HYDROGEN
Hydrogen has the simplest
atomic structure among all the elements around us in Nature. In atomic form it
consists of only one proton and one electron. However, in elemental form it
exists as a diatomic (H2)
molecule and is called di hydrogen.
POSITION OF HYDROGEN IN THE PERIODIC TABLE
Hydrogen is the first element in the periodic table.
However, its placement in the
periodic table has been a subject of discussion in the past. Now As we know
that the elements in the periodic table are arranged according to their electronic configurations. Hydrogen has electronic configuration 1s1.
So, On one hand, its electronic
configuration is similar to the outer electronic configuration (ns1)
of alkali metals, which belong to the first group of the periodic table.
On the other hand, like halogens
(with ns2np5 configuration belonging to the
seventeenth group of the periodic table), it is short by one electron to the
corresponding noble gas configuration, helium (1s2).
Hydrogen, therefore, has resemblance to alkali metals, which lose one electron
to form uni positive ions, as well as with halogens, which gain one electron to
form uni negative ion. Like alkali metals, hydrogen forms oxides, halides and
sulphides. However, unlike alkali metals, it has a very high ionization
enthalpy and does not possess metallic characteristics under normal conditions.
In fact, in terms of ionization enthalpy, hydrogen resembles more with
halogens, ∆i H of Li is 520 kJ mol–1, F is 1680 kJ mol–1
and that of H is 1312 kJ mol–1. Like halogens, it forms a diatomic
molecule, combines with elements to form hydrides and a large number of
covalent compounds.
However, in terms of reactivity, it is very low as compared to
halogens.
Inspite of the fact that
hydrogen, to a certain extent resembles both with alkali metals and halogens,
it differs from them as well. Now the pertinent question arises as where should it be placed in the periodic
table?
Loss of the electron from
hydrogen atom results in nucleus (H+) of ~1.5×10–3 pm
size. This is extremely small as compared to normal atomic and ionic sizes of
50 to 200pm. As a consequence, H+ does not exist freely and is
always associated with other atoms or molecules. Thus, it is unique in
behaviour and is, therefore, best placed separately in the periodic table
DIHYDROGEN, H2
Occurrence
Dihydrogen is the most abundant element in the universe (70% of the
total mass of the universe) and is the principal element in the solar
atmosphere. The giant planets Jupiter and Saturn consist mostly of hydrogen.
However, due to its light nature, it is much less abundant (0.15% by
mass) in the earth’s atmosphere. Of course, in the combined form it constitutes
15.4% of the earth's crust and the oceans. In the combined form besides in
water, it occurs in plant and animal tissues, carbohydrates, proteins,
hydrides including hydrocarbons and many other compounds.
Isotopes of Hydrogen
Hydrogen has three isotopes: protium, 1H1,
deuterium, 1H2
or D and tritium, 1H3 or T.
Q. How these isotopes differ from each other ?
Ans. These isotopes differ from one another in respect of the presence
of neutrons. Ordinary hydrogen, protium, has no neutrons, deuterium (also known as heavy
hydrogen) has one and tritium has two neutrons in the nucleus. In the year
1934, an American scientist, Harold C.
Urey, got Nobel Prize for separating hydrogen isotope of mass number 2 by
physical methods.
Of these isotopes, only tritium
is radioactive and emits low energy α– particles (t½, 12.33 years). Since the isotopes have the same
electronic configuration, they have
almost the same chemical properties.
The only difference is in their rates of
reactions, mainly due to their different enthalpy of bond dissociation.
However, in physical properties these isotopes differ considerably due to their
large mass differences.
PREPARATION OF DIHYDROGEN, H2
Laboratory Preparation of dihydrogen
(i) It is usually prepared by the reaction of granulated zinc with
dilute hydrochloric acid.
Zn + 2H+
→Zn2+ + H2
(ii) It can also be prepared by the reaction of zinc with aqueous
alkali.
Zn + 2NaOH → Na2ZnO2
+ H2
Sodium zincate
Commercial Production of Dihydrogen
The commonly used processes are outlined below:
(i) Electrolysis of acidified water using platinum electrodes gives
hydrogen.
(ii) High purity (>99.95%) dihydrogen is obtained by electrolysing
warm aqueous barium hydroxide solution between nickel electrodes.
