An Elementary Study of Chemistry
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William McPherson >> An Elementary Study of Chemistry
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2. _Plastic sulphur._ When boiling sulphur is poured into cold water it
assumes a gummy, doughlike form, which is quite elastic. This can be
seen in a very striking manner by distilling sulphur from a small,
short-necked retort, such as is represented in Fig. 40, and allowing the
liquid to run directly into water. In a few days it becomes quite
brittle and passes over into ordinary rhombic sulphur.
[Illustration Fig. 40]
~Chemical properties of sulphur.~ When sulphur is heated to its kindling
temperature in oxygen or in the air it burns with a pale blue flame,
forming sulphur dioxide (SO_{2}). Small quantities of sulphur trioxide
(SO_{3}) may also be formed in the combustion of sulphur. Most metals
when heated with sulphur combine directly with it, forming metallic
sulphides. In some cases the action is so energetic that the mass
becomes incandescent, as has been seen in the case of iron uniting with
sulphur. This property recalls the action of oxygen upon metals, and in
general the metals which combine readily with oxygen are apt to combine
quite readily with sulphur.
~Uses of sulphur.~ Large quantities of sulphur are used as a germicide in
vineyards, also in the manufacture of gunpowder, matches, vulcanized
rubber, and sulphuric acid.
COMPOUNDS OF SULPHUR WITH HYDROGEN
~Hydrosulphuric acid~ (H_{2}S). This substance is a gas having the
composition expressed by the formula H_{2}S and is commonly called
hydrogen sulphide. It is found in the vapors issuing from volcanoes, and
in solution in the so-called sulphur waters of many springs. It is
formed when organic matter containing sulphur undergoes decay, just as
ammonia is formed under similar circumstances from nitrogenous matter.
~Preparation.~ Hydrosulphuric acid is prepared in the laboratory by
treating a sulphide with an acid. Iron sulphide (FeS) is usually
employed:
FeS + 2HCl = FeCl_{2} + H_{2}S.
A convenient apparatus is shown in Fig. 41. A few lumps of iron sulphide
are placed in the bottle A, and dilute acid is added in small
quantities at a time through the funnel tube B, the gas escaping
through the tube C.
[Illustration: Fig. 41]
~Explanation of the reaction.~ Iron sulphide is a salt of
hydrosulphuric acid, and this reaction is therefore similar to
the one which takes place when sulphuric acid acts upon a
nitrate. In both cases a salt and an acid are brought together,
and there is a tendency for the reaction to go on until a state
of equilibrium is reached. This equilibrium is constantly
disturbed by the escape of the gaseous acid set free, so that
the reaction goes on until all of the original salt has been
decomposed. The two reactions differ in that the first one is
complete at ordinary temperatures, while in the case of
sulphuric acid acting upon sodium nitrate, the reacting
substances must be heated so as to secure a temperature at
which nitric acid is a gas.
~Physical properties.~ Hydrosulphuric acid is a colorless gas, having a
weak, disagreeable taste and an exceedingly offensive odor. It is rather
sparingly soluble in water at ordinary temperatures, about three volumes
dissolving in one of water. In boiling water it is not soluble at all.
In pure form it acts as a violent poison, and even when diluted largely
with air produces headache, dizziness, and nausea. It is a little
heavier than air, having a density of 1.18.
~Chemical properties.~ The most important chemical properties of
hydrosulphuric acid are the following:
1. _Acid properties._ Hydrosulphuric acid is a weak acid. In solution in
water it turns blue litmus red and neutralizes bases, forming salts
called _sulphides_.
2. _Action on oxygen._ The elements composing hydrosulphuric acid have
each a strong affinity for oxygen, and are not held together very
firmly. Consequently the gas burns readily in oxygen or the air,
according to the equation
H_{2}S + 3O = H_{2}O + SO_{2}.
When there is not enough oxygen for both the sulphur and the hydrogen,
the latter element combines with the oxygen and the sulphur is set free:
H_{2}S + O = H_{2}O + S.
