An Elementary Study of Chemistry

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~Hydrobromic acid (HBr).~ When sulphuric acid acts upon a bromide
hydrobromic acid is set free:

2NaBr + H_{2}SO_{4} = Na_{2}SO_{4} + 2HBr.

At the same time some bromine is set free, as may be seen from the red
fumes which appear, and from the odor. The explanation of this is found
in the fact that hydrobromic acid is much less stable than hydrochloric
acid, and is therefore more easily oxidized. Concentrated sulphuric acid
is a good oxidizing agent, and oxidizes a part of the hydrobromic acid,
liberating bromine:

H_{2}SO_{4} + 2HBr = 2H_{2}O + SO_{2} + 2Br.

~Preparation of pure hydrobromic acid.~ A convenient way to make
pure hydrobromic acid is by the action of bromine upon moist
red phosphorus. This can be done with the apparatus shown in
Fig. 56. Bromine is put into the dropping funnel A, and red
phosphorus, together with enough water to cover it, is placed
in the flask B. By means of the stopcock the bromine is
allowed to flow drop by drop into the flask, the reaction
taking place without the application of heat. The equations are

(1) P + 3Br = PBr_{3},

(2) PBr_{3} + 3H_{2}O = P(OH)_{3} + 3HBr.

[Illustration Fig. 56]

The U-tube C contains glass beads which have been moistened
with water and rubbed in red phosphorus. Any bromine escaping
action in the flask acts upon the phosphorus in the U-tube. The
hydrobromic acid is collected in the same way as hydrochloric
acid.

~Properties.~ Hydrobromic acid very strikingly resembles hydrochloric acid
in physical and chemical properties. It is a colorless, strongly fuming
gas, heavier than hydrochloric acid and, like it, is very soluble in
water. Under standard conditions 1 volume of water dissolves 610 volumes
of the gas. Chemically, the chief point in which it differs from
hydrochloric acid is in the fact that it is much more easily oxidized,
so that bromine is more readily set free from it than chlorine is from
hydrochloric acid.

~Salts of hydrobromic acid,--bromides.~ The bromides are very similar to
the chlorides in their properties. Chlorine acts upon both bromides and
free hydrobromic acid, liberating bromine from them:

KBr + Cl = KCl + Br,

HBr + Cl = HCl + Br.

Silver bromide is extensively used in photography, and the bromides of
sodium and potassium are used as drugs.

~Oxygen compounds.~ No oxides of bromine are surely known, and
bromine does not form so many oxygen acids as chlorine does.
Salts of hypobromous acid (HBrO) and bromic acid (HBrO_{3}) are
known.

IODINE

~Historical.~ Iodine was discovered in 1812 by Courtois in the ashes of
certain sea plants. Its presence was revealed by its beautiful violet
vapor, and this suggested the name iodine (from the Greek for violet
appearance).

~Occurrence.~ In the combined state iodine occurs in very small quantities
in sea water, from which it is absorbed by certain sea plants, so that
it is found in their ashes. It occurs along with bromine in salt springs
and beds, and is also found in Chili saltpeter.

~Preparation.~ Iodine may be prepared in a number of ways, the principal
methods being the following:

1. _Laboratory method._ Iodine can readily be prepared in the laboratory
from an iodide by the method used in preparing bromine, except that
sodium iodide is substituted for sodium bromide. It can also be made by
passing chlorine into a solution of an iodide.

[Illustration: Fig. 57]

2. _Commercial method._ Commercially iodine was formerly prepared from
seaweed (kelp), but is now obtained almost entirely from the deposits of
Chili saltpeter. The crude saltpeter is dissolved in water and the
solution evaporated until the saltpeter crystallizes. The remaining
liquors, known as the "mother liquors," contain sodium iodate
(NaIO_{3}), in which form the iodine is present in the saltpeter. The
chemical reaction by which the iodine is liberated from this compound is
a complicated one, depending on the fact that sulphurous acid acts upon
iodic acid, setting iodine free. This reaction is shown as follows:

2HIO_{3} + 5H_{2}SO_{3} = 5H_{2}SO_{4} + H_{2}O + 2I.

~Purification of iodine.~ Iodine can be purified very
conveniently in the following way. The crude iodine is placed
in an evaporating dish E (Fig. 57), and the dish is set upon
the sand bath S. The iodine is covered with the inverted
funnel F, and the sand bath is gently heated with a Bunsen
burner. As the dish becomes warm the iodine rapidly evaporates
and condenses again on the cold surface of the funnel in
shining crystals.