(iii) It is obtained as a byproduct in the manufacture of sodium
hydroxide and chlorine by the electrolysis of brine
solution. During electrolysis, the reactions that take place are:
at anode: 2Cl–(aq) →Cl2(g)
+ 2e–
at cathode: 2H2O (l) + 2e– →H2(g)
+ 2OH–(aq)
The overall reaction is
2Na+ (aq) + 2Cl–(aq) + 2H2O(l) ----Ă Cl2(g) + H2(g) + 2Na+
(aq) + 2OH–(aq)
(iv) Reaction of steam on hydrocarbons or coke at high temperatures in
the presence of catalyst yields hydrogen.
The mixture of CO and H2 is
called water gas. As this mixture of CO and H2 is used for the
synthesis of methanol and a number of hydrocarbons, it is also called synthesis
gas or 'syngas'.
Nowadays 'syngas' is produced from sewage, saw-dust, scrap wood, newspapers
etc. The process of producing 'syngas' from coal is called 'coal
gasification.
The production of dihydrogen
can be increased by reacting carbon monoxide of syngas mixtures with steam in
the presence of iron chromate as a catalyst.
This is called a water-gas shift reaction. Carbon
dioxide is removed by scrubbing with sodium arsenite solution.
Note--Presently ~77% of the industrial dihydrogen is produced from
petrochemicals, 18% from coal, 4% from the electrolysis of aqueous solutions and
1% from other sources.
PROPERTIES OF DIHYDROGEN
Physical Properties
Dihydrogen is a colourless, odourless, tasteless, combustible gas. It
is lighter than air and insoluble in water.
Chemical Properties
The chemical behaviour of dihydrogen (and for that matter any molecule)
is determined, to a large extent, by bond dissociation enthalpy. The H–H bond
dissociation enthalpy is the highest for a single bond between two atoms of any
element.
Q. What inferences would you draw from this fact?
Ans. It is because of this factor that the dissociation of dihydrogen
into its atoms is only ~0.081% around 2000K which increases to 95.5% at 5000K.
Also, it is relatively inert at room
temperature due to the high H–H bond enthalpy.
Thus, the atomic hydrogen is produced
at a high temperature in an electric arc or under ultraviolet radiation. Since
its orbital is incomplete with 1s1 electronic configuration, it does
combine with almost all the elements. It accomplishes reactions by
(i) loss of the only electron to give H+, (ii) gain of an electron to
form H–, and
(iii) sharing electrons to form a single covalent bond.
The chemistry of dihydrogen can be illustrated by the following
reactions:
Reaction with halogens: It reacts with halogens, X2 to give hydrogen halides, HX,
While the reaction with fluorine occurs even in the dark, with iodine
it requires a catalyst.
Reaction with dioxygen: It reacts with dioxygen to form water. The reaction is highly exothermic.
Reaction with dinitrogen: With dinitrogen forms ammonia.
This is the method for the manufacture of ammonia by the Haber process.
Reactions with metals: With many metals, it combines at high a temperature to yield the corresponding
hydrides
H2(g)
+2M(g) → 2MH(s);
where M is an alkali metal
Reactions with metal ions and metal oxides:
It reduces some metal ions in an aqueous solution and oxides of metals
(less active than iron)
into corresponding metals.
Reactions
with organic compounds: It reacts with many organic compounds
in the presence of catalysts to give useful hydrogenated products of commercial
importance. For example :
(i) Hydrogenation (addition of hydrogen) of vegetable oils using nickel
as a catalyst gives edible fats
(margarine and vanaspati ghee)
(ii) Hydroformylation of olefins yields aldehydes which further undergo
reduction to give alcohols.
Uses of Dihydrogen
---The largest single use of dihydrogen is in the synthesis of ammonia
which is used in the manufacture of nitric acid and nitrogenous fertilizers.
---Dihydrogen is used in the manufacture of vanaspati fat by the
hydrogenation of polyunsaturated vegetable oils like soyabean, cotton seeds
etc.
---It is used in the manufacture of bulk organic chemicals,
particularly methanol.
---It is widely used for the manufacture of metal hydrides
---It is used for the preparation of hydrogen chloride, a highly
useful chemical.
---In metallurgical processes, it is used to reduce heavy metal oxides
to metals.
