemical reac- tions in the atmosphere to form ozone and other air pollutants
ea
7.1-J GROUND-LEVEL OZONE In certain metropolitan regions of the United States, ground-level ozone, or tropospheric ozone, often is simultaneously present in the air with nitric oxide, nitrogen dioxide (Section 10.14) and certain substances called volatile organic compounds (VOCs). The voes are components of the air emissions from petroleum refineries, fuel dispensers, chemical plants, and other industrial facilities . They typically are regarded as organic compounds that boil at temperatures less than approximately 392°F (200°C) . Hence, the substances in gasoline vapor are examples of voes. Figure 7.6 shows that the majority of the atmospheric voes originate when nonchemical industrial processes are conducted.
The oxides of nitrogen are generated during the operation of automobiles. They are components of vehicular exhaust and the plumes emanating from the smokestacks of fossil-fuel-fired power plants. Within the troposphere-or lower atmosphere immediately above the ground-these oxides react photochemically with the voes to produce ozone, At ground level where we live and breathe, ozone and the nitrogen oxides often are_
th e
primary components of polluted air, especially during the daylight hours of summerume, when ample sunlight catalyzes ozone production.
When the ground-level ozone concentration exceeds approximately 100 ppb, weal h
er forecasters declare an ozone alert, which signifies that the ozone concentration in
th e air
is approaching an unhealthful condition. At this concentration affected individuals-cl . ' an especially the elderly-struggle for breath and feel dizzy. Their eyes, noses, throats, . 1 b
. . d d h · · h. wheez ungs ecome 1rntate , an t ey expenence fattgue, a lethargic feeling, coug ing, ing, and hoarseness. s
Th · h 1 · f 1 · h illnesse e m a atton o ow concentrations of ozone is also likely to exacerbate t e o·
f · d. 'd I uff · f h · · · These pe o m 1v1 ua s s ermg rom eart ailments, emphysema or chronic bronch1tts, . 0d 1 lik I h f
. , pain a p e are e y to coug more requently and with greater intensity experience cheSt . ked · · duff ' benhn smus congest10n, an s er severe headaches. Short-term exposure to ozone has e 5ed
with premature death, and long-term cumulative exposure has been linked with an increa
232 Chapter 7 Chem istry of Some Common Elements
Nonchernical industrial processes
51%
Chemical industrial Processes
7%
Fuel combustion
3%
Transportation 37%
risk of fatalities from respiratory causes This risk ri'ses u t 4
o, f 10
b · . t ozon 2 · P o 10 or every pp increase in exposure o e.
7.1-K WORKPLACE REGULATIONS INVOLVING OZONE When the use of ozone is needed in the workplace, OSHA requires employers to limit employee exposure to an ozone concentration in air of 0.1 ppm, averaged over an 8-hour workday.
7.1-L ENVIRONMENTAL REGULATIONS INVOLVING GROUND-LEVEL OZONE
EPA has evaluated the adverse health effects that result from exposure to ground-level ozone. Using the legal authority of the Clean Air Act, EPA regulates ground-level ozone as a criteria air pollutant (Section 1.3-A) by setting its primary and secondary national ambi- ent air quality standards at 0.12 parts per million (235 µg/m 3) as a 1-hour average and 0.079 parts per million ( 157 µg/m 3 ) as an 8-hour average.
On a hot, hum id day, the ozone concentration in urban air can exceed 100 parts per billion . What actions to limit the adverse health effects associated with breathing ozone should be taken by persons who suffer from emphysema?
Solution: Emphysema is a lung ailment associated with the swelli ng of the alveoli and connecting tissues in the lungs. Emphysema sufferers cough frequently, experien ce headaches, and have trouble breathing . These ill effects are exacerbated when they inhale air contaminated with ozone . To limit undue distress when the ozone concentration exceeds 100 parts per billion, emphysema sufferers are advised to remain in an air-conditioned environment, avoid heavy work and exercise, and breathe oxygen fro m a portable so urce. - 7.1-M STRATOSPHERIC OZONE Although the presence of ozone in the troposphere can be a troublewme factor for maintaining good health, the presence of ozone m the stratosphere prnv1des an excep- tional benefit to the inhabitants of Earth. Approximately 10 to 19 miles (16 to 30 km) above Earth's surface ozone occurs naturally in the region of the stratosphere called the ozone layer. As sho:n in Figure 7.7, oxygen is bombarded within the stratosphere by high-energy particles that originate in the sun. The b~mbardment causes s_ome oxygen molecules to dissociate into their individual atoms (·Q·). The oxygen w1thm the ozone
~ael J 1 " L O one exposure and mortality," N. Engl. ]. Med., Vol. 360 (2009 ), erretr et a ., ong-term z Pp. 1085-95.
