HYDROGEN
~Historical.~ The element hydrogen was first clearly recognized as a
distinct substance by the English investigator Cavendish, who in 1766
obtained it in a pure state, and showed it to be different from the
other inflammable airs or gases which had long been known. Lavoisier
gave it the name hydrogen, signifying water former, since it had been
found to be a constituent of water.
~Occurrence.~ In the free state hydrogen is found in the atmosphere, but
only in traces. In the combined state it is widely distributed, being a
constituent of water as well as of all living organisms, and the
products derived from them, such as starch and sugar. About 10% of the
human body is hydrogen. Combined with carbon, it forms the substances
which constitute petroleum and natural gas.
It is an interesting fact that while hydrogen in the free state
occurs only in traces on the earth, it occurs in enormous
quantities in the gaseous matter surrounding the sun and
certain other stars.
~Preparation from water.~ Hydrogen can be prepared from water by several
methods, the most important of which are the following.
1. _By the electric current._ As has been indicated in the preparation
of oxygen, water is easily separated into its constituents, hydrogen and
oxygen, by passing an electric current through it under certain
conditions.
2. _By the action of certain metals._ When brought into contact with
certain metals under appropriate conditions, water gives up a portion
or the whole of its hydrogen, its place being taken by the metal. In the
case of a few of the metals this change occurs at ordinary temperatures.
Thus, if a bit of sodium is thrown on water, an action is seen to take
place at once, sufficient heat being generated to melt the sodium, which
runs about on the surface of the water. The change which takes place
consists in the displacement of one half of the hydrogen of the water by
the sodium, and may be represented as follows:
_ _ _ _
| hydrogen | | sodium |
sodium + | hydrogen |(water) = | hydrogen |(sodium hydroxide) + hydrogen
|_oxygen _| |_oxygen _|
The sodium hydroxide formed is a white solid which remains dissolved in
the undecomposed water, and may be obtained by evaporating the solution
to dryness. The hydrogen is evolved as a gas and may be collected by
suitable apparatus.
Other metals, such as magnesium and iron, decompose water rapidly, but
only at higher temperatures. When steam is passed over hot iron, for
example, the iron combines with the oxygen of the steam, thus displacing
the hydrogen. Experiments show that the change may be represented as
follows:
_ _
| hydrogen | _ _ _ _
iron + | hydrogen |(water) = | iron |(iron oxide) + | hydrogen |
|_oxygen _| |_oxygen _| |_hydrogen_|
The iron oxide formed is a reddish-black compound, identical with that
obtained by the combustion of iron in oxygen.
~Directions for preparing hydrogen by the action of steam on
iron.~ The apparatus used in the preparation of hydrogen from
iron and steam is shown in Fig. 9. A porcelain or iron tube
B, about 50 cm. in length and 2 cm. or 3 cm. in diameter, is
partially filled with fine iron wire or tacks and connected as
shown in the figure. The tube B is heated, slowly at first,
until the iron is red-hot. Steam is then conducted through the
tube by boiling the water in the flask A. The hot iron
combines with the oxygen in the steam, setting free the
hydrogen, which is collected over water. The gas which first
passes over is mixed with the air previously contained in the
flask and tube, and is allowed to escape, _since a mixture of
hydrogen with oxygen or air explodes violently when brought in
contact with a flame_. It is evident that the flask A must be
disconnected from the tube before the heat is withdrawn.
That the gas obtained is different from air and oxygen may be
shown by holding a bottle of it mouth downward and bringing a
lighted splint into it. The hydrogen is ignited and burns with
an almost colorless flame.
[Illustration Fig. 9]
~Preparation from acids~ (_usual laboratory method_). While hydrogen can
be prepared from water, either by the action of the electric current or
by the action of certain metals, these methods are not economical and
are therefore but little used. In the laboratory hydrogen is generally
prepared from compounds known as acids, all of which contain hydrogen.
When acids are brought in contact with certain metals, the metals
dissolve and set free the hydrogen of the acid. Although this reaction
is a quite general one, it has been found most convenient in preparing
hydrogen by this method to use either zinc or iron as the metal and
either hydrochloric or sulphuric acid as the acid. Hydrochloric acid is
a compound consisting of 2.77% hydrogen and 97.23% chlorine, while
sulphuric acid consists of 2.05% hydrogen, 32.70% sulphur, and 65.25%
oxygen.
