An Elementary Study of Chemistry
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William McPherson >> An Elementary Study of Chemistry
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3O_{2} = 2O_{3}.
~Other methods of determining molecular weights.~ It will be noticed that
Avogadro's law gives us a method by which we can determine the relative
weights of the molecules of two gases because it enables us to tell when
we are dealing with an equal number of the two kinds of molecules. If by
any other means we can get this information, we can make use of the
knowledge so gained to determine the molecular weights of the two
substances.
~Raoult's laws.~ Two laws have been discovered which give us just such
information. They are known as Raoult's laws, and can be stated as
follows:
1. _When weights of substances which are proportional to their molecular
weights are dissolved in the same weight of solvent, the rise of the
boiling point is the same in each case._
2. _When weights of substances which are proportional to their molecular
weights are dissolved in the same weight of solvent, the lowering of the
freezing point is the same in each case._
By taking advantage of these laws it is possible to determine when two
solutions contain the same number of molecules of two dissolved
substances, and consequently the relative molecular weights of the two
substances.
~Law of Dulong and Petit.~ In 1819 Dulong and Petit discovered a very
interesting relation between the atomic weight of an element and its
specific heat, which holds true for elements in the solid state. If
equal weights of two solids, say, lead and silver, are heated through
the same range of temperature, as from 10 deg. to 20 deg., it is found that very
different amounts of heat are required. The amount of heat required to
change the temperature of a solid or a liquid by a definite amount
compared with the amount required to change the temperature of an equal
weight of water by the same amount is called its specific heat. Dulong
and Petit discovered the following law: _The specific heat of an element
in the solid form multiplied by its atomic weight is approximately equal
to the constant 6.25._ That is,
at. wt. x sp. ht. = 6.25.
Consequently,
6.25
at. wt. = --------
sp. ht.
This law is not very accurate, but it is often possible by means of it
to decide upon what multiple of the equivalent is the real atomic
weight. Thus the specific heat of iron is found by experiment to be
0.112, and its equivalent is 27.95. 6.25 / 0.112 = 55.8. We see,
therefore, that the atomic weight is twice the equivalent, or 55.9.
~How formulas are determined.~ It will be well in connection with
molecular weights to consider how the formula of a compound is decided
upon, for the two subjects are very closely associated. Some examples
will make clear the method followed.
The molecular weight of a substance containing hydrogen and chlorine was
36.4. By analysis 36.4 parts of the substance was found to contain 1
part of hydrogen and 35.4 parts of chlorine. As these are the simple
atomic weights of the two elements, the formula of the compound must be
HCl.
A substance consisting of oxygen and hydrogen was found to have a
molecular weight of 34. Analysis showed that in 34 parts of the
substance there were 2 parts of hydrogen and 32 parts of oxygen.
Dividing these figures by the atomic weights of the two elements, we get
2 / 1 = 2 for H; 32 / 16 = 2 for O. The formula is therefore H_{2}O_{2}.
A substance containing 2.04% H, 32.6% S, and 65.3% O was found to have a
molecular weight of 98. In these 98 parts of the substance there are 98
x 2.04% = 2 parts of H, 98 x 32.6% = 32 parts of S, and 98 x 65.3% = 64
parts of O. If the molecule weighs 98, the hydrogen atoms present must
together weigh 2, the sulphur atoms 32, and the oxygen atoms 64.
Dividing these figures by the respective atomic weights of the three
elements, we have, for H, 2 / 1 = 2 atoms; for S, 32 / 32 = 1 atom; for
O, 64 / 16 = 4 atoms. Hence the formula is H_{2}SO_{4}.
We have, then, this general procedure: Find the percentage composition
of the substance and also its molecular weight. Multiply the molecular
weight successively by the percentage of each element present, to find
the amount of the element in the molecular weight of the compound. The
figures so obtained will be the respective parts of the molecular weight
due to the several atoms. Divide by the atomic weights of the respective
elements, and the quotient will be the number of atoms present.
~Avogadro's hypothesis and chemical calculations.~ This law simplifies
many chemical calculations.