---Atomic hydrogen and oxy-hydrogen torches find a use for cutting and
welding purposes. Atomic hydrogen atoms
(produced by the dissociation of dihydrogen with the help of an electric
arc) are allowed to recombine on the surface to be welded to generate a
temperature of 4000 K.
---It is used as rocket fuel in space research.
--Dihydrogen is used in fuel cells for generating electrical energy. It has many advantages over conventional
fossil fuels and electric power. It does not produce any pollution and releases greater energy per unit mass of fuel in comparison to gasoline and other fuels.
HYDRIDES
Dihydrogen, under certain reaction conditions, combines with almost all
elements, except noble gases, to form binary compounds, called hydrides.
If ‘E’ is the symbol of an element then hydride can be expressed as EHx (e.g.,
MgH2) or EmHn (e.g., B2H6).
The hydrides are classified into three categories :
(i) Ionic or saline or saltlike hydrides
(ii) Covalent or molecular hydrides
(iii) Metallic or non-stoichiometric hydrides
1 Ionic or Saline Hydrides
These are stoichiometric compounds of dihydrogen formed with most of
the s-block elements which are highly electropositive in character.
However, the significant covalent character is found in the lighter metal hydrides such
as LiH, BeH2 and MgH2. In fact, BeH2 and MgH2
are polymeric in structure. The ionic hydrides are crystalline, non-volatile
and nonconducting in the solid state. However, their melts conduct electricity and
on electrolysis liberate
dihydrogen gas at the anode, which confirms the existence of the H– ion
Saline hydrides react violently with water producing dihydrogen gas.
Lithium hydride is rather unreactive at moderate temperatures with O2
or Cl2. It is, therefore, used in the synthesis of other useful hydrides,
e.g.,
8LiH + Al2Cl6
→ 2LiAlH4 + 6LiCl
2LiH + B2H6
→ 2LiBH4
Covalent or Molecular Hydride
Dihydrogen forms molecular compounds with most of the p-block elements. The most familiar examples are CH4, NH3, H2O and
HF.
Note--For convenience hydrogen compounds of nonmetals have also been
considered as hydrides. Being covalent, they are volatile compounds.
Molecular hydrides are further classified according to the relative
numbers of electrons and bonds in their Lewis structure into :
(i) electron-deficient, An
electron-deficient hydride, as the name suggests, has few electrons for
writing its conventional Lewis structure. Diborane (B2H6)
is an example. In fact all elements of group 13
will form electron-deficient compounds.
Q. What do you expect from their behaviour?
Ans. They act as Lewis acids
i.e., electron acceptors.
(ii) electron-precise, Electron-precise
compounds have the required number of electrons to write their conventional
Lewis structures. All elements of group 14 form such compounds (e.g., CH4)
which are tetrahedral in geometry.
(iii) Electron-rich hydrides. Electron-rich
hydrides have excess electrons which are present as lone pairs. Elements of
groups 15-17 form such compounds. (NH3 has 1- lone pair, H2O
– 2 and HF –3 lone pairs).
Q. What do you expect from the behaviour of such
compounds?
Ans. They will behave as Lewis bases i.e., electron donors. The presence
of lone pairs on highly electronegative
atoms like N, O and F in hydrides results in hydrogen bond formation
between the molecules. This leads to the association of molecules.
Q. Would you expect the hydrides of N, O and F to have lower boiling
points than the hydrides of their subsequent group members? Give reasons.
Ans. No, They have higher boiling points than the hydrides of their
subsequent group members due to hydrogen bonding.
Metallic or Non-stoichiometric (or
Interstitial ) Hydrides
These are formed by many d-block and f-block elements.
However, the metals of groups 7, 8 and 9 do not form hydride. Even from group 6,
only chromium forms CrH. These hydrides conduct heat and electricity though not
as efficiently as their parent metals do. Unlike saline hydrides, they are
almost always nonstoichiometric, being deficient in hydrogen. For example, LaH 2.87, YbH 2.55,
TiH 1.5–1.8, ZrH 1.3–1.75, VH 0.56, NiH 0.6–0.7,
PdH 0.6–0.8 etc. In such hydrides, the law of constant composition
does not hold well.
