An Elementary Study of Chemistry

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2AsH_{3} + 6O = As_{2}O_{3} + 3H_{2}O.

When the supply of air is deficient water and metallic arsenic are
formed:

2AsH_{3} + 3O = 3H_{2}O + 2As.

These reactions make the detection of even minute quantities of arsenic
a very easy problem.

[Illustration: Fig. 72]

~Marsh's test for arsenic.~ The method devised by Marsh for
detecting arsenic is most frequently used, the apparatus being
shown in Fig. 72. Hydrogen is generated in the flask A by the
action of dilute sulphuric acid on zinc, is dried by passing
over calcium chloride in the tube B, and after passing
through the hard-glass tube C is ignited at the jet D. If a
substance containing arsenic is now introduced into the
generator A, the arsenic is converted into arsine by the
action of the nascent hydrogen, and passes to the jet along
with the hydrogen. If the tube C is strongly heated at some
point near the middle, the arsine is decomposed while passing
this point and the arsenic is deposited just beyond the heated
point in the form of a shining, brownish-black mirror. If the
tube is not heated, the arsine burns along with the hydrogen at
the jet. Under these conditions a small porcelain dish crowded
down into the flame is blackened by a spot of metallic arsenic,
for the arsine is decomposed by the heat of the flame, and the
arsenic, cooled below its kindling temperature by the cold
porcelain, deposits upon it as a black spot. Antimony conducts
itself in the same way as arsenic, but the antimony deposit is
more sooty in appearance. The two can also be distinguished by
the fact that sodium hypochlorite (NaClO) dissolves the arsenic
deposit, but not that formed by antimony.

~Oxides of arsenic.~ Arsenic forms two oxides, As_{2}O_{3} and
As_{2}O_{5}, corresponding to those of phosphorus. Of these arsenious
oxide, or arsenic trioxide (As_{2}O_{3}), is much better known, and is
the substance usually called white arsenic, or merely arsenic. It is
found as a mineral, but is usually obtained as a by-product in burning
pyrite in the sulphuric-acid industry. The pyrite has a small amount of
arsenopyrite in it, and when this is burned arsenious oxide is formed as
a vapor together with sulphur dioxide:

2FeAsS + 10O = Fe_{2}O_{3} + As_{2}O_{3} + 2SO_{2}.

The arsenious oxide is condensed in appropriate chambers. It is a rather
heavy substance, obtained either as a crystalline powder or as large,
vitreous lumps, resembling lumps of porcelain in appearance. It is very
poisonous, from 0.2 to 0.3 g. being a fatal dose. It is frequently given
as a poison, since it is nearly tasteless and does not act very rapidly.
This slow action is due to the fact that it is not very soluble, and
hence is absorbed slowly by the system. Arsenious oxide is also used as
a chemical reagent in glass making and in the dye industry.

~Acids of arsenic.~ Like the corresponding oxides of phosphorus, the
oxides of arsenic are acid anhydrides. In solution they combine with
bases to form salts, corresponding to the salts of the acids of
phosphorus. Thus we have salts of the following acids:

H_{3}AsO_{3} arsenious acid.

H_{3}AsO_{4} orthoarsenic acid.

H_{4}As_{2}O_{3} pyroarsenic acid.

HAsO_{3} metarsenic acid.

Several other acids of arsenic are also known. Not all of these can be
obtained as free acids, since they tend to lose water and form the
oxides. Thus, instead of obtaining arsenious acid (H_{3}AsO_{3}), the
oxide As_{2}O_{3} is obtained:

2H_{3}AsO_{3} = As_{2}O_{3} + 3H_{2}O.

Salts of all the acids are known, however, and some of them have
commercial value. Most of them are insoluble, and some of the copper
salts, which are green, are used as pigments. Paris green, which has a
complicated formula, is a well-known insecticide.

~Antidote for arsenical poisoning.~ The most efficient antidote for
arsenic poisoning is ferric hydroxide. It is prepared as needed,
according to the equation

Fe_{2}(SO_{4})_{3} + 3Mg(OH)_{2} = 2Fe(OH)_{3} + 3MgSO_{4}.

~Sulphides of arsenic.~ When hydrogen sulphide is passed into an acidified
solution containing an arsenic compound the arsenic is precipitated as a
bright yellow sulphide, thus:

2H_{3}AsO_{3} + 3H_{2}S = As_{2}S_{3} + 6H_{2}O,

2H_{3}AsO_{4} + 5H_{2}S = As_{2}S_{5} + 8H_{2}O.

In this respect arsenic resembles the metallic elements, many of which
produce sulphides under similar conditions. The sulphides of arsenic,
both those produced artificially and those found in nature, are used as
yellow pigments.

ANTIMONY

~Occurrence.~ Antimony occurs in nature chiefly as the sulphide
(Sb_{2}S_{3}), called stibnite, though it is also found as oxide and as
a constituent of many complex minerals.

~Preparation.~ Antimony is prepared from the sulphide in a very simple
manner. The sulphide is melted with scrap iron in a furnace, when the
iron combines with the sulphur to form a slag, or liquid layer of melted
iron sulphide, while the heavier liquid, antimony, settles to the bottom
and is drawn off from time to time. The reaction involved is represented
by the equation

Sb_{2}S_{3} + 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 deg.) and expands very
noticeably on solidifying.

