DISENO DE CARTERAS

CHEMICAL NAME = iron(III) oxide CAS NUMBER = 1317–60–8 MOLECULAR FORMULA = Fe₂O₃ MOLAR MASS = 159.7 g/mol

COMPOSITION = Fe(69.9%) O(30.1%) MELTING POINT = 1,565°C

BOILING POINT = not reported DENSITY = 5.24 g/cm³

Iron(III) oxide is known in mineral form as hematite, which is the primary form of iron ore.

It is also known simply as iron oxide or ferric oxide and is what is colloquially called rust when referring to the corrosion of iron objects. Th e use of iron marked a signifi cant advance-ment in human history and was the last of the three prehistoric archaeological stages after the Stone and Bronze Ages. Th e period of the Iron Age is not fi xed but varied as technology developed in diff erent geographic areas. In Egypt and the Near East there is evidence that smelted iron items appeared shortly after 2000 b.c.e., although crude iron objects appeared several hundred years before, and items made from iron obtained from meteorites were pres-ent even earlier. In cpres-entral Europe the early Iron Age did not occur until roughly 800 b.c.e., and in the Americas smelted iron was not used until introduced by the European explorers and colonists.

Th e smelting process involves reducing the iron, which is in an oxidized state in iron ore, to elemental iron. To obtain elemental iron, the iron-bearing mineral is heated with carbon so that the oxygen separates from the iron and bonds to the carbon. Smelting involves pro-ducing carbon monoxide by burning charcoal or coke to produce carbon monoxide, which is the reducing agent. Th e carbon monoxide then reacts with iron oxide compounds to form elemental iron. Th e production of iron from iron(III) oxide can be summarized as:

2C_(s) + O_2(g) → 2CO_(g)

3CO_(g) + Fe₂O_3(s)→ 2Fe_(l) + 3CO_2(g)

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Carbon may also act directly as a reducing agent according to the equation: Fe₂O_3(l) + 3C_(s) → 2Fe_(l) + 3CO_(g). Impurities such as silica, calcium, and aluminum are removed by adding lime-stone or other agents in the process. Th e impurities fl oat to the top of the molten iron as slag.

Th e crude iron produced from smelting results in pig iron. Pig iron has a carbon content of 3–5%, a high percentage of impurities, and is very brittle. It is roughly 90% iron. Th e name pig iron comes from the parallel sand bed molds fed by a common runner from the furnace.

Th e parallel sand molds were thought to resemble suckling pigs. Pig iron must be further pro-cessed to reduce its carbon content and impurities to make useable iron and steel. Cast iron resembles pig iron, with a carbon content between 2% and 4%. Cast iron is hard and brittle.

Steel has intermediate carbon content between 0.2% and 2%. Steel is alloyed with an array of other metals to produce many types of steels. For example, stainless steel contains chromium and nickel. High carbon steel has a carbon content of about 1.5%, is very hard, and is used for cutting tools. Wrought iron is almost pure iron, with a carbon content less than 0.2%. It is soft and malleable but has little structural strength.

Iron (III) oxide exists in mineral form as hematite. It is 70% iron and is the primary source of iron ore in the world. About 90% of the iron mined in the United States is hematite. World production of this ore is more than 1 billion tons. Magnetite and taconite are two other primary iron oxide minerals used as iron ore. Th e name hematite comes from the blood-red color of powdered hematite. Th e Greek word hematite means “blood-like.” Some ancients held the belief that hematite was formed in areas where battles were fought and blood was spilled into the earth. Large deposits of hematite have been identifi ed on Mars.

In iron production, iron ores are reduced to produce iron metal. Th e opposite process oc curs when iron metals are oxidized to produce iron oxides or rust. Rust is primarily iron(III) oxide. Iron does not combine directly with oxygen to produce rust but involves the oxida-tion of iron in an electrochemical process. Th ere are two requirements for rust: oxygen and water. Th e necessity of both oxygen and water is illustrated when observing automobiles oper-ated in dry climates and ships or other iron objects recovered from anoxic water. Autos and ships subjected to these conditions show remarkably little rust, the former because of lack of water and the latter because of lack of oxygen.

Iron(III) oxide results from electrochemical reactions occurring on a piece of iron or steel.

