ALLOYS

Sb2S3+ 3Fe = 2Sb + 3FeS.

Physical properties.Antimony is a bluish-white, metallic-looking substance whose density is 6.7. It is highly crystalline, hard, and very brittle. It has a rather low melting point (432°) and expands very noticeably on solidifying.

Chemical properties.In chemical properties antimony resembles arsenic in many particulars. It forms the oxides Sb2O3and Sb2O5, and in addition Sb2O4. It combines with the halogen elements with great energy, burning brilliantly in chlorine to form antimony trichloride (SbCl3). When heated on charcoal with the blowpipe it is oxidized and forms a coating of antimony oxide on the charcoal which has a characteristic bluish-white color.

Stibine(SbH3). The gas stibine (SbH3) is formed under conditions which are very similar to those which produce arsine, and it closely resembles the latter compound, though it is still less stable. It is very poisonous.

Acids of antimony.The oxides Sb_{2}O_{3} and Sb_{2}O_{5} are weak acid anhydrides and are capable of forming two series of acids corresponding in formulas to the acids of phosphorus and arsenic. They are much weaker, however, and are of little practical importance.Sulphides of antimony.Antimony resembles arsenic in that hydrogen sulphide precipitates it as a sulphide when conducted into an acidified solution containing an antimony compound:2SbCl3+ 3H2S = Sb2S3+ 6HCl,2SbCl5+ 5H2S = Sb2S5+ 10HCl.The two sulphides of antimony are called the trisulphide and the pentasulphide respectively. When prepared in this way they are orange-colored substances, though the mineral stibnite is black.

Sulphides of antimony.Antimony resembles arsenic in that hydrogen sulphide precipitates it as a sulphide when conducted into an acidified solution containing an antimony compound:

2SbCl3+ 3H2S = Sb2S3+ 6HCl,2SbCl5+ 5H2S = Sb2S5+ 10HCl.

2SbCl3+ 3H2S = Sb2S3+ 6HCl,

2SbCl5+ 5H2S = Sb2S5+ 10HCl.

The two sulphides of antimony are called the trisulphide and the pentasulphide respectively. When prepared in this way they are orange-colored substances, though the mineral stibnite is black.

Metallic properties of antimony.The physical properties of the element are those of a metal, and the fact that its sulphide is precipitated by hydrogen sulphide shows that it acts like a metal in a chemical way. Many other reactions show that antimony has more of the properties of a metal than of a non-metal. The compound Sb(OH)3, corresponding to arsenious acid, while able to act as a weak acid is also able to act as a weak base with strong acids. For example, when treated with concentrated hydrochloric acid antimony chloride is formed:

Sb(OH)3+ 3HCl = SbCl3+ 3H2O.

A number of elements act in this same way, their hydroxides under some conditions being weak acids and under others weak bases.

Some metals when melted together thoroughly intermix, and on cooling form a homogeneous, metallic-appearing substance called analloy. Not all metals will mix in this way, and in some cases definite chemical compounds are formed and separate out as the mixture solidifies, thus destroying the uniform quality of the alloy. In general the melting point of the alloy is below the average of the melting points of its constituents, and it is often lower than any one of them.

Antimony forms alloys with many of the metals, and its chief commercial use is for such purposes. It imparts to its alloys high density, rather low melting point, and theproperty of expanding on solidification. Such an alloy is especially useful in type founding, where fine lines are to be reproduced on a cast. Type metal consists of antimony, lead, and tin. Babbitt metal, used for journal bearings in machinery, contains the same metals in a different proportion together with a small percentage of copper.

Occurrence.Bismuth is usually found in the uncombined form in nature. It also occurs as oxide and sulphide. Most of the bismuth of commerce comes from Saxony, and from Mexico and Colorado, but it is not an abundant element.

Preparation.It is prepared by merely heating the ore containing the native bismuth and allowing the melted metal to run out into suitable vessels. Other ores are converted into oxides and reduced by heating with carbon.

Physical properties.Bismuth is a heavy, crystalline, brittle metal nearly the color of silver, but with a slightly rosy tint which distinguishes it from other metals. It melts at a low temperature (270°) and has a density of 9.8. It is not acted upon by the air at ordinary temperatures.