~Chemical properties.~ In chemical properties antimony resembles arsenic
in many particulars. It forms the oxides Sb_{2}O_{3} and Sb_{2}O_{5},
and in addition Sb_{2}O_{4}. It combines with the halogen elements with
great energy, burning brilliantly in chlorine to form antimony
trichloride (SbCl_{3}). 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~ (SbH_{3}). The gas stibine (SbH_{3}) 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:

2SbCl_{3} + 3H_{2}S = Sb_{2}S_{3} + 6HCl,

2SbCl_{5} + 5H_{2}S = Sb_{2}S_{5} + 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 = SbCl_{3} + 3H_{2}O.

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

ALLOYS

Some metals when melted together thoroughly intermix, and on cooling
form a homogeneous, metallic-appearing substance called an _alloy_. 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 the property 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.

BISMUTH

~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 deg.)
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 Bi_{2}O_{3}. 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. Some of these melt in hot
water. For example, Wood's metal, consisting of bismuth, lead, tin, and
cadmium, melts at 60.5 deg..

~Compounds of bismuth.~ Unlike the other elements of this group, bismuth
has almost no acid properties. Its chief oxide, Bi_{2}O_{3}, is basic in
its properties. It dissolves in strong acids and forms salts of bismuth:

Bi_{2}O_{3} + 6HCl = 2BiCl_{3} + 3H_{2}O,

Bi_{2}O_{3} + 6HNO_{3} = 2Bi(NO_{3})_{3} + 3H_{2}O.

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.

HYDROLYSIS

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
called _hydrolysis_.

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

BiCl_{3} + 3H_{2}O <--> 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

BiCl_{3} + H_{2}O = Bi(OH)Cl_{2} + HCl,

BiCl_{3} + 2H_{2}O = Bi(OH)_{2}Cl + 2HCl,

BiCl_{3} + 3H_{2}O = Bi(OH)_{3} + 3HCl.

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

Bi(OH)_{2}Cl = BiOCl + H_{2}O.

The salt represented in the last equation is sometimes called bismuth
oxychloride, or bismuthyl chloride. The corresponding nitrate,
BiONO_{3}, 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 called _bismuthyl_. Similar basic
salts are formed by the hydrolysis of antimony salts.

EXERCISES

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?

CHAPTER XXI

SILICON, TITANIUM, BORON

=================================================================
| | | | |
| SYMBOL | ATOMIC | DENSITY | CHLORIDES | OXIDES
| | WEIGHT | | |
____________|________|________|_________|___________|____________
| | | | |
Silicon | Si | 28.4 | 2.35 | SiCl_{4} | SiO_{2}
Titanium | Ti | 48.1 | 3.5 | TiCl_{4} | TiO_{2}
Boron | B | 11.0 | 2.45 | BCl_{3} | B_{2}O_{3}
=================================================================

~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.

SILICON

~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 (SiO_{2}), 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:

SiO_{2} + 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
(SiH_{4}) corresponds in formula to methane (CH_{4}), but its properties
are more like those of phosphine (PH_{3}). 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 SiCl_{4} and SiF_{4}. Of these silicon fluoride is the most
familiar and interesting. As stated in the discussion of fluorine, it is
formed when hydrofluoric acid acts upon silicon dioxide or a silicate.
With silica the reaction is thus expressed:

SiO_{2} + 4HF = SiF_{4} + 2H_{2}O.

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

SiF_{4} + 4H_{2}O = 4HF + Si(OH)_{4}.

The hydrofluoric acid so formed combines with an additional amount of
silicon fluoride, forming the complex fluosilicic acid (H_{2}SiF_{6}),
thus:

2HF + SiF_{4} = H_{2}SiF_{6}.

~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 is
_carborundum_, 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

SiO_{2} + 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 graphite G. Surrounding this core
is a layer of crystallized carborundum C, about 16 in. thick. Outside
this is a shell of amorphous carborundum A. The remaining materials
M are unchanged and are used for a new charge.

[Illustration: Fig. 73]

~Silicon dioxide~ (_silica_) (SiO_{2}). 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 heated to a high temperature with their salts,
especially when the silicates so formed are fusible. The following
equations illustrate this property:

Na_{2}CO_{3} + SiO_{2} = Na_{2}SiO_{3} + CO_{2},

Na_{2}SO_{4} + SiO_{2} = Na_{2}SiO_{3} + SO_{3}.

~Silicic acids.~ Silicon forms two simple acids, orthosilicic acid
(H_{4}SiO_{4}) and metasilicic acid (H_{2}SiO_{3}). 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:

H_{4}SiO_{4} = H_{2}SiO_{3} + H_{2}O.

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

H_{2}SiO_{3} = H_{2}O + SiO_{2}.

~Salts of silicic acids,--silicates.~ A number of salts of the
orthosilicic and metasilicic acids occur in nature. Thus mica
(KAlSiO_{4}) 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

3H_{4}SiO_{4} = H_{4}Si_{3}O_{8} + 4H_{2}O.

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 KAlSi_{3}O_{8}, and is a mixed salt of the acid
H_{4}Si_{3}O_{8}, whose formation is represented in the equation above.
Kaolin has the formula Al_{2}Si_{2}O_{7}.2H_{2}O. 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:

Na_{2}CO_{3} + SiO_{2} = Na_{2}SiO_{3} + CO_{2},

CaO + SiO_{2} = CaSiO_{3}.

[Illustration: Fig. 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 glass A on the hollow
rod B is lowered into the mold, which is then closed by the handles
C. 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.

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