3. _Reducing action._ Owing to the ease with which hydrosulphuric acid
decomposes and the strong affinity of both sulphur and hydrogen for
oxygen, the substance is a strong reducing agent, taking oxygen away
from many substances which contain it.
4. _Action on metals._ Hydrosulphuric acid acts towards metals in a way
very similar to water. Thus, when it is passed over heated iron in a
tube, the reaction is represented by the equation
3Fe + 4H_{2}S = Fe_{3}S_{4} + 8H.
Water in the form of steam, under similar circumstances, acts according
to the equation
3Fe + 4H_{2}O = Fe_{3}O_{4} + 8H.
~Salts of hydrosulphuric acid,--sulphides.~ The salts of hydrosulphuric
acid, called sulphides, form an important class of salts. Many of them
are found abundantly in nature, and some of them are important ores.
They will be frequently mentioned in connection with the metals.
Most of the sulphides are insoluble in water, and some of them are
insoluble in acids. Consequently, when hydrosulphuric acid is passed
into a solution of a salt, it often happens that a sulphide is
precipitated. With copper chloride the equation is
CuCl_{2} + H_{2}S = CuS + 2HCl.
Because of the fact that some metals are precipitated in this way as
sulphides while others are not, hydrosulphuric acid is extensively used
in the separation of the metals in the laboratory.
~Explanation of the reaction.~ When hydrosulphuric acid and
copper chloride are brought together in solution, both copper
and sulphur ions are present, and these will come to an
equilibrium, as represented in the equation
Cu^{+} + S^{-} <--> CuS.
Since copper sulphide is almost insoluble in water, as soon as
a very small quantity has formed the solution becomes
supersaturated, and the excess keeps precipitating until nearly
all the copper or sulphur ions have been removed from the
solution. With some other ions, such as iron, the sulphide
formed does not saturate the solution, and no precipitate
results.
OXIDES OF SULPHUR
Sulphur forms two well-known compounds with oxygen: sulphur dioxide
(SO_{2}), sometimes called sulphurous anhydride; and sulphur trioxide
(SO_{3}), frequently called sulphuric anhydride.
~Sulphur dioxide~ (SO_{2}). Sulphur dioxide occurs in nature in the gases
issuing from volcanoes, and in solution in the water of many springs. It
is likely to be found wherever sulphur compounds are undergoing
oxidation.
~Preparation.~ Three general ways may be mentioned for the preparation of
sulphur dioxide:
1. _By the combustion of sulphur._ Sulphur dioxide is readily formed by
the combustion of sulphur in oxygen or the air:
S + 2O = SO_{2}.
It is also formed when substances containing sulphur are burned:
ZnS + 3O = ZnO + SO_{2}.
2. _By the reduction of sulphuric acid._ When concentrated sulphuric
acid is heated with certain metals, such as copper, part of the acid is
changed into copper sulphate, and part is reduced to sulphurous acid.
The latter then decomposes into sulphur dioxide and water, the complete
equation being
Cu + 2H_{2}SO_{4} = CuSO_{4} + SO_{2} + 2H_{2}O.
3. _By the action of an acid on a sulphite._ Sulphites are salts of
sulphurous acid (H_{2}SO_{3}). When a sulphite is treated with an acid,
sulphurous acid is set free, and being very unstable, decomposes into
water and sulphur dioxide. These reactions are expressed in the
equations
Na_{2}SO_{3} + 2HCl = 2NaCl + H_{2}SO_{3},
H_{2}SO_{3} = H_{2}O + SO_{2}.
~Explanation of the reaction.~ In this case we have two reversible
reactions depending on each other. In the first reaction,
(1) Na_{2}SO_{3} + 2HCl <--> 2NaCl + H_{2}SO_{3},
we should expect an equilibrium to result, for none of the four
substances in the equation are insoluble or volatile when water is
present to hold them in solution. But the quantity of the H_{2}SO_{3} is
constantly diminishing, owing to the fact that it decomposes, as
represented in the equation
(2) H_{2}SO_{3} <--> H_{2}O + SO_{2},
and the sulphur dioxide, being a gas, escapes. No equilibrium can
therefore result, since the quantity of the sulphurous acid is
constantly being diminished because of the escape of sulphur dioxide.