1. _Application to volume relations in gaseous reactions._ Since equal
volumes of gases contain an equal number of molecules, it follows that
when an equal number of gaseous molecules of two or more gases take part
in a reaction, the reaction will involve equal volumes of the gases. In
the equation
C_{2}H_{2}O_{4} = H_{2}O + CO_{2} + CO,
since 1 molecule of each of the gases CO_{2} and CO is set free from
each molecule of oxalic acid, the two substances must always be set free
in equal volumes.
Acetylene burns in accordance with the equation
2C_{2}H_{2} + 5O_{2} = 4CO_{2} + 2H_{2}O.
Hence 2 volumes of acetylene will react with 5 volumes of oxygen to form
4 volumes of carbon dioxide and 2 volumes of steam. That the volume
relations may be correct a gaseous element must be given its molecular
formula. Thus oxygen must be written O_{2} and not 2O.
2. _Application to weights of gases._ It will be recalled that the
molecular weight of a gas is determined by ascertaining the weight of
22.4 l. of the gas. This weight in grams is called the _gram-molecular
weight_ of a gas. If the molecular weight of any gas is known, the
weight of a liter of the gas under standard conditions may be determined
by dividing its gram-molecular weight by 22.4. Thus the gram-molecular
weight of a hydrochloric acid gas is 36.458. A liter of the gas will
therefore weigh 36.458 / 22.4 = 1.627 g.
EXERCISES
1. From the following data calculate the atomic weight of sulphur. The
equivalent, as obtained by an analysis of sulphur dioxide, is 16.03. The
densities and compositions of a number of compounds containing sulphur
are as follows:
NAME DENSITY COMPOSITION BY PERCENTAGE
Hydrosulphuric acid 1.1791 S = 94.11 H = 5.89
Sulphur dioxide 2.222 S = 50.05 O = 49.95
Sulphur trioxide 2.74 S = 40.05 O = 59.95
Sulphur chloride 4.70 S = 47.48 Cl = 52.52
Sulphuryl chloride 4.64 S = 23.75 Cl = 52.53 O = 23.70
Carbon disulphide 2.68 S = 84.24 C = 15.76
2. Calculate the formulas for compounds of the following compositions:
MOLECULAR
WEIGHT
(1) S = 39.07% O = 58.49% H = 2.44% 81.0
(2) Ca = 29.40 S = 23.56 O = 47.04 136.2
(3) K = 38.67 N = 13.88 O = 47.45 101.2
3. The molecular weight of ammonia is 17.06; of sulphur dioxide is
64.06; of chlorine is 70.9. From the molecular weight calculate the
weight of 1 l. of each of these gases. Compare your results with the
table on the back cover of the book.
4. From the molecular weight of the same gases calculate the density of
each, referred to air as a standard.
5. A mixture of 50 cc. of carbon monoxide and 50 cc. of oxygen was
exploded in a eudiometer, (a) What gases remained in the tube after
the explosion? (b) What was the volume of each?
6. In what proportion must acetylene and oxygen be mixed to produce the
greatest explosion?
7. Solve Problem 18, Chapter XVII, without using molecular weights.
Compare your results.
8. Solve Problem 10, Chapter XVIII, without using molecular weights.
Compare your results.
9. The specific heat of aluminium is 0.214; of lead is 0.031. From these
specific heats calculate the atomic weights of each of the elements.
CHAPTER XX
THE PHOSPHORUS FAMILY
==================================================
| | ATOMIC | | MELTING
| SYMBOL | WEIGHT | DENSITY | POINT
-----------+--------+---------+---------+---------
Phosphorus | P | 31.0 | 1.8 | 43.3 deg.
Arsenic | As | 75.0 | 5.73 | ---
Antimony | Sb | 120.2 | 6.7 | 432 deg.
Bismuth | Bi | 208.5 | 9.8 | 270 deg.