Chemical properties.When heated with the blowpipe on charcoal, bismuth gives a coating of the oxide Bi2O3. This has a yellowish-brown color which easily distinguishes it from the oxides formed by other metals. It combines very readily with the halogen elements, powdered bismuth burning readily in chlorine. It is not very easily acted upon by hydrochloric acid, but nitric and sulphuric acids act upon it in the same way that they do upon copper.

Uses.Bismuth finds its chief use as a constituent of alloys, particularly in those of low melting point. Someof these melt in hot water. For example, Wood's metal, consisting of bismuth, lead, tin, and cadmium, melts at 60.5°.

Compounds of bismuth.Unlike the other elements of this group, bismuth has almost no acid properties. Its chief oxide, Bi2O3, is basic in its properties. It dissolves in strong acids and forms salts of bismuth:

Bi2O3+ 6HCl = 2BiCl3+ 3H2O,Bi2O3+ 6HNO3= 2Bi(NO3)3+ 3H2O.

Bi2O3+ 6HCl = 2BiCl3+ 3H2O,

Bi2O3+ 6HNO3= 2Bi(NO3)3+ 3H2O.

The nitrate and chloride of bismuth can be obtained as well-formed colorless crystals. When treated with water the salts are decomposed in the manner explained in the following paragraph.

Many salts such as those of antimony and bismuth form solutions which are somewhat acid in reaction, and must therefore contain hydrogen ions. This is accounted for by the same principle suggested to explain the fact that solutions of potassium cyanide are alkaline in reaction (p. 210). Water forms an appreciable number of hydrogen and hydroxyl ions, and very weak bases such as bismuth hydroxide are dissociated to but a very slight extent. When Bi+++ions from bismuth chloride, which dissociates very readily, are brought in contact with the OH-ions from water, the two come to the equilibrium expressed in the equation

Bi++++ 3OH-<--> Bi(OH)3.

For every hydroxyl ion removed from the solution in this way a hydrogen ion is left free, and the solution becomes acid in reaction.

Reactions of this kind and that described under potassium cyanide are calledhydrolysis.

DEFINITION:Hydrolysis is the action of water upon a salt to form an acid and a base, one of which is very slightly dissociated.

Conditions favoring hydrolysis.While hydrolysis is primarily due to the slight extent to which either the acid or the base formed is dissociated, several other factors have an influence upon the extent to which it will take place.

1.Influence of mass.Since hydrolysis is a reversible reaction, the relative masses of the reacting substances influence the point at which equilibrium will be reached. In the equilibrium

BiCl3+ 3H2O <--> Bi(OH)3+ 3HCl

the addition of more water will result in the formation of more bismuth hydroxide and hydrochloric acid. The addition of more hydrochloric acid will convert some of the bismuth hydroxide into bismuth chloride.

2.Formation of insoluble substances.When one of the products of hydrolysis is nearly insoluble in water the solution will become saturated with it as soon as a very little has been formed. All in excess of this will precipitate, and the reaction will go on until the acid set free increases sufficiently to bring about an equilibrium. Thus a considerable amount of bismuth and antimony hydroxides are precipitated when water is added to the chlorides of these elements. The greater the dilution the more hydroxide precipitates. The addition of hydrochloric acid in considerable quantity will, however, redissolve the precipitate.

Partial hydrolysis.In many cases the hydrolysis of a salt is only partial, resulting in the formation of basic salts instead of the free base. Most of these basic salts are insoluble in water, which accounts for their ready formation. Thus bismuth chloride may hydrolyze by successive steps, as shown in the equations

BiCl3+ H2O = Bi(OH)Cl2+ HCl,BiCl3+ 2H2O = Bi(OH)2Cl + 2HCl,BiCl3+ 3H2O = Bi(OH)3+ 3HCl.

BiCl3+ H2O = Bi(OH)Cl2+ HCl,

BiCl3+ 2H2O = Bi(OH)2Cl + 2HCl,

BiCl3+ 3H2O = Bi(OH)3+ 3HCl.