~Physical properties.~ Sulphur dioxide is a colorless gas, which at
ordinary temperatures is 2.2 times as heavy as air. It has a peculiar,
irritating odor. The gas is very soluble in water, one volume of water
dissolving eighty of the gas under standard conditions. It is easily
condensed to a colorless liquid, and can be purchased in this condition
stored in strong bottles, such as the one represented in Fig. 42.
[Illustration: Fig. 42]
~Chemical properties.~ Sulphur dioxide has a marked tendency to combine
with other substances, and is therefore an active substance chemically.
It combines with oxygen gas, but not very easily. It can, however, take
oxygen away from some other substances, and is therefore a good reducing
agent. Its most marked chemical property is its ability to combine with
water to form sulphurous acid (H_{2}SO_{3}).
~Sulphurous acid~ (H_{2}SO_{3}). When sulphur dioxide dissolves in water
it combines chemically with it to form sulphurous acid, an unstable
substance having the formula H_{3}SO_{3}. It is impossible to prepare
this acid in pure form, as it breaks down very easily into water and
sulphur dioxide. The reaction is therefore reversible, and is expressed
by the equation
H_{2}O + SO_{2} <--> H_{2}SO_{3}.
Solutions of the acid in water have a number of interesting properties.
1. _Acid properties._ The solution has all the properties typical of an
acid. When neutralized by bases, sulphurous acid yields a series of
salts called _sulphites_.
2. _Reducing properties._ Solutions of sulphurous acid act as good
reducing agents. This is due to the fact that sulphurous acid has the
power of taking up oxygen from the air, or from substances rich in
oxygen, and is changed by this reaction into sulphuric acid:
H_{2}SO_{3} + O = H_{2}SO_{4},
H_{2}SO_{3} + H_{2}O_{2} = H_{2}S0_{4} + H_{2}O.
3. _Bleaching properties._ Sulphurous acid has strong bleaching
properties, acting upon many colored substances in such a way as to
destroy their color. It is on this account used to bleach paper, straw
goods, and even such foods as canned corn.
4. _Antiseptic properties._ Sulphurous acid has marked antiseptic
properties, and on this account has the power of arresting
fermentation. It is therefore used as a preservative.
~Salts of sulphurous acid,--sulphites.~ The sulphites, like sulphurous
acid, have the power of taking up oxygen very readily, and are good
reducing agents. On account of this tendency, commercial sulphites are
often contaminated with sulphates. A great deal of sodium sulphite is
used in the bleaching industry, and as a reagent for softening paper
pulp.
~Sulphur trioxide~ (SO_{3}). When sulphur dioxide and oxygen are heated
together at a rather high temperature, a small amount of sulphur
trioxide (SO_{3}) is formed, but the reaction is slow and incomplete.
If, however, the heating takes place in the presence of very fine
platinum dust, the reaction is rapid and nearly complete.
[Illustration: Fig. 43]
~ Experimental preparation of sulphur trioxide.~ The experiment
can be performed by the use of the apparatus shown in Fig. 43,
the fine platinum being secured by moistening asbestos fiber
with a solution of platinum chloride and igniting it in a
flame. The fiber, covered with fine platinum, is placed in a
tube of hard glass, which is then heated with a burner to about
350 deg., while sulphur dioxide and air are passed into the tube.
Union takes place at once, and the strongly fuming sulphur
trioxide escapes from the jet at the end of the tube, and may
be condensed by surrounding the receiving tube with a freezing
mixture.