FIGURE 7 .6 Ground- level ozone is produced when the volatile organic compounds (VOCs) in the air react with the nitrogen oxides discharged in vehicular exhaust. voes are contained primarily within the gaseous dis- charges of power plants, motor vehicles, and petro- leum refineries . (Courtesy of Un ited States Environmen- tal Protection Agency.)
ozone layer The region of the upper atmosphere that is especially plentiful in ozone
Chapter 7 Chemistry of Some Common Elements 233 ' lillJJ
I I
ozone hole A thin spot in the ozone layer resulting from destruc- tion of stratospheric ozone by its chemical reaction with chloro- fluorocarbons and other substances
02 03
03 \a, 02 03 03 03 ·i:i·
03 10 to 19 mi 02 (16 to 30 km)
02 / Ozone layer
03 03 ·i:i·
02 N2 02
02 N2 Lower atrnos h 02 N2 N2 /4 (tropospherei ere 02 N2 N2 N2 02 2
02 02 N2 02 N2 02 02 N2
Earth
FIGURE 7. 7 At the outer edge of the stratosphere is a region c~lled th~ ozone layer, where oxygen is constant! converted into ozone. The ozone layer prevents much of the suns ultraviolet rad1at1on from reaching Earth. Y Atmospheric scientists have discovered that in modern times the amount of stratospheric ozone has been deplet compared with the amount in past times. This depletion allows more harmful ultraviolet radiation to penetrate t ed Earth's surface, increasing the instances of skin cancer and cataracts and weakening immune systems. 0
layer exists as a mixture of oxygen atoms and oxygen molecules. These oxygen species collide to initiate a chemical reaction producing ozone.
0 2(g ) - 2 ·Q·(g) Oxygen molecule Oxygen atoms
Oxygen mo lecule Oxyge n atom Ozone molec ul e
When molecules of stratospheric ozone absorb low-energy ultraviolet radiation, some revert to the ordinary form of oxygen. Under normal conditions, the continuous formation and photodecomposition of ozone compete favorably with one another. This chemical activity produces a steady-state condition in which the rate at which the stratospheric ozone forms from oxygen equals the rate at which it undergoes photodecomposition into oxygen.
In the past, the presence of the stratospheric ozone layer has protected Earth, its occupants, and plant life from overexposure to the harshness of ultraviolet radiation. Today, howevei; the former use of the halon fire extinguishing agents (Section 5.14), Freon refrigerants, foam· blowing agents and coolants (Section 12.15), and other halogen-containing compoundsconnn· ues to adversely affect the environmental quality of our planet. Released into the environment, these compounds diffuse upward to the ozone layer, where their exposure to ultraviolet ra~· tion produces halogen atoms. We note more fully in Section 12.15 that these atoms catalyze e decomposition of ozone and contribute to its overall stratospheric depletion. h •
h . . . b . rosp eric In 1985, atmosp enc sc1ent1sts egan to note a precipitous drop m stra •n ozone, first over Antarctica and then over the Arctic. The areas of the ozone lar~e1s. which this thinning of the ozone layer has been observed are referred to as ozoneE oth's Due to their existence, more ultraviolet radiation from the sun now penetrates a~ern atmosphere compared with the amount that reached Earth before 1985. The cf~and among scientists is that this increased radiation exposure is the cause of certain heat environmental problems such as the following:
Radi a tion weakens plant life and contributes to lower agricultural outputs. 1
nee of Radiation may cause climatic changes that could ultimately disturb the baa aquatic and land ecosystems.
234 Chapter 7 Chemistry of Some Common Elements
1 Th~ overexpos~re to ultraviolet radiation in humans has dramatically increased the inc1denc~ of skm cance~ a?d cataracts and weakened immune systems.
1 In aquatic systems, rad1at1on harms marine life.
7,1-N ENVIRONMENTAL REGULATIONS INVOLVING STRATOSPHERIC OZONE
To ensure the conti_nued survival of life on planet Earth, 43 representatives of the world's industrialized nat10ns agreed in 1987 to coordinate the control of ozone-depleting substances by ~hasi?g out their manufacture and use. This agreement, sponsored by the United Nations, ts known as the Montreal Protocol on Substances That Deplete the Ozone Layer.
The original Montreal Protocol has now been ratified by 196 nations including the United States. It banned the parties from producing and consuming most ozone-depleting substances by January 1, 2010. Even with adherence to the Montreal Protocol, however, scientists project that it will take until 2050 for the ozone layer to completely recover from the world's misuse of these hazardous substances.
Although the United States is a party to the Montreal Protocol, EPA has also used the legal authority of the Clean Air Act to regulate the manufacture, use, and importation of sub- stances that deplete stratospheric ozone. Among these substances are methyl bromide, carbon tetrachloride, 1,1,1-trichloroethane, the halon fire extinguishing agents (Section 5.14), and certain chlorofluorocarbons (Section 12.15).
7.2 HYDROGEN Although Table 4.1 shows that hydrogen ranks ninth in natural abundance by mass on Earth, only traces of this element exist naturally in the free state. Hydrogen is such a reac- tive element that it is found on Earth only in compounds like water, acids, and hydrocar- bons. Throughout the entire universe, however, hydrogen is the most abundant element, 72% by mass.
Elemental hydrogen is an odorless, colorless, tasteless, and nontoxic substance. Its chemical formula is H 2• Although hydrogen exists as a gas at ordinary temperatures and pressures, the gas liquefies when compressed under a pressure greater than 294 psi (2030 kPa) and at a temperature less than -390°F (-234.5°C). Liquid hydrogen is sometimes denoted as LH2.
7.2-A COMMERCIAL USES OF HYDROGEN Elemental hydrogen is commercially available as both the compressed gas and the cryogenic liquid. The chemical industry uses it as a raw material for the production and manufacture of other chemical substances. The petroleum industry uses it to Process crude petroleum fractions (Section 12.13 -C) during the production of petro- !eum fuels, and the aerospace industry uses hydrogen as a fuel for propelling rockets Into space.
Hydrogen is chosen as a rocket fuel by aerospace engineers, because the combustion of a very small mass produces a prodigious amount of energy (see below). It is within the aerospace industry that massive volumes of hydrogen are likely to be encountered in stor- age. For example, at the John F. Kennedy Space Center, Cape Canaveral, Florida, a stag- gering 800,000 gallons (3000 m3) of cryogenic hydrogen is held in a single storage tank.