The basic salt so formed may also lose water, as shown in the equation

Bi(OH)2Cl = BiOCl + H2O.

The salt represented in the last equation is sometimes called bismuth oxychloride, or bismuthyl chloride. The corresponding nitrate, BiONO3, is largely used in medicine under the name of subnitrate of bismuth. In these two compounds the group of atoms, BiO, acts as a univalent metallic radical and is calledbismuthyl. Similar basic salts are formed by the hydrolysis of antimony salts.

1.Name all the elements so far studied which possess allotropic forms.

2.What compounds would you expect phosphorus to form with bromine and iodine? Write the equations showing the action of water on these compounds.

3.In the preparation of phosphine, why is coal gas passed into the flask? What other gases would serve the same purpose?

4.Give the formula for the salt which phosphine forms with hydriodic acid. Give the name of the compound.

5.Could phosphoric acid be substituted for sulphuric acid in the preparation of the common acids?

6.Write the equations for the preparation of the three sodium salts of orthophosphoric acid.

7.Why does a solution of disodium hydrogen phosphate react alkaline?

8.On the supposition that bone ash is pure calcium phosphate, what weight of it would be required in the preparation of 1 kg. of phosphorus?

9.If arsenopyrite is heated in a current of air, what products are formed?

10.(a) Write equations for the complete combustion of hydrosulphuric acid, methane, and arsine. (b) In what respects are the reactions similar?

11.Write the equations for all the reactions involved in Marsh's test for arsenic.

12.Write the names and formulas for the acids of antimony.

13.Write the equations showing the hydrolysis of antimony trichloride; of bismuth nitrate.

14.In what respects does nitrogen resemble the members of the phosphorus family?

SYMBOLATOMIC WEIGHTDENSITYCHLORIDESOXIDESSiliconSi28.42.35SiCl4SiOTitaniumTi48.13.5TiCl4TiOBoronB11.02.45BCl3B2O3

General.Each of the three elements, silicon, titanium, and boron, belongs to a separate periodic family, but they occur near together in the periodic grouping and are very similar in both physical and chemical properties. Since the other elements in their families are either so rare that they cannot be studied in detail, or are best understood in connection with other elements, it is convenient to consider these three together at this point.

The three elements are very difficult to obtain in the free state, owing to their strong attraction for other elements. They can be prepared by the action of aluminium or magnesium on their oxides and in impure state by reduction with carbon in an electric furnace. They are very hard and melt only at the highest temperatures. At ordinary temperatures they are not attacked by oxygen, but when strongly heated they burn with great brilliancy. Silicon and boron are not attacked by acids under ordinary conditions; titanium is easily dissolved by them.

Occurrence.Next to oxygen silicon is the most abundant element. It does not occur free in nature, but its compounds are very abundant and of the greatest importance. It occurs almost entirely in combination with oxygen as silicon dioxide (SiO2), often called silica, or with oxygen and various metals in the form of salts of silicic acids, or silicates. These compounds form a large fraction of the earth's crust. Most plants absorb small amounts of silica from the soil, and it is also found in minute quantities in animal organisms.

Preparation.The element is most easily prepared by reducing pure powdered quartz with magnesium powder:

SiO2+ 2Mg = 2MgO + Si.

Properties.As would be expected from its place in the periodic table, silicon resembles carbon in many respects. It can be obtained in several allotropic forms, corresponding to those of carbon. The crystallized form is very hard, and is inactive toward reagents. The amorphous variety has, in general, properties more similar to charcoal.

Compounds of silicon with hydrogen and the halogens.Silicon hydride (SiH4) corresponds in formula to methane (CH4), but its properties are more like those of phosphine (PH3). It is a very inflammable gas of disagreeable odor, and, as ordinarily prepared, takes fire spontaneously on account of the presence of impurities.

Silicon combines with the elements of the chlorine family to form such compounds as SiCl4and SiF4. Of these silicon fluoride is the most familiar and interesting. As stated in the discussion of fluorine, it is formed whenhydrofluoric acid acts upon silicon dioxide or a silicate. With silica the reaction is thus expressed:

SiO2+ 4HF = SiF4+ 2H2O.