==================================================
~The family.~ The elements constituting this family belong in the same
group with nitrogen and therefore resemble it in a general way. They
exhibit a regular gradation of physical properties, as is shown in the
above table. The same general gradation is also found in their chemical
properties, phosphorus being an acid-forming element, while bismuth is
essentially a metal. The other two elements are intermediate in
properties.
~Compounds.~ In general the elements of the family form compounds having
similar composition, as is shown in the following table:
PH_{3} PCl_{3} PCl_{5} P_{2}O_{3} P_{2}O_{5}
AsH_{3} AsCl_{3} AsCl_{5} As_{2}O_{3} As_{2}O_{5}
SbH_{3} SbCl_{3} SbCl_{5} Sb_{2}O_{3} Sb_{2}O_{5}
.... BiCl_{3} BiCl_{5} Bi_{2}O_{3} Bi_{2}O_{5}
In the case of phosphorus, arsenic, and antimony the oxides are acid
anhydrides. Salts of at least four acids of each of these three elements
are known, the free acid in some instances being unstable. The relation
of these acids to the corresponding anhydrides may be illustrated as
follows, phosphorus being taken as an example:
P_{2}O_{3} + 3H_{2}O = 2H_{3}PO_{3} (phosphorous acid).
P_{2}O_{5} + 3H_{2}O = 2H_{3}PO_{4} (phosphoric acid).
P_{2}O_{5} + 2H_{2}O = H_{4}P_{2}O_{7} (pyrophosphoric acid).
P_{2}O_{5} + H_{2}O = 2HPO_{3} (metaphosphoric acid).
PHOSPHORUS
~History.~ The element phosphorus was discovered by the alchemist Brand,
of Hamburg, in 1669, while searching for the philosopher's stone. Owing
to its peculiar properties and the secrecy which was maintained about
its preparation, it remained a very rare and costly substance until the
demand for it in the manufacture of matches brought about its production
on a large scale.
~Occurrence.~ Owing to its great chemical activity phosphorus never occurs
free in nature. In the form of phosphates it is very abundant and widely
distributed. _Phosphorite_ and _sombrerite_ are mineral forms of calcium
phosphate, while _apatite_ consists of calcium phosphate together with
calcium fluoride or chloride. These minerals form very large deposits
and are extensively mined for use as fertilizers. Calcium phosphate is a
constituent of all fertile soil, having been supplied to the soil by the
disintegration of rocks containing it. It is the chief mineral
constituent of bones of animals, and bone ash is therefore nearly pure
calcium phosphate.
~Preparation.~ Phosphorus is now manufactured from bone ash or a pure
mineral phosphate by heating the phosphate with sand and carbon in an
electric furnace. The materials are fed in at M (Fig. 70) by the feed
screw F. The phosphorus vapor escapes at P and is condensed under
water, while the calcium silicate is tapped off as a liquid at S. The
phosphorus obtained in this way is quite impure, and is purified by
distillation.
[Illustration: Fig. 70]
~Explanation of the reaction.~ To understand the reaction which
occurs, it must be remembered that a volatile acid anhydride is
expelled from its salts when heated with an anhydride which is
not volatile. Thus, when sodium carbonate and silicon dioxide
are heated together the following reaction takes place:
Na_{2}CO_{3} + SiO_{2} = Na_{2}SiO_{3} + CO_{2}.
Silicon dioxide is a less volatile anhydride than phosphoric
anhydride (P_{2}O_{5}), and when strongly heated with a
phosphate the phosphoric anhydride is driven out, thus:
Ca_{3}(PO_{4})_{2} + 3SiO_{2} = 3CaSiO_{3} + P_{2}O_{5}.
If carbon is added before the heat is applied, the P_{2}O_{5}
is reduced to phosphorus at the same time, according to the
equation
P_{2}O_{5} + 5C = 2P + 5CO.
~Physical properties.~ The purified phosphorus is a pale yellowish,
translucent, waxy solid which melts at 43.3 deg. and boils at 269 deg.. It can
therefore be cast into any convenient form under warm water, and is
usually sold in the market in the form of sticks. It is quite soft and
can be easily cut with a knife, but this must always be done while the
element is covered with water, since it is extremely inflammable, and
the friction of the knife blade is almost sure to set it on fire if cut
in the air. It is not soluble in water, but is freely soluble in some
other liquids, notably in carbon disulphide. Its density is 1.8.