For more than half a century, physicists and engineers have also been investigating ways to successfully develop a thermonuclear reactor, in which the nuclei of hydrogen atoms fuse and produce heavier nuclei by converting mass into energy. Nuclear fusion is the phenome- non that powers the sun and other stars; it was also the means used on Earth to detonate the hydrogen bomb pictured in Figure 7.8. However, the energy released during the detonation
Montreal Protocol The international agreement that pro- vides for phasing out the worldwide produc- tion, manufacture, and use of chlorofluorocar- bons and other ozone- depleting substances
Hydrogen gas
LH2
Chapter 7 Chemistry of Some Common Elements 235
FIGURE 7 .8 The explo- sion o f an experi mental t hermonucl ear device (hydrogen bomb) on Eniw eto k on Oct ober 3 1 19 52 . (Courtesy of Un ited ' States Department of Energy. Nevada Operations Office, Las Vegas, Nevada.)
of a hydrogen bomb is essentially uncontrolled. To serve as a commercially viable energy source, self-sustaining fusion reactions must be controlled on a large sca~e. If a thermonu- 1 clear reactor could be successfully developed, hydrogen would most likely become the energy source of the future. 3 However, serious technical obstacles in achieving a sustainable, I controlled nuclear power source now exist. Commercialized nuclear fusion is not antici- pated to begin until late in this century, if at all.
When hydrogen burns in an atmosphere of pure oxygen, the accompanying heatol combustion is 61,000 Btu/lb (141,790 kJ/kg). This is the most energy for the least mass I that evolves when any substance burns. This energy is used for launching spacecra& and cutting and welding metals and glass. Jewelry manufacturers, for example, use the oxyhy· drogen torch illustrated in Figure 7.9 to craft rings and bracelets made of platinum, a metal that melts at 3191°F (1755°C). Compressed hydrogen and oxygen are separacelr stored within steel cylinders and then pressure-fed through tubing into a mixing chamber within the torch in a 2:1 ratio. This mixture is then discharged from the nozzle, where combustion of these gases occurs.
3Research relating to the development of a fusion-demonstration reactor is now being conducted in a mula· national effort called the International Thermonuclear Experimental Reactor Consortium. The United Sra: European Union, Russia, India, Japan, South Korea, and China are members of the consortium that funds ~- I project. In 2006, representatives of these seven partner nations agreed to build the plant at Cadarache 1~ ;~;O. ern France, where 1t ts now under construction and slated to be finished in 2018 and begin operation b) boll<
The underlying technology in demonstrating fusion involves the use of a doughnut-shaped magnen~ "' called a tokamak, which confines the hydrogen as it is heated to 180 million degrees. At this superheat "n;.
h h d · I . 1. 0, uuo perature, t e y rogen exists as p asma (see footnote 10 Section 2.1) and its nuclei fuse ro form he iutn, ·u., d ff.. h b d .. . . Jhconsofll an. su tctent energy t at c_an e use to generate electnctty. If the mitial tests are successfu , t e
estimates that the construction of a commercial-scale fusion power plant could begin by midcentur)', bl¥"' IhU"dS fu. · ... tnota . . n t e _n_1te ta_t~s, _s,on research 1s also underway at a number of government fac1httes, mos eW Me.,i-O-
236 Chapter 7 Chemistry of Some Common Elements Nattonal lgmt1on Factlity, Livermore, California, and Los Alamos National Laboratory, Los Alamos, N I
_j.
- ~~""""--..LJ.'°3:'::1:~:: Hydrogen Oxygen
FI GURE 7.9 _Hydroge n and oxygen separately enter the mi xi ng chamber of this oxyhydrogen torch from the right. A 2:1 mixture disch arges from the nozzle at the left at a rate that causes the flame to burn at the tip of the torch . When this mixtu re is ignited, temperatures rang ing between 3 300 and 4400°F (1800 and 2400°C ) are achieve d.
Hydrogen is also used commercially in connection with the chemical process called hydrogenation. At elevat~d temperatures and pressures, and generally in the presence of a catalyst, hydrogen com?mes with certain organic compounds to form commercially use- ful products. Vegetable 01ls a re hydrogenated, for example, for use as shortening and raw materials for the manufacture of soaps and lubricants.
Within the chemical industry, h ydrogen is used to produce metallic and nonmetallic hydrides. Hydrogen combines with elemental sodium, for example, to produce sodium hydride.
2Na(s) + H2(g) ----+ 2NaH (s) Sodi um Hydrogen Sodium hydride
Ir is also used to produce compounds such as hydrogen chloride and ammonia .
Hydrogen C hl orine Hydroge n chl o,ide
Hydrogen Nitrogen Ammon ia
7.2-8 HYDROGEN AS AN ALTERNATIVE MOTOR FUEL In the mid-2000s, there were persuasive reasons for believing that compressed hydrogen would become a popular fuel for powering automobiles and other surface vehicles . In particular, the prospective use of hydrogen as a vehicular fuel would reduce air pollution and our reliance on fossil fuel resources from foreign countries. The use of hydrogen as a vehicular fuel would truly revolutionize the automotive industry and theoretically pro- duce a cleaner environment.