Earlier it was thought that in these hydrides, hydrogen occupies interstices in the metal
lattice producing distortion without any change in its type. Consequently, they were termed as interstitial hydrides.
However, recent studies have shown
that except for hydrides of
Ni, Pd, Ce and Ac, other hydrides of this class have lattices different from that of the parent metal. The property of
absorption of hydrogen on
transition metals is widely used in catalytic reduction/hydrogenation reactions for the preparation of a large number of
compounds.
Note--Some of the metals (e.g., Pd, Pt) can accommodate a very large
volume of hydrogen and, therefore, can be used as its storage media. This
property has a high potential for hydrogen storage and as a source of
energy.
WATER
A major part of all living organisms is made up of water. The human body
has about 65% and some plants have as much as 95% water. It is a crucial
compound for the survival of all life forms. It is a solvent of great
importance. The distribution of water over the earth’s surface is not uniform.
Physical Properties of Water
It is a colourless and tasteless liquid.. The unusual properties of water in the condensed phase (liquid and solid states) are due to the presence
of extensive hydrogen bonding between water molecules. This
leads to a high freezing point, high
boiling point, high heat of
vaporisation and high heat of fusion in comparison to H2S and H2Se. In comparison to
other liquids, water has a higher specific heat, thermal conductivity, surface
tension,
dipole moment and dielectric constant, etc. These properties allow
water to play a key role in the biosphere high heat of vaporisation
and heat capacity are responsible for the moderation of the climate
and body temperature of living beings. It is an excellent solvent for the transportation of ions and molecules required for plant and animal
metabolism. Due to hydrogen bonding with polar molecules, even covalent
compounds like alcohol and carbohydrates dissolve in water.
Structure of Water
In the gas
phase water is a bent molecule with a bond angle of
104.5°, and O–H bond length of 95.7 pm as shown in Fig (a). It is a highly polar molecule, (Fig (b)). Its an orbital
overlap picture.
Fig. (a)
The bent structure of water; (b) the water molecule as a dipole and (c) the
orbital overlap picture in the water molecule.
In the liquid phase, water
molecules are associated together by hydrogen bonds.
The
crystalline form of water is ice. At atmospheric pressure, ice crystallises in the hexagonal form, but at very low
temperatures it condenses to
cubic form. The density of ice is less
than that of water. Therefore, an ice cube
floats on water. In the winter season ice formed on the surface of a lake
provides thermal insulation which ensures the survival of the aquatic life.
This fact is of great ecological significance.
Structure of Ice
Ice has a highly ordered three-dimensional hydrogen-bonded structure.
Examination of ice crystals with X-rays shows that each oxygen atom is surrounded
tetrahedrally by four other oxygen atoms at a distance of 276 pm.
Hydrogen bonding gives the ice a rather open type structure with wide
holes. These holes can hold some other molecules of appropriate size interstitially.
Chemical Properties of Water
Water reacts with a large number of substances.
(1) Amphoteric Nature: It has the ability to act as an acid as well as a base i.e., it behaves
as an amphoteric substance. In the Brönsted since it acts as an acid with NH3
and a base with H2S.
(2) Redox Reactions Involving Water: Water can be easily reduced to dihydrogen by highly electropositive
metals.
Thus, it is a great source of dihydrogen.
Water is oxidised to O2 during photosynthesis.
6CO2(g) + 12H2O(l) → C6H12O6(aq)
+ 6H2O(l) + 6O2(g)
With fluorine also it is oxidised to O2.
2F2(g) + 2H2O(l)
→ 4H+ (aq) + 4F–(aq) + O2(g)
(3) Hydrolysis Reaction: Due to the high dielectric constant, it has a very strong hydrating
tendency. It dissolves many ionic compounds. However, certain covalent and some
ionic compounds are hydrolysed in water.
(4) Hydrates Formation: From aqueous solutions, many salts can be crystallised as hydrated
salts. Such an association of water is of different types viz.,
Hard and Soft Water
Rainwater is almost pure (may contain some dissolved gases from the
atmosphere). Being a good solvent, when it flows on the surface of the earth,
it dissolves many salts. The presence of calcium and magnesium salts in the form of hydrogen carbonate, chloride and sulphate in water makes water ‘hard’.
Hard water does not
give a lather with soap. Water free from soluble salts of calcium and
magnesium is called Soft water. It gives lather with soap easily.