The changes which take place in the preparation of hydrogen from zinc
and sulphuric acid (diluted with water) may be represented as follows:
_ _ _ _
| hydrogen |(sulphuric | zinc |(zinc
zinc + | sulphur | acid) = | sulphur | sulphate) + hydrogen
|_oxygen _| |_oxygen _|
In other words, the zinc has taken the place of the hydrogen in
sulphuric acid. The resulting compound contains zinc, sulphur, and
oxygen, and is known as zinc sulphate. This remains dissolved in the
water present in the acid. It may be obtained in the form of a white
solid by evaporating the liquid left after the metal has passed into
solution.
When zinc and hydrochloric acid are used the following changes take
place:
_ _ _ _
| hydrogen |(hydrochloric | zinc |(zinc
zinc + |_chlorine_| acid) = |_chlorine_| chloride) + hydrogen
When iron is used the changes which take place are exactly similar to
those just given for zinc.
[Illustration Fig. 10.]
~Directions for preparing hydrogen from acids.~ The preparation
of hydrogen from acids is carried out in the laboratory as
follows: The metal is placed in a flask or wide-mouthed bottle
A (Fig. 10) and the acid is added slowly through the funnel
tube B. The metal dissolves in the acid, while the hydrogen
which is liberated escapes through the exit tube C and is
collected over water. It is evident that the hydrogen which
passes over first is mixed with the air from the bottle A.
Hence care must be taken not to bring a flame near the exit
tube, since, as has been stated previously, such a mixture
explodes with great violence when brought in contact with a
flame.
~Precautions.~ Both sulphuric acid and zinc, if impure, are
likely to contain small amounts of arsenic. Such materials
should not be used in preparing hydrogen, since the arsenic
present combines with a portion of the hydrogen to form a very
poisonous gas known as arsine. On the other hand, chemically
pure sulphuric acid, i.e. sulphuric acid that is entirely free
from impurities, will not act upon chemically pure zinc. The
reaction may be started, however, by the addition of a few
drops of a solution of copper sulphate or platinum
tetrachloride.
~Physical properties.~ Hydrogen is similar to oxygen in that it is a
colorless, tasteless, odorless gas. It is characterized by its extreme
lightness, being the lightest of all known substances. One liter of the
gas weighs only 0.08984 g. On comparing this weight with that of an
equal volume of oxygen, viz., 1.4285 g., the latter is found to be 15.88
times as heavy as hydrogen. Similarly, air is found to be 14.38 times as
heavy as hydrogen. Soap bubbles blown with hydrogen rapidly rise in the
air. On account of its lightness it is possible to pour it upward from
one bottle into another. Thus, if the bottle A (Fig. 11) is filled
with hydrogen, placed mouth downward by the side of bottle _B_, filled
with air, and is then gradually inverted under B as indicated in the
figure, the hydrogen will flow upward into bottle _B_, displacing the
air. Its presence in bottle B may then be shown by bringing a lighted
splint to the mouth of the bottle, when the hydrogen will be ignited by
the flame. It is evident, from this experiment, that in order to retain
the gas in an open bottle the bottle must be placed mouth downward.
[Illustration Fig. 11]
Hydrogen is far more difficult to liquefy than any other gas, with the
exception of helium, a rare element recently found to exist in the
atmosphere. The English scientist Dewar, however, in 1898 succeeded not
only in obtaining hydrogen in liquid state but also as a solid. Liquid
hydrogen is colorless and has a density of only 0.07. Its boiling point
under atmospheric pressure is -252°. Under diminished pressure the
temperature has been reduced to -262°. The solubility of hydrogen in
water is very slight, being still less than that of oxygen.
Pure hydrogen produces no injurious results when inhaled. Of course one
could not live in an atmosphere of the gas, since oxygen is essential to
respiration.
~Chemical properties.~ At ordinary temperatures hydrogen is not an active
element. A mixture of hydrogen and chlorine, however, will combine with
explosive violence at ordinary temperature if exposed to the sunlight.
The union can be brought about also by heating. The product formed in
either case is hydrochloric acid. Under suitable conditions hydrogen
combines with nitrogen to form ammonia, and with sulphur to form the
foul-smelling gas, hydrogen sulphide. The affinity of hydrogen for
oxygen is so great that a mixture of hydrogen and oxygen or hydrogen
and air explodes with great violence when heated to the kindling
temperature (about 612°). Nevertheless under proper conditions hydrogen
may be made to burn quietly in either oxygen or air. The resulting
hydrogen flame is almost colorless and is very hot. The combustion of
the hydrogen is, of course, due to its union with oxygen. The product of
the combustion is therefore a compound of hydrogen and oxygen. That this
compound is water may be shown easily by experiment.