~Properties of sulphur trioxide.~ Sulphur trioxide is a colorless liquid,
which solidifies at about 15 deg. and boils at 46 deg.. A trace of moisture
causes it to solidify into a mass of silky white crystals, somewhat
resembling asbestos fiber in appearance. In contact with the air it
fumes strongly, and when thrown upon water it dissolves with a hissing
sound and the liberation of a great deal of heat. The product of this
reaction is sulphuric acid, so that sulphur trioxide is the anhydride of
that acid:
SO_{3} + H_{2}O = H_{2}SO_{4}.
~Catalysis.~ It has been found that many chemical reactions, such as the
union of sulphur dioxide with oxygen, are much influenced by the
presence of substances which do not themselves seem to take a part in
the reaction, and are left apparently unchanged after it has ceased.
These reactions go on very slowly under ordinary circumstances, but are
greatly hastened by the presence of the foreign substance. Substances
which hasten very slow reactions in this way are said to act as
catalytic agents or _catalyzers_, and the action is called _catalysis_.
Just how the action is brought about is not well understood.
DEFINITION: _A catalyzer is a substance which changes the velocity of a
reaction, but does not change its products._
~Examples of Catalysis.~ We have already had several instances of such
action. Oxygen and hydrogen combine with each other at ordinary
temperatures in the presence of platinum powder, while if no catalytic
agent is present they do not combine in appreciable quantities until a
rather high temperature is reached. Potassium chlorate, when heated with
manganese dioxide, gives up its oxygen at a much lower temperature than
when heated alone. Hydrogen dioxide decomposes very rapidly when
powdered manganese dioxide is sifted into its concentrated solution.
On the other hand, the catalytic agent sometimes retards chemical
action. For example, a solution of hydrogen dioxide decomposes more
slowly when it contains a little phosphoric acid than when perfectly
pure. For this reason commercial hydrogen dioxide always contains
phosphoric acid.
Many reactions are brought about by the catalytic action of traces of
water. For example, phosphorus will not burn in oxygen in the absence of
all moisture. Hydrochloric acid will not unite with ammonia if the
reagents are perfectly dry. It is probable that many of the chemical
transformations in physiological processes, such as digestion, are
assisted by certain substances acting as catalytic agents. The principle
of catalysis is therefore very important.
~Sulphuric acid~ (_oil of vitriol_) (H_{2}SO_{4}). Sulphuric acid is one
of the most important of all manufactured chemicals. Not only is it one
of the most common reagents in the laboratory, but enormous quantities
of it are used in many of the industries, especially in the refining of
petroleum, the manufacture of nitroglycerin, sodium carbonate, and
fertilizers.
~Manufacture of sulphuric acid.~ 1. _Contact process_. The reactions
taking place in this process are represented by the following equations:
SO_{2} + O = SO_{3},
SO_{3} + H_{2}O = H_{2}SO_{4}.
To bring about the first of these reactions rapidly, a catalyzer is
employed, and the process is carried out in the following way: Large
iron tubes are packed with some porous material, such as calcium and
magnesium sulphates, which contains a suitable catalytic substance
scattered through it. The catalyzers most used are platinum powder,
vanadium oxide, and iron oxide. Purified sulphur dioxide and air are
passed through the tubes, which are kept at a temperature of about 350 deg..
Sulphur trioxide is formed, and as it issues from the tube it is
absorbed in water or dilute sulphuric acid. The process is continued
until all the water in the absorbing vessel has been changed into
sulphuric acid, so that a very concentrated acid is made in this way. An
excess of the trioxide may dissolve in the strong sulphuric acid,
forming what is known as _fuming sulphuric acid_.
2. _Chamber process._ The method of manufacture exclusively employed
until recent years, and still in very extensive use, is much more
complicated. The reactions are quite involved, but the conversion of
water, sulphur dioxide, and oxygen into sulphuric acid is accomplished
by the catalytic action of oxides of nitrogen. The reactions are brought
about in large lead-lined chambers, into which oxides of nitrogen,
sulphur dioxide, steam, and air are introduced in suitable proportions.