Hard water forms scum/precipitate with soap. Soap containing sodium
stearate (C17H35COONa) reacts with hard water to precipitate
out Ca/Mg-stearate
Hence hard water is unsuitable
for laundry. It is harmful to boilers as well, because of the deposition of salts
in the form of scale. This reduces
the efficiency of the boiler. The hardness of water is of two types:
(i) temporary hardness, and (ii) permanent hardness.
Temporary Hardness
Temporary hardness is due to the presence of magnesium and calcium hydrogen carbonates. It can be removed by:
(i) Boiling: During boiling, the soluble Mg(HCO3)2 is
converted into insoluble Mg(OH)2 and Ca(HCO3)2
is changed to insoluble CaCO3. It is because of the high solubility
product of Mg(OH)2 as compared to that of MgCO3, that Mg(OH)2
is precipitated. These precipitates can be removed by filtration. The filtrate thus
obtained will be soft water.
(ii) Clark’s method: In this method calculated amount of lime is added to hard water. It precipitates
out calcium carbonate and magnesium hydroxide which can be filtered off.
Permanent Hardness It is due to the presence
of soluble salts of magnesium and calcium in the form of chlorides and
sulphates in water. Permanent hardness is not removed by boiling. It can be removed
by the following methods:
(i) Treatment with washing soda (sodium carbonate):
Washing soda reacts with soluble calcium and magnesium chlorides
and sulphates in hard
water to form insoluble carbonates.
(ii) Calgon’s method: Sodium hexametaphosphate (Na6P6O18),
commercially called ‘Calgon’, when added to hard water, the following reactions
take place.
The complex anion keeps the Mg 2+ and Ca 2+ ions
in solution.
(iii) Ion-exchange method: This method is also called the zeolite/permutit process. Hydrated sodium
aluminium silicate is zeolite/permutit. For the sake of simplicity, sodium
aluminium silicate (NaAlSiO4) can be written as NaZ. When this is
added to hard water, exchange reactions take place.
Permutit/zeolite is said to be exhausted when all the sodium in it is
used up. It is regenerated for further use by treating it with an aqueous sodium
chloride solution.
(iv) Synthetic resins method: Nowadays hard water is softened by using synthetic cation exchangers.
This method is more efficient than the zeolite process. Cation exchange resins contain
large organic molecules with - SO3H
group and are water insoluble. Ion exchange resin (RSO3H) is
changed to RNA by treating it with NaCl. The resin exchanges Na+
ions with Ca 2+ and Mg 2+ ions present in hard water to make
the water soft. Here R is resin anion.
The resin can be regenerated by adding an aqueous NaCl solution.
Pure de-mineralised
(de-ionized) water free from all soluble mineral salts is
obtained by passing water successively through a cation exchange (in the H+
form) and anion exchange (in the OH– form) resins:
In this cation exchange process, H+ exchanges for Na+,
Ca2+, Mg2+ and other cations present in water. This
process results in proton release and thus makes the water acidic. In the anion
exchange process.
OH–exchanges for anions like Cl–, HCO3–,
SO42–etc. present in water. OH– ions, thus,
liberated neutralise the H+ ions set free in the cation exchange.
The exhausted cation and anion exchange resin beds are regenerated by
treatment with dilute acid and alkali solutions respectively.
HYDROGEN PEROXIDE (H2O2)
Hydrogen peroxide is an important chemical used in the pollution control
treatment of domestic and industrial effluents.
Preparation It can be prepared by the following methods.
(i) Acidifying barium peroxide and removing excess water by evaporation
under reduced pressure gives hydrogen peroxide.
(ii) Peroxodisulphate, obtained by electrolytic oxidation of
acidified sulphate solutions at high current density,
on hydrolysis yields hydrogen peroxide.
This method is now used for the laboratory preparation of D2O2.
(iii) Industrially it is
prepared by the autooxidation of
2-alklylanthraquinols.
In this case, 1% H2O2 is formed. It is extracted
with water and concentrated to ~30% (by mass) by distillation under reduced pressure.
It can be further concentrated to ~85% by careful distillation under low pressure.
The remaining water can be frozen out to obtain pure H2O2.