It is a very volatile, invisible, poisonous gas. In contact with water it is partially decomposed, as shown in the equation

SiF4+ 4H2O = 4HF + Si(OH)4.

The hydrofluoric acid so formed combines with an additional amount of silicon fluoride, forming the complex fluosilicic acid (H2SiF6), thus:

2HF + SiF4= H2SiF6.

Silicides.As the name indicates, silicides are binary compounds consisting of silicon and some other element. They are very stable at high temperatures, and are usually made by heating the appropriate substances in an electric furnace. The most important one iscarborundum, which is a silicide of carbon of the formula CSi. It is made by heating coke and sand, which is a form of silicon dioxide, in an electric furnace, the process being extensively carried on at Niagara Falls. The following equation represents the reaction

SiO2+ 3C = CSi + 2CO.

The substance so prepared consists of beautiful purplish-black crystals, which are very hard. Carborundum is used as an abrasive, that is, as a material for grinding and polishing very hard substances. Ferrosilicon is a silicide of iron alloyed with an excess of iron, which finds extensive use in the manufacture of certain kinds of steel.

Manufacture of carborundum.The mixture of materials is heated in a large resistance furnace for about thirty-six hours. After the reaction is completed there is left a core of graphiteG. Surrounding this core is a layer of crystallized carborundumC, about 16 in. thick. Outside this is a shell of amorphous carborundumA. The remaining materialsMare unchanged and are used for a new charge.

Fig. 73Fig. 73

Silicon dioxide(silica) (SiO2). This substance is found in a great variety of forms in nature, both in the amorphous and in the crystalline condition. In the form of quartz it is found in beautifully formed six-sided prisms, sometimes of great size. When pure it is perfectly transparent and colorless. Some colored varieties are given special names, as amethyst (violet), rose quartz (pale pink), smoky or milky quartz (colored and opaque). Other varieties of silicon dioxide, some of which also contain water, are chalcedony, onyx, jasper, opal, agate, and flint. Sand and sandstone are largely silicon dioxide.

Properties.As obtained by chemical processes silicon dioxide is an amorphous white powder. In the crystallized state it is very hard and has a density of 2.6. It is insoluble in water and in most chemical reagents, and requires the hottest oxyhydrogen flame for fusion. Acids, excepting hydrofluoric acid, have little action on it, and it requires the most energetic reducing agents to deprive it of oxygen. It is the anhydride of an acid, and consequently it dissolves in fused alkalis to form silicates. Being nonvolatile, it will drive out most other anhydrides when heatedto a high temperature with their salts, especially when the silicates so formed are fusible. The following equations illustrate this property:

Na2CO3+ SiO2= Na2SiO3+ CO2,Na2SO4+ SiO2= Na2SiO3+ SO3.

Na2CO3+ SiO2= Na2SiO3+ CO2,

Na2SO4+ SiO2= Na2SiO3+ SO3.

Silicic acids.Silicon forms two simple acids, orthosilicic acid (H4SiO4) and metasilicic acid (H2SiO3). Orthosilicic acid is formed as a jelly-like mass when orthosilicates are treated with strong acids such as hydrochloric. On attempting to dry this acid it loses water, passing into metasilicic or common silicic acid:

H4SiO4= H2SiO3+ H2O.

Metasilicic acid when heated breaks up into silica and water, thus:

H2SiO3= H2O + SiO2.

Salts of silicic acids,—silicates.A number of salts of the orthosilicic and metasilicic acids occur in nature. Thus mica (KAlSiO4) is a salt of orthosilicic acid.

Polysilicic acids.Silicon has the power to form a great many complex acids which may be regarded as derived from the union of several molecules of the orthosilicic acid, with the loss of water. Thus we have

3H4SiO4= H4Si3O8+ 4H2O.

These acids cannot be prepared in the pure state, but their salts form many of the crystalline rocks in nature. Feldspar, for example, has the formula KAlSi3O8, and is a mixed salt of the acid H4Si3O8, whose formation is represented in the equation above. Kaolin has the formula Al2Si2O7·2H2O. Many other examples will be met in the study of the metals.