[Illustration Fig. 12]
~Directions for burning hydrogen in air.~ The combustion of
hydrogen in air may be carried out safely as follows: The
hydrogen is generated in the bottle A (Fig. 12), is dried by
conducting it through the tube X, filled with some substance
(generally calcium chloride) which has a great attraction for
moisture, and escapes through the tube T, the end of which is
drawn out to a jet. The hydrogen first liberated mixes with the
air contained in the generator. If a flame is brought near the
jet before this mixture has all escaped, a violent and very
dangerous explosion results, since the entire apparatus is
filled with the explosive mixture. On the other hand, if the
flame is not applied until all the air has been expelled, the
hydrogen is ignited and burns quietly, since only the small
amount of it which escapes from the jet can come in contact
with the oxygen of the air at any one time. By holding a cold,
dry bell jar or bottle over the flame, in the manner shown in
the figure, the steam formed by the combustion of the hydrogen
is condensed, the water collecting in drops on the sides of the
jar.
~Precautions.~ In order to avoid danger it is absolutely necessary to
prove that the hydrogen is free from air before igniting it. This can be
done by testing small amounts of the escaping gas. A convenient and safe
method of doing this is to fill a test tube with the gas by inverting it
over the jet. The hydrogen, on account of its lightness, collects in the
tube, displacing the air. After holding it over the jet for a few
moments in order that it may be filled with the gas, the tube is gently
brought, mouth downward, to the flame of a burner placed not nearer than
an arm's length from the jet. If the hydrogen is mixed with air a slight
explosion occurs, but if pure it burns quietly in the tube. The
operation is repeated until the gas burns quietly, when the tube is
quickly brought back over the jet for an instant, whereby the escaping
hydrogen is ignited by the flame in the tube.
[Illustration. Fig. 13]
~A mixture of hydrogen and oxygen is explosive.~ That a mixture of
hydrogen and air is explosive may be shown safely as follows: A cork
through which passes a short glass tube about 1 cm. in diameter is
fitted air-tight into the tubule of a bell jar of 2 l. or 3 l. capacity.
(A thick glass bottle with bottom removed may be used.) The tube is
closed with a small rubber stopper and the bell jar filled with
hydrogen, the gas being collected over water. When entirely filled with
the gas the jar is removed from the water and supported by blocks of
wood in order to leave the bottom of the jar open, as shown in Fig. 13.
The stopper is now removed from the tube in the cork, and the hydrogen,
which on account of its lightness escapes from the tube, is at once
lighted. As the hydrogen escapes, the air flows in at the bottom of the
jar and mixes with the remaining portion of the hydrogen, so that a
mixture of the two soon forms, and a loud explosion results. The
explosion is not dangerous, since the bottom of the jar is open, thus
leaving room for the expansion of the hot gas.
Since air is only one fifth oxygen, the remainder being inert gases, it
may readily be inferred that a mixture of hydrogen with pure oxygen
would be far more explosive than a mixture of hydrogen with air. Such
mixtures should not be made except in small quantities and by
experienced workers.
~Hydrogen does not support combustion.~ While hydrogen is readily
combustible, it is not a supporter of combustion. In other words,
substances will not burn in it. This may be shown by bringing a lighted
candle supported by a stiff wire into a bottle or cylinder of the pure
gas, as shown in Fig. 14. The hydrogen is ignited by the flame of the
candle and burns at the mouth of the bottle, where it comes in contact
with the oxygen in the air. When the candle is thrust up into the gas,
its flame is extinguished on account of the absence of oxygen. If slowly
withdrawn, the candle is relighted as it passes through the layer of
burning hydrogen.
[Illustration: Fig. 14]
[Illustration: Fig. 15]
~Reduction.~ On account of its great affinity for oxygen, hydrogen has the
power of abstracting it from many of its compounds. Thus, if a stream of
hydrogen, dried by passing through the tube B (Fig. 15), filled with
calcium chloride, is conducted through the tube C containing some
copper oxide, heated to a moderate temperature, the hydrogen abstracts
the oxygen from the copper oxide. The change may be represented as
follows:
hydrogen + {copper} {hydrogen}
{oxygen}(copper oxide) = {oxygen }(water) + copper
The water formed collects in the cold portions of the tube C near its
end. In this experiment the copper oxide is said to undergo reduction.