~Chemical properties.~ Exposed to the air phosphorus slowly combines with
oxygen, and in so doing emits a pale light, or phosphorescence, which
can be seen only in a dark place. The heat of the room may easily raise
the temperature to the kindling point of phosphorus, when it burns with
a sputtering flame, giving off dense fumes of oxide of phosphorus. It
burns with dazzling brilliancy in oxygen, and combines directly with
many other elements, especially with sulphur and the halogens. On
account of its great affinity for oxygen it is always preserved under
water.
Phosphorus is very poisonous, from 0.2 to 0.3 gram being a fatal dose.
Ground up with flour and water or similar substances, it is often used
as a poison for rats and other vermin.
~Precaution.~ The heat of the body is sufficient to raise
phosphorus above its kindling temperature, and for this reason
it should always be handled with forceps and never with the
bare fingers. Burns occasioned by it are very painful and slow
in healing.
~Red phosphorus.~ On standing, yellow phosphorus gradually undergoes a
remarkable change, being converted into a dark red powder which has a
density of 2.1. It no longer takes fire easily, neither does it dissolve
in carbon disulphide. It is not poisonous and, in fact, seems to be an
entirely different substance. The velocity of this change increases with
rise in temperature, and the red phosphorus is therefore prepared by
heating the yellow just below the boiling point (250 deg.-300 deg.). When
distilled and quickly condensed the red form changes back to the yellow.
This is in accordance with the general rule that when a substance
capable of existing in several allotropic forms is condensed from a gas
or crystallized from the liquid state, the more unstable variety forms
first, and this then passes into the more stable forms.
~Matches.~ The chief use of phosphorus is in the manufacture of
matches. Common matches are made by first dipping the match
sticks into some inflammable substance, such as melted
paraffin, and afterward into a paste consisting of (1)
phosphorus, (2) some oxidizing substance, such as manganese
dioxide or potassium chlorate, and (3) a binding material,
usually some kind of glue. On friction the phosphorus is
ignited, the combustion being sustained by the oxidizing agent
and communicated to the wood by the burning paraffin. In
sulphur matches the paraffin is replaced by sulphur.
In safety matches _red_ phosphorus, an oxidizing agent, and
some gritty material such as emery is placed on the side of the
box, while the match tip is provided as before with an
oxidizing agent and an easily oxidized substance, usually
antimony sulphide. The match cannot be ignited easily by
friction, save on the prepared surface.
~Compounds of phosphorus with hydrogen.~ Phosphorus forms several
compounds with hydrogen, the best known of which is phosphine (PH_{3})
analogous to ammonia (NH_{3}).
~Preparation of phosphine.~ Phosphine is usually made by heating
phosphorus with a strong solution of potassium hydroxide, the reaction
being a complicated one.
[Illustration: Fig. 71]
The experiment can be conveniently made in the apparatus shown
in Fig. 71. A strong solution of potassium hydroxide together
with several small bits of phosphorus are placed in the flask
A, and a current of coal gas is passed into the flask through
the tube B until all the air has been displaced. The gas is
then turned off and the flask is heated. Phosphine is formed in
small quantities and escapes through the delivery tube, the
exit of which is just covered by the water in the vessel C.
Each bubble of the gas as it escapes into the air takes fire,
and the product of combustion (P_{2}O_{5}) forms beautiful
small rings, which float unbroken for a considerable time in
quiet air. The pure phosphine does not take fire spontaneously.
When prepared as directed above, impurities are present which
impart this property.
~Properties.~ Phosphine is a gas of unpleasant odor and is exceedingly
poisonous. Like ammonia it forms salts with the halogen acids. Thus we
have phosphonium chloride (PH_{4}Cl) analogous to ammonium chloride
(NH_{4}Cl). The phosphonium salts are of but little importance.