Iron metals are never uniform; there are always minor irregularities in both the composition and physical structure of the metal. Th ese minute diff erences give rise to the anodic and cathodic regions associated with rust formation. During the rusting process, iron is oxidized at the anode according to the reaction: Fe_(s) → Fe²⁺ + 2e⁻. Th e electrons from this reaction fl ow through the metal to an area of the metal where oxygen is reduced according to the reac-tion: O_2(g) + 4H⁺_(aq) + 4e⁻ → 2H₂O_(l). Th e hydrogen ions in this reaction are provided mainly by carbonic acid from the dissolution of atmospheric carbon dioxide in the water. Th e water acts as an electrolyte connecting the anode and cathode regions of the metal. Iron ions, Fe²⁺, migrate through the electrolyte and in the process are further oxidized by dissolved oxygen in the water to Fe³⁺. Iron (III) oxide is formed at the cathode according to the equation:

4Fe²⁺_(aq) + O_2(g) + (4 + 2x)H₂O_(l)→ 4Fe₂O₃ •+ xH₂O_(s) + 8H⁺_(aq)

Th e “x” in this equation indicates a positive whole number corresponding to the variable amount of water molecules that takes part in the reaction. Th e rusting process is illustrated in Figure 52.1. Th e area of rust formation diff ers from where the oxidation of iron takes place.

162 ^| Th e 100 Most Important Chemical Compounds

As noted, water serves as an electrolyte through which iron ions migrate. Th is explains why vehicles rust much more rapidly in regions where road salts are used to melt winter ice. Th e salts improve the conductivity of the electrolyte, thereby accelerating the corrosive process.

Figure 52.1 Formation of rust.

Most metals undergo corrosion of some form except for the so-called noble metals of gold, platinum, and palladium. Metals such as aluminum and zinc have an even greater tendency than iron to oxidize, but the oxide layer on these metals forms an impervious protective layer.

Th is protects the metal below from further oxidation. Iron (III) oxide is highly porous and as a result rust does not protect the underlying metal from further corrosion.

A number of methods are used to reduce and prevent corrosion. Th e most common method is to paint iron materials so that the metals are protected from water and oxygen.

Alloying iron with other metals is a common means to reduce corrosion; for example, stainless steel is an alloy of iron and chromium. Iron may also be protected by coating it with another

Figure 52.2 Cathodic protection, magnesium is connected to an iron object and undergoes oxidation, protecting the iron from rust.

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metal. Galvanizing refers to applying a coating of zinc to protect the underlying metal. Also, because it is a more active metal, zinc oxidizes rather than iron.

Connecting iron objects to a more active metal is called cathodic protection. Cathodic protection is widely used to protect underground storage tanks, ship hulls, bridges, and bur-ied pipes. One of the most common forms of cathodic protection is to connect the object to magnesium. When magnesium is connected to an iron object, magnesium rather than iron becomes the anode in the oxidation process. In cathodic protection bars of magnesium are connected either directly or by wire to the iron structure. Because the metal connected to the iron corrodes over time, it is called the sacrifi cial anode. Sacrifi cial anodes must eventually be replaced if they are to continue to protect the structure. Th e basic chemistry of cathodic protection is illustrated in Figure 52.2.

53. Isooctane

CHEMICAL NAME = 2,2,4-trimethylpentane MOLECULAR FORMULA = C₈H₁₈

MOLAR MASS = 114.2 g/mol

COMPOSITION = C(84.1%) H(15.9%) MELTING POINT = −107.5°C

BOILING POINT = 99.3°C DENSITY = 0.69 g/cm³

Isooctane is a fl ammable liquid isomer of octane. It is best known for defi ning the octane number to rate the antiknock quality of gasoline, which is related to engine performance.

Knock is a descriptive term used to describe the sound produced by an engine subject to ineffi cient combustion. Combustion in the cylinders is precisely timed to produce a smooth uniform combustion in the cylinder. Th e ignition spark produces a small explosion of the fuel-air mixture, producing a fl ame front that moves smoothly and rapidly through the cylinder. Knock occurs when a fuel-air mixture autoignites (spontaneously combusts pre-maturely) in a region of the cylinder before the fl ame front arrives. Th e autoignition occurs because combustion compresses the fuel mixture in localized areas of the cylinder, causing an increase in pressure and temperature in these areas. Th erefore knock can be viewed as the spontaneous combustion of fuel occurring prematurely. Knocking results in ineffi cient engine operation and loss of power. In severe cases knocking results in overheating and engine damage. Th e octane rating of a gasoline is a measure of its ability to resist autoigni-tion, which causes the knock.

Th e history of gasoline as a fuel parallels the development of the automobile and the internal combustion engine. At the start of the 20th century, gasoline was an undesirable by-product in kerosene production. Gasoline supply exceeded its demand, but this changed within a decade, as the automobile became a major form of transportation. Gasoline was obtained directly from distillation of crude oil (this was termed straight-run gasoline), but this process could not meet the increase demand (Figure 53.1). By 1913, cracking pro-cesses began to increase supply, and this was followed in subsequent years with advances

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in processes such as reforming, polymerization, and alkylation to both increase supply and improve quality. Th ese advances continue today, with greater demand to fuel cleaner burning more effi cient vehicles.