In hydrogen-powered automobiles, compressed hydrogen directly replaces the gaso- line used in gasoline-powered vehicles. It is an example of an alternative motor fuel. In 1984, Daimler Benz first demonstra ted the use of hydrogen to power the Mercedes Benz 280 Te automobile.
hydrogenat ion The addition of hydrogen to a substance
alternative moto r fuel Any of the sub- stances or mixtures that can power vehicles as an alternative to the use of motor gasoline or diesel oil
Hydrogen may also be encountered as an alternati ve motor fuel in the form of a metallic hydride, or metal hydride. A metal powder is generally contained in an alu- minum cylinder with a pressure-relief valve and a coupling for connecting to a system for potential use. When hydrogen is charged into the cylinder, it is absorbed into the metal and p r oduces the corresponding metal hydride. Because the hydrogen is only loosely bonded to the metal, it is desorbed readily for use upon demand . In this sense, fuel cell Any device 11 Performs similarly to the stora ge and withdrawal of electricity from a battery. that produces electricity
.Although the energy from burning hydrogen may be used directly to power motor th rough a chemical
v h I react ion between a d:v" es, hydrogen may also be used indirect!! in the form of fuel cells . These are source fuel and an
ices that produce electricity throu g h a chemica l react10n between a source fuel and oxidizer
Chapter 7 Chemistry of Some Common Elements 237
I I
I I
FI G URE 7 .10 Hydrogen and oxygen react to produce energy in a hydrogen fuel cell. These reactants are provided to the cell from external sources . The flow of electrons in the cell produces electricity, which is the source of power. When the electricity is used to power a motor vehicle, the hydrogen is regarded as an alternative motor fuel.
hyd rogen f uel cell A device that produces energy by the reaction of hydrogen and oxygen
FIGURE 7 .11 At a hydrogen refueling sta- t ion, hydrogen fuel is stored either as a liquid or a gas under pressure. As hydrogen fuel is dis- pensed from the pump, its pressure typically ranges from 5000 psig (34,456 kPa) to 10,000 psig (68,911 kPa). (Courtesy of Air Products, Allentown, Pennsylvania.)
e;:- 0
,,., E~ (hydrogen) - • .,
,' H2 (g) - 2H + (aq) + 2e - Electrodes H 6
0
! 11lb Air 't> - 'ff (oxygen) ..,
H20 -,.
an oxidizer. The cross section of a hydrogen fuel cell is shown in Figure 7.10. Hydro- gen fuel cells are manufactured tiny enough to power a cell phone and large enough t power a car or truck. The reactants in a hydrogen fuel cell are hydrogen and atmos~ pheric oxygen, which react to produce water and energy. The water is discharged t the atmosphere and the energy is converted into electricity, which powers the vehicle~ Although the sales of vehicles powered by hydrogen fuel cells has annually increased in the United States, gasoline-powered vehicles are still the most widespread form of transportation.
The direct use of hydrogen gas for powering motor vehicles is now limited to areas equipped with hydrogen-refueling stations like the one shown in Figure 7.11. These sta- tions are primarily located in California, where the current use of hydrogen as an automo- tive motor fuel is greatest. Each refueling station has equipment from which the hydrogen
238 Chapter 7 Chemistry of Some Common Elements
l is dispensed to customers. At 16 c FR § 3 fue · es retail hydroge n distributors t · · r/ 06 -12, the U.S. Federal Trade Commission
requtrser that resembles the follow· 0
a ix an orange-and-black label on the hydrogen dis pen tng:
HYDROGEN MINIMUM
98% HYDROGEN
Th. label identifies hydrogen as the alt • is 98 '¾ by volum Th U S F ernative motor fuel and provides its minimum fuel
ranng as O
d d e. h' e · · ederal Trade Commission also requires new-vehicle manufacturers an use -ve 1cle dealers to affix the 1 b 1 • 'bl f f h h'
h · ed b h d a e on a v1s1 e sur ace o eac ve 1-cle t at 1s power y y rogen.
7.2-C PRODUCTION OF HYDROGEN Hydrogen is produced for commercial use by the following methods:
In the firS t
method, steam is first passed over red-hot coke or coal under high temperature and pressure. This_ process, called coal gasification, produces a mixture of gases called water gas, synthesis gas, or syngas. This mixture consists mainly of carbon monoxide and hydrogen.
Carbon monoxide Hydroge n
The hydrogen is isolated from the carbon monoxide, or the carbon monoxide is converted to carbon dioxide, by passing the water gas with additional steam over a catalyst such as iron(III) oxide.
Carbon monoxide Water Carbon dioxide Hydrogen
The carbon dioxide in the resulting mixture is passed through an alkaline solution. The second industrial method of producing hydrogen involves the chemical action
of steam on methane at high temperatures.
Waler Hydrogen Carbon monox ide
The water gas produced by the chemical reaction is treated by either of the methods previ- ously noted for isolating carbon monoxide-free hydrogen.