Glass.When sodium and calcium silicates, together with silicon dioxide, are heated to a very high temperature, the mixture slowly fuses to a transparent liquid, which on cooling passes into the solid called glass. Instead of starting with sodium and calcium silicates it is more convenient and economical to heat sodium carbonate (or sulphate) and lime with an excess of clean sand, the silicates being formed during the heating:

Na2CO3+ SiO2= Na2SiO3+ CO2,CaO + SiO2= CaSiO3.

Na2CO3+ SiO2= Na2SiO3+ CO2,

CaO + SiO2= CaSiO3.

Fig. 74Fig. 74

The mixture is heated below the fusing point for some time, so that the escaping carbon dioxide may not spatter the hot liquid; the heat is then increased and the mixture kept in a state of fusion until all gases formed in the reaction have escaped.

Molding and blowing of glass.The way in which the melted mixture is handled in the glass factory depends upon the character of the article to be made. Many articles, such as bottles, are made by blowing the plastic glass into hollow molds of the desired shape. The mold is first opened, as shown in Fig. 74. A lump of plastic glassAon the hollow rodBis lowered into the mold, which is then closed by the handlesC. By blowing into the tube the glass is blown into the shape of the mold. The mold is then opened and the bottle lifted out. The neck of the bottle must be cut off at the proper place and the sharp edges rounded off in a flame.

Other objects, such as lamp chimneys, are made by getting a lump of plastic glass on the end of a hollow iron rod and blowing it into the desired shape without the help of a mold, great skill being required in the manipulation of the glass. Window glass is made by blowing large hollow cylinders about 6 ft. long and 1-1/2 ft. in diameter. These are cut longitudinally, and are then placed in an oven and heated until they soften, when they are flattened out into plates (Fig. 75). Plate glass is cast into flat slabs, which are then ground and polished to perfectly plane surfaces.

Varieties of glass.The ingredients mentioned above make a soft, easily fusible glass. If potassium carbonate is substituted for the sodium carbonate, the glass is much harder and less easily fused; increasing the amount of sand has somewhat the same effect. Potassium glass is largely used in making chemical glassware, since it resists the action of reagents better than the softer sodium glass. If lead oxide is substituted for the whole or a part of the lime, the glass is very soft, but has a high index of refraction and is valuable for making optical instruments and artificial jewels.

Fig. 75Fig. 75

Coloring of glass.Various substances fused along with the glass mixture give characteristic colors. The amber color of common bottles is due to iron compounds in the glass; in other cases iron colors the glass green. Cobalt compounds color it deep blue; those of manganese give it an amethyst tint and uranium compounds impart a peculiar yellowish green color. Since iron is nearly always present in the ingredients, glass is usually slightly yellow. This color can be removed by adding the proper amount of manganese dioxide, for the amethyst color of manganese and the yellow of iron together produce white light.

Nature of glass.Glass is not a definite chemical compound and its composition varies between wide limits. Fused glass is really a solution of various silicates, such as those of calcium and lead, in fused sodium or potassium silicate. A certain amount of silicon dioxide is also present. This solution is then allowed to solidify under such conditions of cooling that the dissolved substances do not separate from the solvent. The compounds which are used to color the glass are sometimes converted into silicates, which then dissolve in the glass, giving it a uniform color. In other cases, as in the milky glasses which resemble porcelain in appearance, the color or opaqueness is due to the finely divided color material evenly distributed throughout the glass, but not dissolved in it. Milky glass is made by mixing calcium fluoride, tin oxide, or some other insoluble substance in the melted glass. Copper or gold in metallic form scattered through glass gives it shades of red.

Titanium is a very widely distributed element in nature, being found in almost all soils, in many rocks, and even in plant and animal tissues. It is not very abundant in any one locality, and it possesses little commercial value save in connection with the iron industry. Its most common ore is rutile (TiO2), which resembles silica in many respects.In both physical and chemical properties titanium resembles silicon, though it is somewhat more metallic in character. This resemblance is most marked in the acids of titanium. It not only forms metatitanic and orthotitanic acids but a great variety of polytitanic acids as well.