Gasoline is a mixture containing more than 500 diff erent hydrocarbons and a variety of other compounds. Th e major hydrocarbon groups making up gasoline include straight-alkanes (n-paraffi ns), branched-alkanes (isoparaffi ns), alkenes (olefi ns), branched-alkenes (isoolefi ns), and aromatics (the words in parentheses indicate terms used by the petrochemical industry for classifi cation). As early as the 1880s, it was noted that knocking in internal combustion engines was related to the type of fuel. While engineers worked on engine designs to reduce knock, chemists approached the problem by examining fuel composition and additives to reduce knock. In the early 1920s, Th omas Midgley (1889–1944) and his team of research-ers at General Motors discovered that tetraethyl lead (Pb(C₂H₅)₄) added to gasoline reduced knock. Th is discovery enabled the car producers to increase compression ratios in engines, increasing engine performance. Tetraethyl lead was the preferred antiknock additive until the 1960s, when environmental concerns about lead pollution rose. Also, catalysts (plantinum, rhodium, palladium) used in catalytic converters to decrease air pollutants from vehicles were poisoned by lead.

To quantify the degree to which a fuel promoted or retarded knock, a committee with representatives from the gasoline and automobile industries was established in 1927. General Motors built an experimental single cylinder engine in which the compression ratio could be varied. A chemist from the Ethyl Corporation named Graham Edgar (1887–1955) experi-mented by burning diff erent fuels in the engine and recording the knock characteristics. One of these fuels was n-heptane acquired from sap from the Jeff rey pine tree. Edgar determined that heptane performed poorly in engine tests. At this time, Edgar synthesized isooctane as a compound with good antiknock qualities. Edgar suggested that the rating of a fuel could be measured by referencing it to n-heptane, which was assigned a rating of 0, and to isooctane,

Figure 53.1 Distillation column diagram. Drawing by Rae Déjur.

166 ^| Th e 100 Most Important Chemical Compounds

which was assigned a rating of 100. By choosing these compounds, fuels of Edgar’s time would have octane ratings ranging between 40 and 60.

During the next several years, a number of methods were proposed for rating knock of fuels. Diff erent methods were proposed for diff erent operating conditions, but two are gener-ally used today: the Motor method and Research method. Th e Motor method is for an engine that operates at 900 rpm, a higher engine temperature of 149°C (300°F), and heavier load.

Th e motor method is more refl ective of high-speed freeway driving. Th e Research method performed at 600 rpm and variable ambient operating temperature more closely approxi-mates conditions at low speed or starting from rest. Th e tests are performed with a standard one-cylinder engine made by Waukesha Engine in Wisconsin using an instrument called a knockmeter. Th e knock of a test fuel is compared to reference fuels that contain a blend of isooctane and n-heptane. Once the knock is matched against the reference fuel, the octane rating is given by the percentage of isooctane in the reference fuel. For example, if the fuel gives the same knock as a reference fuel with 90% isooctane and 10% n-heptane, its octane number is 90. Often the formula R+M/2 is displayed with the octane rating on gas pumps.

Th is signifi es that the octane rating is an average of ratings determined from both the Motor and Research methods. Typically, a gasoline’s Research octane number (RON) is higher than its Motor octane number (MON). For fuels with octane numbers exceeding 100, reference fuels of isooctane containing tetraethyl lead are used.

Th e actual octane (C₈H₁₈) composition of gasoline is very low, generally comprising only a few percent by mass. Th e octane rating of a fuel depends on its composition. Isooctane has an octane rating of 100; normal octane has an octane rating of −18. Normal alkanes (n-paraffi ns) have the lowest octane ratings of the major hydrocarbons found in gasoline, and the octane number decreases as the number of carbon in the chain increases. N-butane has a RON of 113, but the RON of the next alkane, n-pentane, drops to 62, n-hexane is 19, and the reference alkane n-heptane is defi ned as 0. Branched compounds have higher octane rat-ings than straight-chain compounds. Since 1930, many chemical processes, such as alkylation and polymerization, have been developed to increase the production of branched compounds in refi nery operations. High octane numbers in gasoline are those associated with the alkenes (olefi ns) and aromatics, especially akyl benzene compounds. For example, 2-pentene has a RON of 154. Benzene itself has a RON of 98, but 1,3,5-trimethylbenzene has a RON of 170.

Th e highest octane numbers in gasoline are associated with cyclic alkenes, but these account for only a minute fraction of gasoline.