Hydrogen is also produced by a multistep, complex reaction between propane and steam that is denoted by the following overall equation:
C3H 8(g) + 6H2O(g) --> 3CO2(g) + I0H2(g) Propane Wa ter Carbon dioxide Hydrogen
The carbon dioxide in the resulting mixture is passed through an alkaline solution. Hydrogen is also produced by the reaction of steam on methanol vapor using a
copper oxide/zinc oxide catalyst as follows:
CH 3OH(g) + H2O(g) --> CO2(g) + 3H2(g) Meth anol Water Carbon diox id e Hydrogen
Once again, the carbon dioxide is passed through an alkaline solution.
coal gasif ication The chemical process that produces a mixture of carbon monoxide and hydrogen when steam is exposed to coal at high temperature and pressure
wate r gas (synthes is gas; syngas) The mixture of carbon monoxide and hydro- gen produced by blow- ing steam through a bed of red-hot coke
Chapter 7 Chemistry of Some Common Elements 239 Ii' I ~
lifting power The natural ability of a gas that is less dense than air to move upwards
TABLE 7.4 Physical Properties of Elemental Hydrogen
Melting point -434.S'F (-259.2'C)
Boiling point -423.0'F (-252.8°()
Specific gravity (gas) at 68'F (20'C) 0 .071
Vapor density (air - 1) 0.069
Autoignition point 1058'F (570'C)
Lower flammable limit 4% by volume
Upper flammable limit 75% by volume
Liquid-to-gas expansion ratio 848
7.2-D PROPERTIES OF HYDROGEN Elemental hydrogen possesses several interesting physi~al properties, some of which listed in Table 7.4. A notable property is its vapor density-only 0.07 compared w·ih are
1 l"ft' I a~ Becau~e this value is so low, hy~roge_n posse~s7s a natura I mg po~er or buoyan · potential. When expressed in metric uruts, the _lifting power of hydro~en 1s equal to the dJ. ference in mass between 1 liter of air and 1 liter of hydrogen a~ a given ~emperature and pressure. At 32°F (0°C) and 14.7 psi (10_1.3 kPa), the mass of 1 hter ?f ~1r 1s 1.2930 grams, whereas the mass of 1 liter of hydrogen 1s 0.0899 grams; hence, the hftmg power of hydro- gen in air at this temperature and pressure is 1.2930 g/L - 0.0899 ~~, _or 1.2031 g/L, As we note in Section 7.2-H, this lifting power once was used to keep dmg1bles aloft.
Another important property of hydrogen is associated with the tiny size of its mole- cules. Hydrogen molecules are extremely small when compared with other molecules. They are so tiny that they easily leak through openings such as pipe joints and valve con- nections. To prevent loss during storage and transport, the steel cylinders, tubes, and tanks used to hold the compressed gas must be uniquely designed.
Another important property of hydrogen is the relatively high rate at which it diffuses into the air. When released from a tank or container, hydrogen diffuses into the air more rapidly than any other substance; that is, at the same temperature, hydrogen moves faster than other gases or vapors.
Hydrogen is a flammable gas that burns when its concentration in air is between 4% and 75% by volume. A concentration within this wide range can readily be achieved when hydrogen is released from its container or tank. Nonetheless, because it dissipates so rapidly, unconfined hydrogen does not remain in any single area for long.
Hydrogen burns in air to form water vapor. 2Hz(g) + Oz(g) --+ 2H20(g)
Hydrogen Oxygen Water
Given its low vapor density and high diffusion rate, the combustion reaction occurs as th
e gas moves upward. Hydrogen burns with an almost nonluminous flame that is especially difficult to observe during daylight hours.
7 .2-E HYDROGEN AND THE RISK OF FIRE AND EXPLOSION The physical properties of hydrogen give rise to two potential scenarios relating to its risk of fire and explosion:
When released indoors or into an enclosure where the gas can accumulate, the pres· ence of the hydrogen poses a pronounced risk of fire and explosion. . dissi· When rele_ase? outdoors _or ~n a manner that enables the hydrogen t~ read
1 \ 11 .
pate, the hkehhood that it will form a flammable mixture is comparauvelY sJll
240 Chapter 7 Chemistry of Some Common Elements
.,,--- · h h H drogen diffus es into t e.arr at t e rate of 0.09s in.2/s (0.634 cni2/s) at 32'F (0'C ). What does the magnitude of i s rate reveal about the likelihood that a fl amm able mixture of hyd rogen and air will be produced under most
1 .d nt scenarios that occur outside building s? acc1 e solution: This inform ation indicates that .hydrogen diff uses very rapidly into the surrounding air . In fact, uncon - fi ned hydrogen ;roves ~ore rapidly into arr than any other gas. Hydrogen is also fl amm abl e over a wide concen- tration range (4 1/• to.lS 1/o by volume). Because hydrogen does not re main loca lized for long, a flammable mixture of hydrogen and arr is not 0rd in arr ly produced under most accident scenari os that occu r outside buildings.
Because hydrogen rises in the air, hydrogen-sensing devices often are installed near the ceilings of enclosures m which hydrogen could be inadvertently released, as from a leaking storage tank mto a roon:i , These devices activate exhaust fans capable of evacuat- ing the hydrogen mto the outside air at very rapid speeds. When emergency response crews are called to such incidents, they should enter the room only after the hydrogen has been dispelled and its concentration measures less than 4 % by volume,
7.2-F CHEMICAL REACTIONS THAT GENERATE HYDROGEN Although hydrogen may be encountered in cylinders and storage tanks, it also may be inadvertently generated as the product of chemical reactions. Certain metals generate hydrogen by displacing it from water and acid solutions. These displacement reactions occur at rates that depend on the nature of the metal.
Most alkali metals and alkaline earth metals react with water and acids to produce hydrogen. Certain other metals react much more slowly with water and acids, Nonethe- less, these reactions still can pose the risk of fire and explosion, because the hydrogen produced may absorb the heat of reaction and burst spontaneously into flame.