This process, in which a solid is converted into a vapor and is
again condensed into a solid without passing through the liquid
state, is called _sublimation_.

~Physical properties.~ Iodine is a purplish-black, shining, heavy solid
which crystallizes in brilliant plates. Even at ordinary temperatures it
gives off a beautiful violet vapor, which increases in amount as heat is
applied. It melts at 107 deg. and boils at 175 deg.. It is slightly soluble in
water, but readily dissolves in alcohol, forming a brown solution
(tincture of iodine), and in carbon disulphide, forming a violet
solution. The element has a strong, unpleasant odor, though by no means
as irritating as that of chlorine and bromine.

~Chemical properties.~ Chemically iodine is quite similar to chlorine and
bromine, but is still less active than bromine. It combines directly
with many elements at ordinary temperatures. At elevated temperatures it
combines with hydrogen, but the reaction is reversible and the compound
formed is quite easily decomposed. Both chlorine and bromine displace it
from its salts:

KI + Br = KBr + I,

KI + Cl = KCl + I.

When even minute traces of iodine are added to thin starch paste a very
intense blue color develops, and this reaction forms a delicate test for
iodine. Iodine is extensively used in medicine, especially in the form
of a tincture. It is also largely used in the preparation of dyes and
organic drugs, iodoform, a substance used as an antiseptic, has the
formula CHI_{3}.

~Hydriodic acid (HI).~ This acid cannot be prepared in pure condition by
the action of sulphuric acid upon an iodide, since the hydriodic acid
set free is oxidized by the sulphuric acid just as in the case of
hydrobromic acid, but to a much greater extent. It can be prepared in
exactly the same way as hydrobromic acid, iodine being substituted for
bromine. It can also be prepared by passing hydrosulphuric acid into
water in which iodine is suspended. The equation is

H_{2}S + 2I = 2HI + S.

The hydriodic acid formed in this way dissolves in the water.

~Properties and uses.~ Hydriodic acid resembles the corresponding acids of
chlorine and bromine in physical properties, being a strongly fuming,
colorless gas, readily soluble in water. Under standard conditions 1
volume of water dissolves about 460 volumes of the gas. It is, however,
more unstable than either hydrochloric or hydrobromic acids, and on
exposure to the air it gradually decomposes in accordance with the
equation

2HI + O = H_{2}O + 2I.

Owing to the slight affinity between iodine and hydrogen the acid easily
gives up its hydrogen and is therefore a strong reducing agent. This is
seen in its action on sulphuric acid.

The salts of hydriodic acid, the iodides, are, in general, similar to
the chlorides and bromides. Potassium iodide (KI) is the most familiar
of the iodides and is largely used in medicine.

~Oxygen compounds.~ Iodine has a much greater affinity for oxygen
than has either chlorine or bromine. When heated with nitric
acid it forms a stable oxide (I_{2}O_{5}). Salts of iodic acid
(HIO_{3}) and periodic acid (HIO_{4}) are easily prepared, and
the free acids are much more stable than the corresponding
acids of the other members of this family.

GAY-LUSSAC'S LAW OF VOLUMES

In the discussion of the composition of hydrochloric acid it was stated
that one volume of hydrogen combines with one volume of chlorine to form
two volumes of hydrochloric acid. With bromine and iodine similar
combining ratios hold good. These facts recall the simple volume
relations already noted in the study of the composition of steam and
ammonia. These relations may be represented graphically in the following
way:

+---+ +----+ +------+ +------+
| H | + | Cl | = | H Cl | + | H Cl |
+---+ +----+ +------+ +------+

+---+ +---+ +---+ +--------+ +--------+
| H | | H | + | O | = | H_{2}O | + | H_{2}O |
+---+ +---+ +---+ +--------+ +--------+

+---+ +---+ +---+ +---+ +--------+ +--------+
| H | | H | | H | + | N | = | NH_{3} | + | NH_{3} |
+---+ +---+ +---+ +---+ +--------+ +--------+

In the early part of the past century Gay-Lussac, a distinguished French
chemist, studied the volume relations of many combining gases, and
concluded that similar relations always hold. His observations are
summed up in the following law: _When two gases combine chemically there
is always a simple ratio between their volumes, and between the volume
of either one of them and that of the product, provided it is a gas._ By
a simple ratio is meant of course the ratio of small whole numbers, as
1 : 2, 2 : 3.

EXERCISES

1. How do we account for the fact that liquid hydrofluoric acid is not
an electrolyte?

2. Why does sulphuric acid liberate hydrofluoric acid from its salts?

3. In the preparation of chlorine, what advantages are there in treating
manganese dioxide with a mixture of sodium chloride and sulphuric acid
rather than with hydrochloric acid?