54. Isoprene

CHEMICAL NAME = 2-methyl-1,3 butadiene

CAS NUMBER = 78–79–5 MOLECULAR FORMULA = C₅H₈ MOLAR MASS = 68.1

COMPOSITION = C(88.2%) H(11.8%) MELTING POINT = −146°C

BOILING POINT = 34.1°C DENSITY = 0.68 g/cm³

Isoprene is a volatile colorless liquid monomer that is the basic building block of rubber.

Natural rubber items date back to at least 1500 b.c.e. Spanish explorers discovered native Central and South Americans using rubber in the 16th century for waterproofi ng, balls, and bindings. Europeans were intrigued with rubber products brought back by explorers and the substance remained a curiosity for two centuries. Joseph Priestley, one of the founders of modern chemistry, noted in 1770 that the substance could be used as an eraser if writing was rubbed with it, and so the name “rubber” came into use. In 1826, Michael Faraday (1791–

1867) established the chemical formula of isoprene as C₅H₈. William Gregory (1803–1858) distilled rubber, obtaining isoprene in 1835. In 1860, Charles Hanson Greville Williams (1829–1910) isolated the monomer isoprene from rubber, showing that rubber was a polymer of isoprene.

Th e isoprene molecule structure shown above is just one form in which it exists. Rotation can occur around the C-C single bond, giving rise to a structure where the double bonds exist on the same side of the molecule:

168 ^| Th e 100 Most Important Chemical Compounds

Rubber results from the polymerization of isoprene to form polyisoprene. Th e result-ing structure dictates the properties of the rubber. Natural rubber has a cis 1,4 structure (Figure 54.1). Th is means that the carbon atoms that form the chain attach to the same side of the chain at the 1 and 4 positions. Th e cis structure gives rubber its elasticity. Polyisoprene also exists in a trans 1,3 confi guration. In the trans confi guration, the addition takes place on opposite sides of the carbon chain. Two substances that exhibit the polyisoprene trans struc-ture are gutta-percha and balata. Gutta-percha and balata, like rubber, are derived from the milky latex extracted from certain plants. Th ese substances crystallize at a higher temperature than natural rubber and are partially crystallized at normal room temperatures. On cooling, gutta-percha and balata acquire a hard, leathery texture. Gutta-percha and balata have tradi-tionally been used for sheathing of underwater cables and for the cover of golf balls.

Figure 54.1 Isoprene structures.

Natural rubber is an elastomer. An elastomer is an amorphous polymer that has the abil-ity to stretch and return to its original shape above a certain temperature. Natural rubber’s properties are temperature dependent. At temperatures above 60°C, natural rubber becomes soft and sticky and at temperatures of −50°C and below, it becomes hard and brittle. To enhance natural rubber’s usefulness requires that various additives be combined with the latex obtained from rubber plants. Adding ammonia to the latex prevents coagulation and allows the latex to be shipped and processed as a liquid. Charles Goodyear (1800–1860) was attempting to improve the quality of rubber in 1839 when he accidentally dropped rubber to which sulfur had been added on a hot stove. He discovered that the product had superior qualities compared to other natural rubbers of his day. Th e process Goodyear discovered was vulcanization. Goodyear was awarded a patent for the process in 1844 (U.S. Patent Number 3633). Vulcanized rubber is more elastic, stronger, and more resistant to light and chemical exposure. Goodyear never reaped the rewards of his discovery and died in poverty. Half a century later, the Goodyear Tire Company was named after him and is currently the larg-est tire producer in the United States and third in the world behind Michelin (France) and Bridgestone (Japan).

Goodyear discovered vulcanization, but he was unaware of the chemistry of the process.

Rubber’s elastic properties are due to the structure of isoprene. Rubber consists of coiled iso-prene polymers. When rubber is stretched, the polyisoiso-prene coils are straightened in a direc-tion parallel to the direcdirec-tion of stretching. Once the stretching force is removed, the isoprene polymers return to their coiled structure. In the stretched position, the isoprene polymers can slide past each other, especially as the temperature increases. Vulcanization produces cross-links of sulfur-to-sulfur bonds between isoprene polymers. Th is allows elongation of rubber without sliding and gives the rubber a greater degree of elasticity over a wider temperature

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range. Cross-linking by adding just a few tenths of a percent sulfur greatly improves rubber’s usefulness. Soft rubbers, such as that found in rubber bands, have a sulfur content of approximately 1–3%, and hard rubbers contain as much as 35% sulfur.

range. Cross-linking by adding just a few tenths of a percent sulfur greatly improves rubber’s usefulness. Soft rubbers, such as that found in rubber bands, have a sulfur content of approximately 1–3%, and hard rubbers contain as much as 35% sulfur.