~Reactions of the chamber process.~ In a very general way, the
various reactions which take place in the lead chambers may be
expressed in two equations. In the first reaction sulphur
dioxide, nitrogen peroxide, steam, and oxygen unite, as shown
in the equation
(1) 2SO_{2} + 2NO_{2} + H_{2}O + O = 2SO_{2} (OH) (NO_{2}).
The product formed in this reaction is called nitrosulphuric
acid or "chamber crystals." It actually separates on the walls
of the chambers when the process is not working properly. Under
normal conditions, it is decomposed as fast as it is formed by
the action of excess of steam, as shown in the equation
(2) 2SO_{2} (OH) (NO_{2}) + H_{2}O + O = 2H_{2}SO_{4} + 2NO_{2}.
The nitrogen dioxide formed in this reaction can now enter into
combination with a new quantity of sulphur dioxide, steam, and
oxygen, and the series of reactions go on indefinitely. Many
other reactions occur, but these two illustrate the principle
of the process.
The relation between sulphuric acid and nitrosulphuric acid can be seen
by comparing their structural formulas:
O= -OH O= -OH
S S
O= -OH O= -NO_{2}
The latter may be regarded as derived from the former by the
substitution of the nitro group (NO_{2}) for the hydroxyl group (OH).
[Illustration: Fig. 44]
~The sulphuric acid plant.~ Fig. 44 illustrates the simpler parts of a
plant used in the manufacture of sulphuric acid by the chamber process.
Sulphur or some sulphide, as FeS_{2}, is burned in furnace A. The
resulting sulphur dioxide, together with air and some nitrogen peroxide,
are conducted into the large chambers, the capacity of each chamber
being about 75,000 cu. ft. Steam is also admitted into these chambers at
different points. These compounds react to form sulphuric acid,
according to the equations given above. The nitrogen left after the
withdrawal of the oxygen from the admitted air escapes through the
Gay-Lussac tower X. In order to prevent the escape of the oxides of
nitrogen regenerated in the reaction, the tower is filled with lumps of
coke, over which trickles concentrated sulphuric acid admitted from Y.
The nitrogen peroxide dissolves in the acid and the resulting solution
collects in H. This is pumped into E, where it is mixed with dilute
acid and allowed to trickle down through the chamber D (Glover tower),
which is filled with some acid-resisting rock. Here the nitrogen
peroxide is expelled from the solution by the action of the hot gases
entering from A, and together with them enters the first chamber
again. The acid from which the nitrogen peroxide is expelled collects in
F. Theoretically, a small amount of nitrogen peroxide would suffice to
prepare an unlimited amount of sulphuric acid; practically, some of it
escapes, and this is replaced by small amounts admitted at B.
The sulphuric acid so formed, together with the excess of condensed
steam, collect upon the floor of the chambers in the form of a liquid
containing from 62% to 70% of sulphuric acid. The product is called
_chamber acid_ and is quite impure; but for many purposes, such as the
manufacture of fertilizers, it needs no further treatment. It can be
concentrated by boiling it in vessels made of iron or platinum, which
resist the action of the acid, nearly all the water boiling off. Pure
concentrated acid can be made best by the contact process, while the
chamber process is cheaper for the dilute impure acid.
~Physical properties.~ Sulphuric acid is a colorless, oily liquid, nearly
twice as heavy as water. The ordinary concentrated acid contains about
2% of water, has a density of 1.84, and boils at 338 deg.. It is sometimes
called _oil of vitriol_, since it was formerly made by distilling a
substance called _green vitriol_.
~Chemical properties.~ Sulphuric acid possesses chemical properties which
make it one of the most important of chemical substances.
1. _Action as an acid._ In dilute solution sulphuric acid acts as any
other acid, forming salts with oxides and hydroxides.
2. _Action as an oxidizing agent._ Sulphuric acid contains a large
percentage of oxygen and is, like nitric acid, a very good oxidizing
agent. When the concentrated acid is heated with sulphur, carbon, and
many other substances, oxidation takes place, the sulphuric acid
decomposing according to the equation
H_{2}SO_{4} = H_{2}SO_{3} + O.