In both physical and chemical properties titanium resembles silicon, though it is somewhat more metallic in character. This resemblance is most marked in the acids of titanium. It not only forms metatitanic and orthotitanic acids but a great variety of polytitanic acids as well.

Occurrence.Boron is never found free in nature. It occurs as boric acid (H3BO3), and in salts of polyboric acids, which usually have very complicated formulas.

Preparation and properties.Boron can be prepared from its oxide by reduction with magnesium, exactly as in the case of silicon. It resembles silicon very strikingly in its properties. It occurs in several allotropic forms, is very hard when crystallized, and is rather inactive toward reagents. It forms a hydride, BH3, and combines directly with the elements of the chlorine family. Boron fluoride (BF3) is very similar to silicon fluoride in its mode of formation and chemical properties.

Boric oxide(B2O3). Boron forms one well-known oxide, B2O3, called boric anhydride. It is formed as a glassy mass by heating boric acid to a high temperature. It absorbs water very readily, uniting with it to form boric acid again:

B2O3+ 3H2O = 2H3BO3.

In this respect it differs from silicon dioxide, which will not combine directly with water.

Boric acid(H3BO3). This is found in nature in considerable quantities and forms one of the chief sources of boron compounds. It is found dissolved in the water of hot springs in some localities, particularly in Italy. Being volatile with steam, the vapor which escapes from these springs has some boric acid in it. It is easily obtained from these sources by condensation and evaporation, the necessary heat being supplied by other hot springs.

Boric acid crystallizes in pearly flakes, which are greasy to the touch. In the laboratory it is easily prepared by treating a strong, hot solution of borax with sulphuric acid. Boric acid being sparingly soluble in water crystallizes out on cooling:

Na2B4O7+ 5H2O + H2SO4= Na2SO4+ 4H3BO3.

The substance is a mild antiseptic, and on this account is often used in medicine and as a preservative for canned foods and milk.

Metaboric and polyboric acids.When boric acid is gently heated it is converted into metaboric acid (HBO2):

H3BO3= HBO2+ H2O.

On heating metaboric acid to a somewhat higher temperature tetraboric acid (H2B4O7) is formed:

4HBO2= H2B4O7+ H2O.

Many other complex acids of boron are known.

Borax.Borax is the sodium salt of tetraboric acid, having the formula Na2B4O7·10 H2O. It is found in some arid countries, as southern California and Tibet, but is now made commercially from the mineral colemanite, which is the calcium salt of a complex boric acid. When this is treated with a solution of sodium carbonate, calciumcarbonate is precipitated and borax crystallizes from the solution.

When heated borax at first swells up greatly, owing to the expulsion of the water of crystallization, and then melts to a clear glass. This glass has the property of easily dissolving many metallic oxides, and on this account borax is used as a flux in soldering, for the purpose of removing from the metallic surfaces to be soldered the film of oxide with which they are likely to be covered. These oxides often give a characteristic color to the clear borax glass, and borax beads are therefore often used in testing for the presence of metals, instead of the metaphosphoric acid bead already described.

The reason that metallic oxides dissolve in borax is that borax contains an excess of acid anhydride, as can be more easily seen if its formula is written 2NaBO2+ B2O3. The metallic oxide combines with this excess of acid anhydride, forming a mixed salt of metaboric acid.

Borax is extensively used as a constituent of enamels and glazes for both metal ware and pottery. It is also used as a flux in soldering and brazing, and in domestic ways it serves as a mild alkali, as a preservative for meats, and in a great variety of less important applications.

1.Account for the fact that a solution of borax in water is alkaline.

2.What weight of water of crystallization does 1 kg. of borax contain?

3.When a concentrated solution of borax acts on silver nitrate a borate of silver is formed. If the solution of borax is dilute, however, an hydroxide of silver forms. Account for this difference in behavior.

The metals.The elements which remain to be considered are known collectively as the metals. They are also called the base-forming elements, since their hydroxides are bases. A metal may therefore be defined as an element whose hydroxide is a base. When a base dissolves in water the hydroxyl groups form the anions, while the metallic element forms the cations. From this standpoint a metal can be defined as an element capable of forming simple cations in solution.