Many metals possess the capability of releasing hydrogen from acids. For instance, metallic tin and aluminum displace hydrogen from aqueous solutions of hydrochloric acid and sulfuric acid, respectively, as follows:
Sn(s) + 2HC l(aq) --> SnClz(aq) + Hz(g) Tin Hydrochloric ac id Tin(ll ) chlori de Hydroge n
2Al (s) + 3H2S04(aq) --> Al2(S04l3 (aq) + 3Hi(g) Aluminum Sulfuric acid Aluminum sulfate Hydrogen
The rates at which metals displace hydrogen from water and acidic solutions can be deter- mined experimentally. When the metals are then arranged by decreasing reaction rates, the compilation in Table 7.5 is obtained. This arrangement of the metals is called an activity series. Its significance is summarized as follows: 1 The metals listed at the top of the table are so chemically reactive that they react with
both water and acids. 1 The metals below magnesium release hydrogen from steam and acid solutions , but
not from liquid water. 1 The metals below iron in the series do not displace hydrogen from steam, even when
the temperature is elevated. 1 The metals below lead possess insufficient chemical reactivity to release hydrogen
from either water or acids.
Hydrogen is also displaced by certain metals from solutions of sodium hydroxide. The common metals exhibiting this chemistry are aluminum, zinc, and lead, but these reactions occur slowly.
activity series An arrangement of the metals in order of their decreasing ability to generate hydrogen by chemical reaction with water and acids
Chapter 7 Chemistry of Some Common Elements 241
lead-acid storage battery The collective group of individual cells composed of lead and lead oxide plates and immersed in a solution of sulfuric acid having a specific gravity ranging from 1.25 to 1.30
TABLE 7.5 Activity series of Some Metals
Metals that react with water to produce Hi
Metals that react with water and acids to produce H2
Metals that react with neither water nor acids to produce H2
7 .2-G HYDROGEN GENERATION WHEN CHARGING LEAD-ACID STORAGE BATTERIES
Cesiurn
Lithiurn
Rubidiurn
Potassiurn Bari urn
Sodiurn
Calciurn
Magnesium
Alurninurn
Manganese Zinc
Chromium
Iron
Nickel
Tin
Lead
Bismuth
Copper
Mercury
Silver
Platinum
Gold
Most of us are familiar with the lead-acid storage battery as the power source in virtu· ally all motor vehicles, including electric hybrid vehicles. A battery differs from a fuel cell in that its reactants are generated by chemical action. On the other hand, each fuel-cell reactant must be continuously provided to a fuel cell from an external source.
A lead-acid storage battery produces electric current by means of two rever~ible chemical reactions commonly referred to as charging and discharging. A battery is said to be charged when an outside current has been delivered through it; then, as the batter)' discharges, it produces electricity by means of chemical reactions involving the subStance'. that were formed during the charging process. Hydrogen is one of them. When the batte;i is being charged, bubbles of hydrogen accumulate near the negative plate; simultaneous Y, bubbles of oxygen accumulate near its positive plate. As they are produced, both gases dissipate into the atmosphere. . ul·
When banks of lead-acid storage batteries like those shown in Figure 7.12 ar~ s;r Jy taneo usly charged within an enclosure, however, the concentration of hydrogen is
1 een
to exceed its lower flammable limit. Within an enclosed room, for instance, byd(°!% rises and concentrates along the length of the ceiling. Because a concentration of on y en· b I f h d
. ff ' . . . d uate v y vo ume o y rogen 1s su 1cient to produce a flammable mixture m air, a eq • s 10 ·1 · h Id I b ·d d · · batterie ti auon s ou a w ays e prov, e w1thm rooms containing banks of storage
242 Chapter 7 Chem istry of Some Common Elements
prevent the accumulation of hydrogen and minimize an unreasonable risk of fire and explosion. OSHA requires the owners and operators of enclosures in which lead-acid storage batteries are charged to post warning signs like the following:
i ·tMM!il 7.2-H THE HINDENBURG
-BATTERY- CHARGING AREA
NO SMOKING
Perhaps the best-known incident involving the burning of hydrogen was the destruction of the German dirigible named the Hindenburg. Although the combustion of diesel fuel
FIGURE 7 .12 This battery-charging station is a location at which banks of individual batteries are simu ltaneously charged . Hydrogen gas is produced during the battery- charging process . When the battery-charging station is located inside a large ventilated room, the production of hydrogen is a relatively small problem, but when it is located inside an enclosure, the hydrogen may reach a concentration within its flammable range. Then, the accumulation of hydrogen poses the risk of fire and explosion . (Photo by Eugene Meyer and courtesy of Interstate Batteries of Las Vegas, Las Vegas, Nevada .)
powered its engines, hydrogen stored in large gas bags held this airship aloft. Germany I had intended to use the Hindenburg to inaugurate a new era in fast transatlantic travel, but the airship mysteriously caught fire and exploded while attempting its landing I approach at Lakehurst, New Jersey. Given the presence of hydrogen onboard, the imrne- 11 diate sentiment linked a hydrogen leak with the burning of the airship. In Section 9.3-D, we shall examine another likely cause .
The tragedy of the Hindenburg caused Germany and other nations to discontinue the use of hydrogen as a buoyant gas in airships. Today, helium is used to provide lifting power in airships, and it is a safer choice because it is a nonflammable gas. It possesses about 93 % of the lifting power of hydrogen.