~Oxides of phosphorus.~ Phosphorus forms two well-known oxides,--the
trioxide (P_{2}O_{3}) and the pentoxide (P_{2}O_{5}), sometimes called
phosphoric anhydride. When phosphorus burns in an insufficient supply of
air the product is partially the trioxide; in oxygen or an excess of air
the pentoxide is formed. The pentoxide is much the better known of the
two. It is a snow-white, voluminous powder whose most marked property is
its great attraction for water. It has no chemical action upon most
gases, so that they can be very thoroughly dried by allowing them to
pass through properly arranged vessels containing phosphorus pentoxide.
~Acids of phosphorus.~ The important acids of phosphorus are the
following:
H_{3}PO_{3} phosphorous acid.
H_{3}PO_{4} phosphoric acid.
H_{4}P_{2}O_{7} pyrophosphoric acid.
HPO_{3} metaphosphoric acid.
These may be regarded as combinations of the oxides of phosphorus with
water according to the equations given in the discussion of the
characteristics of the family.
1. _Phosphorous acid_ (H_{3}PO_{3}). Neither the acid nor its salts are
at all frequently met with in chemical operations. It can be easily
obtained, however, in the form of transparent crystals when phosphorus
trichloride is treated with water and the resulting solution is
evaporated:
PCl_{3} + 3H_{2}O = H_{3}PO_{3} + 3HCl.
Its most interesting property is its tendency to take up oxygen and pass
over into phosphoric acid.
2. _Orthophosphoric acid (phosphoric acid)_ (H_{3}PO_{4}). This acid can
be obtained by dissolving phosphorus pentoxide in boiling water, as
represented in the equation
P_{2}O_{5} + 3H_{2}O = 2H_{3}PO_{4}.
It is usually made by treating calcium phosphate with concentrated
sulphuric acid. The calcium sulphate produced in the reaction is nearly
insoluble, and can be filtered off, leaving the phosphoric acid in
solution. Very pure acid is made by oxidizing phosphorus with nitric
acid. It forms large colorless crystals which are exceedingly soluble in
water. Being a tribasic acid, it forms acid as well as normal salts.
Thus the following compounds of sodium are known:
NaH_{2}PO_{4} monosodium hydrogen phosphate.
Na_{2}HPO_{4} disodium hydrogen phosphate.
Na_{3}PO_{4} normal sodium phosphate.
These salts are sometimes called respectively primary, secondary, and
tertiary phosphates. They may be prepared by bringing together
phosphoric acid and appropriate quantities of sodium hydroxide.
Phosphoric acid also forms mixed salts, that is, salts containing two
different metals. The most familiar compound of this kind is microcosmic
salt, which has the formula Na(NH_{4})HPO_{4}.
_Orthophosphates._ The orthophosphates form an important class of salts.
The normal salts are nearly all insoluble and many of them occur in
nature. The secondary phosphates are as a rule insoluble, while most of
the primary salts are soluble.
3. _Pyrophosphoric acid_ (H_{4}P_{2}O_{7}). On heating orthophosphoric
acid to about 225 deg. pyrophosphoric acid is formed in accordance with the
following equation:
2H_{3}PO_{4} = H_{4}P_{2}O_{7} + H_{2}O.
It is a white crystalline solid. Its salts can be prepared by heating a
secondary phosphate:
2Na_{2}HPO_{4} = Na_{4}P_{2}O_{7} + H_{2}O.
4. _Metaphosphoric acid (glacial phosphoric acid)_ (HPO_{3}). This acid
is formed when orthophosphoric acid is heated above 400 deg.:
H_{3}PO_{4} = HPO_{3} + H_{2}O.
It is also formed when phosphorus pentoxide is treated with cold water:
P_{2}O_{5} + H_{2}O = 2HPO_{3}.
It is a white crystalline solid, and is so stable towards heat that it
can be fused and even volatilized without decomposition. On cooling from
the fused state it forms a glassy solid, and on this account is often
called glacial phosphoric acid. It possesses the property of dissolving
small quantities of metallic oxides, with the formation of compounds
which, in the case of certain metals, have characteristic colors. It is
therefore used in the detection of these metals.