The distinction between a metal and a non-metal is not a very sharp one, since the hydroxides of a number of elements act as bases under some conditions and as acids under others. We have seen that antimony is an element of this kind.

Occurrence of metals in nature.A few of the metals are found in nature in the free state. Among these are gold, platinum, and frequently copper. They are usually found combined with other elements in the form of oxides or salts of various acids. Silicates, carbonates, sulphides, and sulphates are the most abundant salts. All inorganic substances occurring in nature, whether they contain a metal or not, are calledminerals. Those minerals from which a useful substance can be extracted are calledoresof the substance. These two terms are most frequently used in connection with the metals.

Extraction of metals,—metallurgy.The process of extracting a metal from its ores is called the metallurgy of the metal. The metallurgy of each metal presents peculiarities of its own, but there are several methods of general application which are very frequently employed.

1.Reduction of an oxide with carbon.Many of the metals occur in nature in the form of oxides. When these oxides are heated to a high temperature with carbon the oxygen combines with it and the metal is set free. Iron, for example, occurs largely in the form of the oxide Fe2O3. When this is heated with carbon the reaction expressed in the following equation takes place:

Fe2O3+ 3 C = 2 Fe + 3 CO.

Many ores other than oxides may be changed into oxides which can then be reduced by carbon. The conversion of such ores into oxides is generally accomplished by heating, and this process is calledroasting. Many carbonates and hydroxides decompose directly into the oxide on heating. Sulphides, on the other hand, must be heated in a current of air, the oxygen of the air entering into the reaction. The following equations will serve to illustrate these changes in the case of the ores of iron:

FeCO3= FeO + CO2,2Fe(OH)3= Fe2O3+ 3H2O,2FeS2+ 11O = Fe2O3+ 4SO2.

FeCO3= FeO + CO2,

2Fe(OH)3= Fe2O3+ 3H2O,

2FeS2+ 11O = Fe2O3+ 4SO2.

2.Reduction of an oxide with aluminium.Not all oxides, however, can be reduced by carbon. In such cases aluminium may be used. Thus chromium may be obtained in accordance with the following equation:

Cr2O3+ 2 Al = 2 Cr + Al2O3.

This method is a comparatively new one, having been brought into use by the German chemist Goldschmidt; hence it is sometimes called the Goldschmidt method.

3.Electrolysis.In recent years increasing use is being made of the electric current in the preparation of metals. In some cases the separation of the metal from its compounds is accomplished by passing the current through a solution of a suitable salt of the metal, the metal usually being deposited upon the cathode. In other cases the current is passed through a fused salt of the metal, the chloride being best adapted to this purpose.

Electro-chemical industries.Most of the electro-chemical industries of the country are carried on where water power is abundant, since this furnishes the cheapest means for the generation of electrical energy. Niagara Falls is the most important locality in this country for such industries, and many different electro-chemical products are manufactured there. Some industries depend upon electrolytic processes, while in others the electrical energy is used merely as a source of heat in electric furnaces.

Preparation of compounds of the metals.Since the compounds of the metals are so numerous and varied in character, there are many ways of preparing them. In many cases the properties of the substance to be prepared, or the material available for its preparation, suggest a rather unusual way. There are, however, a number of general principles which are constantly applied in the preparation of the compounds of the metals, and a clear understanding of them will save much time and effort in remembering the details in any given case. The most important of these general methods for the preparation of compounds are the following:

1.By direct union of two elements.This is usually accomplished by heating the two elements together. Thus the sulphides, chlorides, and oxides of a metal can generally be obtained in this way. The following equations serve as examples of this method:

Fe + S = FeS,Mg + O = MgO,Cu + 2Cl = CuCl2.

Fe + S = FeS,

Mg + O = MgO,

Cu + 2Cl = CuCl2.

2.By the decomposition of a compound.This decomposition may be brought about either by heat alone or by the combined action of heat and a reducing agent. Thus when the nitrate of a metal is heated the oxide of the metal is usually obtained. Copper nitrate, for example, decomposes as follows:

Cu(NO3)2= CuO + 2NO2+ O.

Similarly the carbonates of the metals yield oxides, thus:

CaCO3= CaO + CO2.