Chapter 7 Chemistry of Some Common Elements 243
7 .2-1 WORKPLACE REGULATIONS INVOLVING HYDROGEN When it is intended for use at manufacturing and processing plants, hydrogen is encountered as a confined gas within cylinders or storage tanks. The capacif&enerally tanks used to store hydrogen range from less than 3000 cubic feet (85 m3 ) to ov~:s of the cubic feet (425 m3 ). At 29 C.F.R. §§1910.103(b)(2)(1)(a)-(d), OSHA regulates th ~5,0oo tions in relation to the position of buildings and other storage tanks so they a eir loca,
· · d 1 H d re read· accessible to delivery equipment and authonze personne · Y rogen storage tly must be located aboveground but not beneath electric power lines, and away fro sys.tellls
1
for other flammable gases and flammable liquids. To alert individuals to the pr i1l P1Ping . . k b 1 esenc this flammable gas, OSHA also requires the tan s to e permanent Y placarded Wi; of
sign that reads as follows: ha
! •
HYDROGEN FLAMMABLE GAS NO SMOKING OR
OPEN FLAMES
7 .2 -J TRANSPORTING HYDROGEN When shippers intend to transport compressed or cryogenic hydrogen, DOT requires them to identify it as shown in Table 7.6 on the accompanying shipping paper. All label- ing, marking, and placarding requirements apply.
When hydrogen is transported in bulk by highway or rail, its name must be displayed on two opposing sides of the tankcar used for shipment. When it is transported by rail in a DOT-113 tankcar, DOT requires the carrier to display FLAMMABLE GAS placards on squares having a white background and black border.
To meet demand in high-use areas of the United States, hydrogen is also transferred by pipeline from production plants to user facilities . For example, hydrogen is supplied to the petroleum refineries and petrochemical plants located along the coast of the Gulf of
TABLE 7.6 Shipping Descriptions of Hydrogen
HYDROGEN
Compressed hydrogen
Cryogenic hydrogen
Hydrogen in a metal hydride storage system
244 Chapter 7 Chemistry of Some Common Elements
SHIPPING DESCRIPTION
UN1049, Hydrogen, compressed, 2.1
UN1966, Hydrogen, refrigerated liquid, 2, 1
UN3468, Hydrogen in a metal hydride storage system, 2.1
or 'd storage UN3468, Hydrogen in a metal hydn e
system contained in equipment, 2.1
or d 'de storage UN3468, Hydrogen in a metal hy n
system packed with equipment, 2.1
M )(ico from a supply pipeline that stretches from the Houston Ship Channel to New o:ieans and connects to a 600-mile (965-km) pipeline network.
2. 1( RESPONDING TO INCIDENTS INVOLVING 7· A RELEASE OF HYDROGEN AS a practical matter, combating ~n ongoi_ng fire involving hydrogen is rarely success_ful unless the flow of hydrogen fro~ its cont~1~ment vessel can be stopped withou_t exposing
sonnel to undue nsk. _For this reason, It 1s often best to permit a hydrogen fire to burn ~~hoot interference until t~e fuel is enti~ely exhausted. Because a hydrogen-fueled fire is JikelY to caus~ secondary fires, appropnate emergency response actions should include
ention of its spread. prevWhen LH2 ha~ ?~en released, the use of a water fog is generally warranted. It prevents or reduces the possibility that hydrogen will concentrate in the nearby atmosphere and ignite.
7,3 CHLORINE Elemental chlorine is not found naturally, but chlorine is found on Earth to the extent of 0.19 % by ma~s in a variety of_compounds including sodium chloride, potassium chloride, calcium chloride, and magnesmm chloride.
At room conditions, chlorine exists as a yellow-green gas with a characteristic pene- rrating and irritating odor. It is encountered as a gas and a liquefied compressed gas. The element is about 2½ times heavier than air and is highly poisonous when inhaled . Several other physical properties of chlorine are noted in Table 7 .7.
Elemental chlorine should not be confused with solid "chlorine" products used to treat the water in residential and municipal swimming pools. Although the latter products frequently are called chlorine, they actually are oxidizing agents that generate chlorine by chemical action within the pools. Their properties are more appropriately discussed in Chapter 11.
7 .3-A PRODUCTION AND COMMERCIAL USES OF ELEMENTAL CHLORINE
For commercial use, elemental chlorine is prepared by passing an electric current through either molten sodium chloride or an aqueous solution of sodium chloride or magnesium chloride. When aqueous sodium chloride is used, sodium hydroxide and hydrogen are simultaneously produced.
2NaCl (aq) + 2H20(/) --> 2Na0H(aq) + H2(g) + Cl2(g) Sodi um chl oride Wale r Sod ium hydroxide Hydrogen Chlorine
TABLE 7.7 Physical Properties of Elemental Chlorine
Melting point -150'F (-101'()
Boiling point -30'F (-35'C)
Specific gravity (gas) at 68'F (20'C) 1.56
Specific gravity (liquid) at 6.86 atm 1.41
Vapor density (air - 1) 2.49
Liquid-to-gas expansion ratio 457 .6
Chlorine
Chapter 7 Chemistry of Some Common Elements 245
pulmonary edema The excessive accumula- tion of fluid within the lungs
TABLE 7.8 Adverse Health Effects Associated with Breathing Chlorinea
TIME EXPOSURE FOR CONCENTRATION
1-8 hr exposure at< 0.5 ppm 1-hr exposure at 0.5-2 ppm
1-hr exposure at 2-20 ppm
1-hr exposure at> 20 ppm
1-hr exposure at 34 ppm
ADVERSE HEALTH EFFECT
No signs or symptoms of adverse effects Strong odor; slight irritation of nose th ' roat eyes • and
Burning of eyes or throat; coughing and cha . sensations krng
Sense of suffocation;_t~hest padinh; shortness of breath; nausea; vomI rng; an oarseness
Pulmonary edema; sudden death; bronchos (closure of the larynx) Pasrn
8 U.S. Army Center for Health Promotion and Preventative Medicine (USACHPPM) T~c_hnical Guide 230, "Environ mental Health Risk Assessment and Chemical Exposure Guidelines for Deployed Military Personnel" (U.S. Arm~ Public Health Command, June 2010).