4. Why must chlorine water be kept in the dark?

5. What is the derivation of the word nascent?

6. What substances studied are used as bleaching agents? To what is the
bleaching action due in each case?

7. What substances studied are used as disinfecting agents?

8. What is meant by the statement that hydrochloric acid is one of the
strongest acids?

9. What is the meaning of the phrase _aqua regia_?

10. Cl_{2}O is the anhydride of what acid?

11. A solution of hydriodic acid on standing turns brown. How is this
accounted for?

12. How can bromine vapor and nitrogen peroxide be distinguished from
each other?

13. Write the equations for the reaction taking place when hydriodic
acid is prepared from iodine, phosphorus, and water.

14. From their behavior toward sulphuric acid, to what class of agents
do hydrobromic and hydriodic acids belong?

15. Give the derivation of the names of the elements of the chlorine
family.

16. Write the names and formulas for the binary acids of the group in
the order of the stability of the acids.

17. What is formed when a metal dissolves in each of the following?
nitric acid; dilute sulphuric acid; concentrated sulphuric acid;
hydrochloric acid; aqua regia.

18. How could you distinguish between a chloride, a bromide, and an
iodide?

19. What weight of sodium chloride is necessary to prepare sufficient
hydrochloric acid to saturate 1 l. of water under standard conditions?

20. On decomposition 100 l. of hydrochloric acid would yield how many
liters of hydrogen and chlorine respectively, the gases being measured
under the same conditions? Are your results in accord with the
experimental facts?

CHAPTER XVII

CARBON AND SOME OF ITS SIMPLER COMPOUNDS

~The family.~ Carbon stands at the head of a family of elements in the
fourth group in the periodic table. The resemblances between the
elements of this family, while quite marked, are not so striking as in
the case of the elements of the chlorine family. With the exception of
carbon, these elements are comparatively rare, and need not be taken up
in detail in this chapter. Titanium will be referred to again in
connection with silicon which it very closely resembles.

~Occurrence.~ Carbon is found in nature in the uncombined state in several
forms. The diamond is practically pure carbon, while graphite and coal
are largely carbon, but contain small amounts of other substances. Its
natural compounds are exceedingly numerous and occur as gases, liquids,
and solids. Carbon dioxide is its most familiar gaseous compound.
Natural gas and petroleum are largely compounds of carbon with hydrogen.
The carbonates, especially calcium carbonate, constitute great strata of
rocks, and are found in almost every locality. All living organisms,
both plant and animal, contain a large percentage of this element, and
the number of its compounds which go to make up all the vast variety of
animate nature is almost limitless. Over one hundred thousand definite
compounds containing carbon have been prepared. In the free state carbon
occurs in three allotropic forms, two of which are crystalline and one
amorphous.

~Crystalline carbon.~ Crystalline carbon occurs in two forms,--diamond and
graphite.

1. _Diamond._ Diamonds are found in considerable quantities in several
localities, especially in South Africa, the East Indies, and Brazil. The
crystals belong to the regular system, but the natural stones do not
show this very clearly. When found they are usually covered with a rough
coating which is removed in the process of cutting. Diamond cutting is
carried on most extensively in Holland.

The density of the diamond is 3.5, and, though brittle, it is one of the
hardest of substances. Black diamonds, as well as broken and imperfect
stones which are valueless as gems, are used for grinding hard
substances. Few chemical reagents have any action on the diamond, but
when heated in oxygen or the air it blackens and burns, forming carbon
dioxide.

Lavoisier first showed that carbon dioxide is formed by the combustion
of the diamond; and Sir Humphry Davy in 1814 showed that this is the
only product of combustion, and that the diamond is pure carbon.

~The diamond as a gem.~ The pure diamond is perfectly transparent
and colorless, but many are tinted a variety of colors by
traces of foreign substances. Usually the colorless ones are
the most highly prized, although in some instances the color
adds to the value; thus the famous Hope diamond is a beautiful
blue. Light passing through a diamond is very much refracted,
and to this fact the stone owes its brilliancy and sparkle.

~Artificial preparation of diamonds.~ Many attempts have been
made to produce diamonds artificially, but for a long time
these always ended in failure, graphite and not diamonds being
the product obtained. The French chemist Moissan, in his
extended study of chemistry at high temperatures, finally
succeeded (1893) in making some small ones. He accomplished
this by dissolving carbon in boiling iron and plunging the
crucible containing the mixture into water, as shown in Fig.
58. Under these conditions the carbon crystallized in the iron
in the form of the diamond. The diamonds were then obtained by
dissolving away the iron in hydrochloric acid.