Most of the hydroxides form an oxide and water when heated:

2Al(OH)3= Al2O3+ 3H2O.

When heated with carbon, sulphates are reduced to sulphides, thus:

BaSO4+ 2C = BaS + 2CO2.

3.Methods based on equilibrium in solution.In the preparation of compounds the first requisite is that the reactions chosen shall be of such a kind as will go on to completion. In the chapter on chemical equilibrium it was shown that reactions in solution may become complete in either of three ways: (1) a gas may be formed which escapes from solution; (2) an insoluble solid may be formed which precipitates; (3) two different ions may combine to formundissociated molecules. By the judicious selection of materials these principles may be applied to the preparation of a great variety of compounds, and illustrations of such methods will very frequently be found in the subsequent pages.

4.By fusion methods.It sometimes happens that substances which are insoluble in water and in acids, and which cannot therefore be brought into double decomposition in the usual way, are soluble in other liquids, and when dissolved in them can be decomposed and converted into other desired compounds. Thus barium sulphate is not soluble in water, and sulphuric acid, being less volatile than most other acids, cannot easily be driven out from this salt When brought into contact with melted sodium carbonate, however, it dissolves in it, and since barium carbonate is insoluble in melted sodium carbonate, double decomposition takes place:

Na2CO3+ BaSO4= BaCO3+ Na2SO4.

On dissolving the cooled mixture in water the sodium sulphate formed in the reaction, together with any excess of sodium carbonate which may be present, dissolves. The barium carbonate can then be filtered off and converted into any desired salt by the processes already described.

5.By the action of metals on salts of other metals.When a strip of zinc is placed in a solution of a copper salt the copper is precipitated and an equivalent quantity of zinc passes into solution:

Zn + CuSO4= Cu + ZnSO4.

In like manner copper will precipitate silver from its salts:

Cu + Ag2SO4= 2Ag + CuSO4.

It is possible to tabulate the metals in such a way that any one of them in the table will precipitate any one following it from its salts. The following is a list of some of the commoner metals arranged in this way:

ZincIronTinLeadCopperBismuthMercurySilverGold

According to this table copper will precipitate bismuth, mercury, silver, or gold from their salts, and will in turn be precipitated by zinc, iron, tin, or lead. Advantage is taken of this principle in the purification of some of the metals, and occasionally in the preparation of metals and their compounds.

Important insoluble compounds.Since precipitates play so important a part in the reactions which substances undergo, as well as in the preparation of many chemical compounds, it is important to know what substances are insoluble. Knowing this, we can in many cases predict reactions under certain conditions, and are assisted in devising ways to prepare desired compounds. While there is no general rule which will enable one to foretell the solubility of any given compound, nevertheless a few general statements can be made which will be of much assistance.

1.Hydroxides.All hydroxides are insoluble save those of ammonium, sodium, potassium, calcium, barium, and strontium.

2.Nitrates.All nitrates are soluble in water.

3.Chlorides.All chlorides are soluble save silver and mercurous chlorides. (Lead chloride is but slightly soluble.)

4.Sulphates.All sulphates are soluble save those of barium, strontium, and lead. (Sulphates of silver and calcium are only moderately soluble.)

5.Sulphides.All sulphides are insoluble save those of ammonium, sodium, and potassium. The sulphides of calcium, barium, strontium, and magnesium are insoluble in water, but are changed by hydrolysis into acid sulphides which are soluble. On this account they cannot be prepared by precipitation.

6.Carbonates, phosphates, and silicates.All normal carbonates, phosphates, and silicates are insoluble save those of ammonium, sodium and potassium.

1.Write equations representing four different ways for preparing Cu(NO3)2.

2.Write equations representing six different ways for preparing ZnSO4.

3.Write equations for two reactions to illustrate each of the three ways in which reactions in solutions may become complete.

4.Give one or more methods for preparing each of the following compounds: CaCl2, PbCl2, BaSO4, CaCO3, (NH4)2S, Ag2S, PbO, Cu(OH)2(for solubilities, see last paragraph of chapter). State in each case the general principle involved in the method of preparation chosen.

Back to Index Next