Throughout the civilized world, large volumes of elemental chlorine are required annually for the following commercial applications:
A raw material for the production and manufacture of a wide range of chlorine- containing compounds used as solvents, pesticides, dyes, bleaching agents, plastics, refrigerants, and other commercial products A microbicide for treatment of drinking water and wastewater to prevent waterborne infectious diseases A bleaching agent of paper pulp and certain textiles
7 .3-B ILL EFFECTS CAUSED BY INHALING CHLORINE Exposure to elemental chlorine poses the threat of inhalation toxicity, its principal risk. The exposure initially causes coughing, dizziness, nausea, headache, and severe inflamma- tion of the eyes, nose, and throat. Prolonged (>1 hr) exposure to moderate concentra- tions potentially causes congestion of the lungs, which can give rise to the onset of pulmonary edema, obstruction of the airways, and painful and difficult breathing. Other ill effects are listed in Table 7.8.
7 .3-C CHLORINE AS A CHEMICAL WARFARE AGENT On a somber note, Germany used chlorine during World War I as a chemical warf~re agent (Section 13.11). The worst incident occurred at Ypres, Belgium in 1915. Taktn7 advantage of ~ts highly pois~nous nat~re, the Germans unleashed the gas as a we~pontie mass destruct10n. The chlorine was discharged from 5730 pressurized cylinders mt? f wind, which carried the gas into the trenches. As a poisonous gas with a vapor den5ir_y o 2.49 (air = 1 ), the gas caused mass casualties by maintaining ground-level concentranons capable of causing death when inhaled. . • h
The reported loss of life at Ypres ranged from 7000 to 15,000 people. The Brttl~e retaliated by discharging chlorine from 5100 cylinders at German troops during rhe Ba~rd of Loos. However, meteorological conditions caused the chlorine gas to move back row the British troops, resulting in 2632 British casualties, including 7 deaths. f oi·
In 1925, the ~eague of Nations too~ first step toward eliminating the ~se eff!cs, sonous gases dunng warfare between c1v1hzed nations. As a consequence of its
246 Chapter 7 Chemistry of Some Common Elements
130 nations ratified the Geneva Protocol which prohibits the use of chlorine and ver h · 1 ' o poisonous gases as c em1ca warfare agents. Known fo rmally as the Proto col for th e 0thr;bition of the Use in War of Asphyxiating, Pois onous, or Other Gases, and of Bacte- pr~o ;cal Methods ~f Warf~re, it h_as successfully prevented civilized countries from using r10. g ous gases against their wartime enemies. poison
Geneva Protocol • The international agreement that bans the use of chemical weapons against wartime enemies
SOLVED EXERCISE 7 .4
~ kcar contains 1 ton (0 .9 t ) of chlorine as a liquefied compressed gas . The following informat ion is im- A. don its exterior surface : pnnte
PROX28829 DOT-1 0SAS00W
LD LMT LTWT NEW
5000 LB 2000 LB 08 97
2273 KG 909 KG
use the data in Table 7. 7_ to ca lculate the approximate volume in cub ic feet of gaseous ch lorine that is generated when the contents of this tank are suddenly released into the atm osphere during a transportation mishap, and ascertain how this amount impacts emergency responders when t he wi nd speed is <6 mi/hr (<1 0 km/hr).
Solution: In Section 3 .7-A, we learned that " LD LMT" and " LT WT " are the abbreviations for the load limit and light weight of a rail tankcar, respectively. Hence, the imprinted information indicates that the tankcar weighs 2000 pounds (909 kg) when empty and may be used to safely transport up t o 5000 pounds (2273 kg) of liquid chlorine.
using the specific gravity of liquid chlorine in Table 7.7, we calculate the density of t he liquid as follows:
lb 1.41 X 62.4 ft3 = 88 .0 lb/ft3
The volume occupied by 1 ton of liquid chlorine is then calculated to be 22 . 7 cubic f eet:
lb 1 ft3 1 m X 2000tA X 88 _0 lb= 22. 7 ft
3
using its liquid-to-gas expansion ratio, we determine that 22 .7 cub ic feet of liquid chlorine expands to a gaseous vol ume of 10,388 cubic feet.
22.7 ft 3(11 q uld) X 45 7. 6 = 10,388ft3(gas) One ton of chlorine per 10,388 cubic feet is equivalent to an undiluted concent rati on in air of 0.19 lb/ft3 , or
3000 parts per million. 4 Because the wind speed is relativel y low, this concentration remains virtually unchanged du ring the response action. Table 7.8 indicates that the inhalat ion of 34 pa rts per million is likely to cause sudden death . The inhalation of 3000 parts per million is unquestionably fatal.
7.3-D CHEMICAL REACTIVITY OF CHLORINE Like oxygen, elemental chlorine is a nonflammable gas capable of supporting combus- tion. The oxidizing ability of chlorine is apparent from its reaction with hydrogen, which burns in a chlorine atmosphere to form hydrogen chloride.
H z(g ) + C[i(g) - 2HCl(g) Hydroge n Chlo,i nc Hydrogen c hloride
,;---------- hA concentration of 0.19 lb/ft3 is converted to its equivalent in part per million through use of the program at
ttp ://www.unitconversion.org .
Chapter 7 Chemistry of Some Common Elements 247