[Illustration: Fig. 58]

2. _Graphite._ This form of carbon is found in large quantities,
especially in Ceylon, Siberia, and in some localities of the United
States and Canada. It is a shining black substance, very soft and greasy
to the touch. Its density is about 2.15. It varies somewhat in
properties according to the locality in which it is found, and is more
easily attacked by reagents than is the diamond. It is also manufactured
by heating carbon with a small amount of iron (3%) in an electric
furnace. It is used in the manufacture of lead pencils and crucibles, as
a lubricant, and as a protective covering for iron in the form of a
polish or a paint.

~Amorphous carbon.~ Although there are many varieties of amorphous carbon
known, they are not true allotropic modifications. They differ merely in
their degree of purity, their fineness of division, and in their mode of
preparation. These substances are of the greatest importance, owing to
their many uses in the arts and industries. As they occur in nature, or
are made artificially, they are nearly all impure carbon, the impurity
depending on the particular substance in question.

1. _Pure carbon._ Pure amorphous carbon is best prepared by charring
sugar. This is a substance consisting of carbon, hydrogen, and oxygen,
the latter two elements being present in the ratio of one oxygen atom to
two of hydrogen. When sugar is strongly heated the oxygen and hydrogen
are driven off in the form of water and pure carbon is left behind.
Prepared in this way it is a soft, lustrous, very bulky, black powder.

2. _Coal and coke._ Coals of various kinds were probably formed from
vast accumulations of vegetable matter in former ages, which became
covered over with earthy material and were thus protected from rapid
decay. Under various natural agencies the organic matter was slowly
changed into coal. In anthracite these changes have gone the farthest,
and this variety of coal is nearly pure carbon. Soft or bituminous coals
contain considerable organic matter besides carbon and mineral
substances. When heated strongly out of contact with air the organic
matter is decomposed and the resulting volatile matter is driven off in
the form of gases and vapors, and only the mineral matter and carbon
remain behind. The gaseous product is chiefly illuminating gas and the
solid residue is _coke_. Some of the coke is found as a dense cake on
the sides and roof of the retort. This is called retort carbon and is
quite pure.

3. _Charcoal._ This is prepared from wood in the same way that coke is
made from coal. When the process is carried on in retorts the products
expelled by the heat are saved. Among these are many valuable substances
such as wood alcohol and acetic acid. Where timber is abundant the
process is carried out in a wasteful way, by merely covering piles of
wood with sod and setting the wood on fire. Some wood burns and the heat
from this decomposes the wood not burned, forming charcoal from it. The
charcoal, of course, contains the mineral part of the wood from which it
is formed.

4. _Bone black._ This is sometimes called animal charcoal, and is made
by charring bones and animal refuse. The organic part of the materials
is thus decomposed and carbon is left in a very finely divided state,
scattered through the mineral part which consists largely of calcium
phosphate. For some uses this mineral part is removed by treatment with
hydrochloric acid and prolonged washing.

5. _Lampblack._ Lampblack and soot are products of imperfect combustion
of oil and coal, and are deposited from a smoky flame on a cold surface.
The carbon in this form is very finely divided and usually contains
various oily materials.

~Properties.~ While the various forms of carbon differ in many properties,
especially in color and hardness, yet they are all odorless, tasteless
solids, insoluble in water and characterized by their stability towards
heat. Only in the intense heat of the electric arc does carbon
volatilize, passing directly from the solid state into a vapor. Owing to
this fact the inside surface of an incandescent light bulb after being
used for some time becomes coated with a dark film of carbon. It is not
acted on at ordinary temperatures by most reagents, but at a higher
temperature it combines directly with many of the elements, forming
compounds called _carbides_. When heated in the presence of sufficient
oxygen it burns, forming carbon dioxide.

~Uses of carbon.~ The chief use of amorphous carbon is for fuel to furnish
heat and power for all the uses of civilization. An enormous quantity of
carbon in the form of the purer coals, coke, and charcoal is used as a
reducing agent in the manufacture of the various metals, especially in
the metallurgy of iron. Most of the metals are found in nature as
oxides, or in forms which can readily be converted into oxides. When
these oxides are heated with carbon the oxygen is abstracted, leaving
the metal. Retort carbon and coke are used to make electric light
carbons and battery plates, while lampblack is used for indelible inks,
printer's ink, and black varnishes. Bone black and charcoal have the
property of absorbing large volumes of certain gases, as well as smaller
amounts of organic matter; hence they are used in filters to remove
noxious gases and objectionable colors and odors from water. Bone black
is used extensively in the sugar refineries to remove coloring matter
from the impure sugars.