_Reduction may therefore be defined as the process of withdrawing oxygen
from a compound._
~Relation of reduction to oxidation.~ At the same time that the copper
oxide is reduced it is clear that the hydrogen is oxidized, for it
combines with the oxygen given up by the copper oxide. The two processes
are therefore very closely related, and it usually happens that when one
substance is oxidized some other substance is reduced. That substance
which gives up its oxygen is called an _oxidizing agent_, while the
substance which unites with the oxygen is called a _reducing agent_.
~The oxyhydrogen blowpipe.~ This is a form of apparatus used for burning
hydrogen in pure oxygen. As has been previously stated, the flame
produced by the combustion of hydrogen in the air is very hot. It is
evident that if pure oxygen is substituted for air, the temperature
reached will be much higher, since there are no inert gases to absorb
the heat. The oxyhydrogen blowpipe, used to effect this combination,
consists of a small tube placed within a larger one, as shown in Fig.
16.
[Illustration: Fig. 16]
The hydrogen, stored under pressure, generally in steel cylinders, is
first passed through the outer tube and ignited at the open end of the
tube. The oxygen from a similar cylinder is then conducted through the
inner tube, and mixes with the hydrogen at the end of the tube. In order
to produce the maximum heat, the hydrogen and oxygen must be admitted to
the blowpipe in the exact proportion in which they combine, viz., 2
volumes of hydrogen to 1 of oxygen, or by weight, 1 part of hydrogen to
7.94 parts of oxygen. The intensity of the heat may be shown by bringing
into the flame pieces of metal such as iron wire or zinc. These burn
with great brilliancy. Even platinum, having a melting point of 1779°,
may be melted by the heat of the flame.
While the oxyhydrogen flame is intensely hot, it is almost non-luminous.
If directed against some infusible substance like ordinary lime (calcium
oxide), the heat is so intense that the lime becomes incandescent and
glows with a brilliant light. This is sometimes used as a source of
light, under the name of _Drummond_ or _lime light_.
[Illustration: Fig. 17]
~The blast lamp.~ A similar form of apparatus is commonly used in the
laboratory as a source of heat under the name _blast lamp_ (Fig. 17).
This differs from the oxyhydrogen blowpipe only in the size of the
tubes. In place of the hydrogen and oxygen the more accessible coal gas
and air are respectively used. The former is composed largely of a
mixture of free hydrogen and gaseous compounds of carbon and hydrogen.
While the temperature of the flame is not so high as that of the
oxyhydrogen blowpipe, it nevertheless suffices for most chemical
operations carried out in the laboratory.
~Uses of hydrogen.~ On account of its cost, hydrogen is but little used
for commercial purposes. It is sometimes used as a material for the
inflation of balloons, but usually the much cheaper coal gas is
substituted for it. Even hot air is often used when the duration of
ascension is very short. It has been used also as a source of heat and
light in the oxyhydrogen blowpipe. Where the electric current is
available, however, this form of apparatus has been displaced almost
entirely by the electric light and electric furnace, which are much more
economical and more powerful sources of light and heat.
EXERCISES
1. Will a definite weight of iron decompose an unlimited weight of
steam?
2. Why is oxygen passed through the inner tube of the oxyhydrogen
blowpipe rather than the outer?
3. In Fig. 14, will the flame remain at the mouth of the tube?
4. From Fig. 15, suggest a way for determining experimentally the
quantity of water formed in the reaction.
5. Distinguish clearly between the following terms: oxidation,
reduction, combustion, and kindling temperature.
6. Is oxidation always accompanied by reduction?
7. What is the source of heat in the lime light? What is the exact use
of lime in this instrument?
8. In Fig. 12, why is it necessary to dry the hydrogen by means of the
calcium chloride in the tube X?
9. At what pressure would the weight of 1 l. of hydrogen be equal to
that of oxygen under standard conditions?
10. (a) What weight of hydrogen can be obtained from 150 g. of
sulphuric acid? (b) What volume would this occupy under standard
conditions? (c) The density of sulphuric acid is 1.84. What volume
would the 150 g. of the acid occupy?
11. How many liters of hydrogen can be obtained from 50 cc. of sulphuric
acid having a density of 1.84?
12. Suppose you wish to fill five liter bottles with hydrogen, the gas
to be collected over water in your laboratory, how many cubic
centimeters of sulphuric acid would be required?