3. _Action on metals._ In dilute solution sulphuric acid acts upon many
metals, such as zinc, forming a sulphate and liberating hydrogen. When
the concentrated acid is employed the hydrogen set free is oxidized by a
new portion of the acid, with the liberation of sulphur dioxide. With
copper the reactions are expressed by the equations
(1) Cu + H_{2}SO_{4} = CuSO_{4} + 2H,
(2) H_{2}SO_{4} + 2H = H_{2}SO_{3} + H_{2}O,
(3) H_{2}SO_{3} = H_{2}O + SO_{2}.
By combining these equations the following one is obtained:
Cu + 2H_{2}SO_{4} = CuSO_{4} + SO_{2} + 2H_{2}O.
4. _Action on salts._ We have repeatedly seen that an acid of high
boiling point heated with the salt of some acid of lower boiling point
will drive out the low boiling acid. The boiling point of sulphuric acid
(338 deg.) is higher than that of almost any common acid; hence it is used
largely in the preparation of other acids.
5. _Action on water._ Concentrated sulphuric acid has a very great
affinity for water, and is therefore an effective dehydrating agent.
Gases which have no chemical action upon sulphuric acid can be freed
from water vapor by bubbling them through the strong acid. When the acid
is diluted with water much heat is set free, and care must be taken to
keep the liquid thoroughly stirred during the mixing, and to pour the
acid into the water,--never the reverse.
Not only can sulphuric acid absorb water, but it will often withdraw the
elements hydrogen and oxygen from a compound containing them,
decomposing the compound, and combining with the water so formed. For
this reason most organic substances, such as sugar, wood, cotton, and
woolen fiber, and even flesh, all of which contain much oxygen and
hydrogen in addition to carbon, are charred or burned by the action of
the concentrated acid.
~Salts of sulphuric acid,--sulphates.~ The sulphates form a very important
class of salts, and many of them have commercial uses. Copperas (iron
sulphate), blue vitriol (copper sulphate), and Epsom salt (magnesium
sulphate) serve as examples. Many sulphates are important minerals,
prominent among these being gypsum (calcium sulphate) and barytes
(barium sulphate).
~Thiosulphuric acid~ (H_{2}S_{2}O_{3}); ~Thiosulphates.~ Many other
acids of sulphur containing oxygen are known, but none of them
are of great importance. Most of them cannot be prepared in a
pure state, and are known only through their salts. The most
important of these is thiosulphuric acid.
When sodium sulphite is boiled with sulphur the two substances
combine, forming a salt which has the composition represented
in the formula Na_{2}S_{2}O_{3}:
Na_{2}SO_{3} + S = Na_{2}S_{2}O_{3}.
The substance is called sodium thiosulphate, and is a salt of
the easily decomposed acid H_{2}S_{2}O_{3}, called
thiosulphuric acid. This reaction is quite similar to the
action of oxygen upon sulphites:
Na_{2}SO_{3} + O = Na_{2}SO_{4}.
More commonly the salt is called sodium hyposulphite, or merely
"hypo." It is a white solid and is extensively used in
photography, in the bleaching industry, and as a disinfectant.
~Monobasic and dibasic acids.~ Such acids as hydrochloric and nitric
acids, which have only one replaceable hydrogen atom in the molecule, or
in other words yield one hydrogen ion in solution, are called monobasic
acids. Acids yielding two hydrogen ions in solution are called dibasic
acids. Similarly, we may have tribasic and tetrabasic acids. The three
acids of sulphur are dibasic acids. It is therefore possible for each of
them to form both normal and acid salts. The acid salts can be made in
two ways: the acid may be treated with only half enough base to
neutralize it,--
NaOH + H_{2}SO_{4} = NaHSO_{4} + H_{2}O;
or a normal salt may be treated with the free acid,--
Na_{2}SO_{4} + H_{2}SO_{4} = 2NaHSO_{4}.
Acid sulphites and sulphides may be made in the same ways.
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