~Chemistry of carbon compounds.~ Carbon is remarkable for the very large
number of compounds which it forms with the other elements, especially
with oxygen and hydrogen. Compounds containing carbon are more numerous
than all others put together, and the chemistry of these substances
presents peculiarities not met with in the study of other substances.
For these reasons the systematic study of carbon compounds, or of
_organic chemistry_ as it is usually called, must be deferred until the
student has gained some knowledge of the chemistry of other elements. An
acquaintance with a few of the most familiar carbon compounds is,
however, essential for the understanding of the general principles of
chemistry.

~Compounds of carbon with hydrogen,--the hydrocarbons.~ Carbon unites with
hydrogen to form a very large number of compounds called _hydrocarbons_.
Petroleum and natural gas are essentially mixtures of a great variety of
these hydrocarbons. Many others are found in living plants, and still
others are produced by the decay of organic matter in the absence of
air. Only two of them, methane and acetylene, will be discussed here.

~Methane~ (_marsh gas_) (CH_{4}). This is one of the most important of
these hydrocarbons, and constitutes about nine tenths of natural gas. As
its name suggests, it is formed in marshes by the decay of vegetable
matter under water, and bubbles of the gas are often seen to rise when
the dead leaves on the bottom of pools are stirred. It also collects in
mines, and, when mixed with air, is called _fire damp_ by the miners
because of its great inflammability, damp being an old name for a gas.
It is formed when organic matter, such as coal or wood, is heated in
closed vessels, and is therefore a principal constituent of coal gas.

~Preparation.~ Methane is prepared in the laboratory by heating sodium or
calcium acetate with soda-lime. Equal weights of fused sodium acetate
and soda-lime are thoroughly dried, then mixed and placed in a
good-sized, hard-glass test tube fitted with a one-holed stopper and
delivery tube. The mixture is gradually heated, and when the air has
been displaced from the tube the gas is collected in bottles by
displacement of water. Soda-lime is a mixture of sodium and calcium
hydroxides. Regarding it as sodium hydroxide alone, the equation is

NaC_{2}H_{3}O_{2} + NaOH = Na_{2}CO_{3} + CH_{4}.

~Properties.~ Methane is a colorless, odorless gas whose density is 0.55.
It is difficult to liquefy, boiling at -155 deg. under standard pressure,
and is almost insoluble in water. It burns with a pale blue flame,
liberating much heat, and when mixed with oxygen is very explosive.

~Davy's safety lamp.~ In 1815 Sir Humphry Davy invented a lamp for the use
of miners, to prevent the dreadful mine explosions then common, due to
methane mixed with air. The invention consisted in surrounding the upper
part of the common miner's lamp with a mantle of wire gauze and the
lower part with glass (Fig. 59). It has been seen that two gases will
not combine until raised to their kindling temperature, and if while
combining they are cooled below this point, the combination ceases. A
flame will not pass through a wire gauze because the metal, being a good
conductor of heat, takes away so much heat from the flame that the gases
are cooled below the kindling temperature. When a lamp so protected is
brought into an explosive mixture the gases inside the wire mantle burn
in a series of little explosions, giving warning to the miner that the
air is unsafe.

[Illustration: Fig. 59]

~Acetylene~ (C_{2}H_{2}). This is a colorless gas usually having a
disagreeable odor due to impurities. It is now made in large quantities
from calcium carbide (CaC_{2}). This substance is formed when coal and
lime are heated together in an electric furnace. When treated with water
the carbide is decomposed, yielding acetylene:

CaC_{2} + 2H_{2}O = C_{2}H_{2} + Ca(OH)_{2}.

Under ordinary conditions the gas burns with a very smoky flame; in
burners constructed so as to secure a large amount of oxygen it burns
with a very brilliant white light, and hence is used as an illuminant.

~Laboratory preparation.~ The gas can be prepared readily in a generator
such as is shown in Fig. 60. The inner tube contains fragments of
calcium carbide, while the outer one is filled with water. As long as
the stopcock is closed the water cannot rise in the inner tube. When the
stopcock is open the water rises, and, coming into contact with the
carbide in the inner tube, generates acetylene. This escapes through the
stopcock, and after the air has been expelled may be lighted as it
issues from the burner.

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