While the secondary phosphates, on heating, give salts of pyrophosphoric
acid, the primary phosphates yield salts of metaphosphoric acid. The
equations representing these reactions are as follows:
2Na_{2}HPO_{4} = Na_{4}P_{3}O_{7} + H_{2}O,
NaH_{2}PO_{4} = NaPO_{3} + H_{2}O.
~Fertilizers.~ When crops are produced year after year on the same field
certain constituents of the soil essential to plant growth are removed,
and the soil becomes impoverished and unproductive. To make the land
once more fertile these constituents must be replaced. The calcium
phosphate of the mineral deposits or of bone ash serves well as a
material for restoring phosphorus to soils exhausted of that essential
element; but a more soluble substance, which the plants can more readily
assimilate, is desirable. It is better, therefore, to convert the
insoluble calcium phosphate into the soluble primary phosphate before it
is applied as fertilizer. It will be seen by reference to the formulas
for the orthophosphates (see page 244) that in a primary phosphate only
one hydrogen atom of phosphoric acid is replaced by a metal. Since the
calcium atom always replaces two hydrogen atoms, it might be thought
that there could be no primary calcium phosphate; but if the calcium
atom replaces one hydrogen atom from each of two molecules of phosphoric
acid, the salt Ca(H_{2}PO_{4})_{2} will result, and this is a primary
phosphate. It can be made by treatment of the normal phosphate with the
necessary amount of sulphuric acid, calcium sulphate being formed at the
same time, thus:
Ca_{3}(PO_{4})_{2} + 2H_{2}SO_{4} = Ca(H_{2}PO_{4})_{2} + 2CaSO_{4}.
The resulting mixture is a powder, which is sold as a fertilizer under
the name of "superphosphate of lime."
ARSENIC
~Occurrence.~ Arsenic occurs in considerable quantities in nature as the
native element, as the sulphides realgar (As_{2}S_{2}) and orpiment
(As_{2}S_{3}), as oxide (As_{2}O_{3}), and as a constituent of many
metallic sulphides, such as arsenopyrite (FeAsS).
~Preparation.~ The element is prepared by purifying the native arsenic, or
by heating the arsenopyrite in iron tubes, out of contact with air,
when the reaction expressed by the following equation occurs:
FeAsS = FeS + As.
The arsenic, being volatile, condenses in chambers connected with the
heated tubes. It is also made from the oxide by reduction with carbon:
2As_{2}O_{3} + 3C = 4As + 3CO_{2}.
~Properties.~ Arsenic is a steel-gray, metallic-looking substance of
density 5.73. Though resembling metals in appearance, it is quite
brittle, being easily powdered in a mortar. When strongly heated it
sublimes, that is, it passes into a vapor without melting, and condenses
again to a crystalline solid when the vapor is cooled. Like phosphorus
it can be obtained in several allotropic forms. It alloys readily with
some of the metals, and finds its chief use as an alloy with lead, which
is used for making shot, the alloy being harder than pure lead. When
heated on charcoal with the blowpipe it is converted into an oxide which
volatilizes, leaving the charcoal unstained by any oxide coating. It
burns readily in chlorine gas, forming arsenic trichloride,--
As + 3Cl = AsCl_{3}.
Unlike most of its compounds, the element itself is not poisonous.
~Arsine~ (AsH_{3}). When any compound containing arsenic is brought into
the presence of nascent hydrogen, arsine (AsH_{3}), corresponding to
phosphine and ammonia, is formed. The reaction when oxide of arsenic is
so treated is
As_{2}O_{3} + 12H = 2AsH_{3} + 3H_{2}O.
Arsine is a gas with a peculiar garlic-like odor, and is intensely
poisonous. A single bubble of pure gas has been known to prove fatal. It
is an unstable compound, decomposing into its elements when heated to a
moderate temperature. It is combustible, burning with a pale
bluish-white flame to form arsenic trioxide and water when air is in
excess:
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