Physical Properties-- In the pure state H2O2 is an almost colourless (very
pale blue) liquid.. H2O2 is miscible with water in all proportions
and forms a hydrate H2O2.H2O (mp 221K).
A 30% solution of H2O2 is marketed
as ‘100 volume hydrogen peroxide. It means that one millilitre of 30%
H2O2 solution will give 100 V of oxygen at STP.
Commercially, it is marketed as 10 V, which means it contains 3% H2O2
Structure-- Hydrogen peroxide has a non-planar structure.
The molecular dimensions in the gas phase and solid phase.
Fig. (a) H2O2 structure in the gas phase, the dihedral angle is 111.5°. (b) H2O2 structure
in the solid phase at 110K, the dihedral angle is 90.2°.
Chemical Properties--It acts as an
oxidising as well as a reducing agent in both acidic and alkaline media.
Simple
reactions are described below.
(i) Oxidising action in
acidic medium-
Second reaction is used to brighten the old oil paintings.
(ii) Reducing action in
acidic medium
(iii) Oxidising action in
basic medium
(iv) Reducing action in
basic medium
Storage----H2O2 decomposes
slowly on exposure to light.
In the presence of metal
surfaces or traces of alkali (present in glass containers), the above reaction
is catalysed. It is, therefore, stored
in wax-lined glass or plastic vessels in dark. Urea can be added as a stabiliser. It is kept away from dust
because dust can induce explosive decomposition of the compound.
Uses---Its wide-scale use has led to a tremendous increase
in the industrial production of H2O2. Some uses
are --
(i) In daily life it is used as a hair bleach and as a mild
disinfectant. As an antiseptic, it is sold in the market as perhydrol.
(ii) It is used to manufacture chemicals like sodium perborate and
per-carbonate, which are used in high-quality detergents.
(iii) It is used in the synthesis of hydroquinone, tartaric acid and
certain food products and pharmaceuticals (cephalosporin) etc.
(iv) It is employed in the industries as a bleaching agent for
textiles, paper pulp, leather, oils, fats, etc.
(v) Nowadays it is also used in Environmental (Green) Chemistry. For
example, in pollution control treatment of domestic and industrial effluents,
oxidation of cyanides, restoration of aerobic conditions to sewage wastes, etc.
HEAVY WATER, D2O
It is extensively used as a
moderator in nuclear reactors and in exchange reactions for the study of reaction mechanisms. It can
be prepared by exhaustive
electrolysis of water or as a
by-product in some fertilizer industries.
It is used for the preparation of other deuterium compounds.
DIHYDROGEN AS A FUEL
Dihydrogen releases large quantities of heat during combustion. On a mass-for-mass basis, dihydrogen can release more energy than petrol (about three times). Moreover,
pollutants in the combustion of dihydrogen
will be less than in petrol. The only pollutants
will be the oxides of dinitrogen (due to the presence of dinitrogen as an impurity
with dihydrogen). This, of course, can
be minimized by injecting a small amount of water into the
cylinder to lower the
temperature so that the reaction between dinitrogen and dioxygen may not take
place. However, the mass of the containers in which dihydrogen will be kept
must be taken into consideration. A
cylinder of compressed dihydrogen weighs about 30 times as much as a tank of
petrol containing the same amount of energy. Also, dihydrogen gas is converted into a liquid state by
cooling to 20K. This would require
expensive insulated tanks. Tanks of
metal alloys like NaNi5, Ti–TiH2, Mg–MgH2 etc. are in use for the storage of
dihydrogen in small quantities.
These limitations have
prompted researchers to search for alternative techniques to use dihydrogen in
an efficient way.
In this view Hydrogen Economy
is an alternative. The basic principle of a hydrogen economy is the
transportation and storage of energy in the form of liquid or gaseous dihydrogen. The advantage of a hydrogen economy is
that energy is transmitted in the form of dihydrogen and not as electric power.
It is for the first time in the history of India that a pilot project using
dihydrogen as fuel was launched in October 2005 for running automobiles. Initially, 5% dihydrogen has been mixed in CNG for use in four-wheeler vehicles. The percentage
of dihydrogen would be gradually increased to reach the optimum level. Nowadays,
it is also used in fuel cells for the generation of electric power. It is expected
that economically viable and safe sources of dihydrogen will be identified in
the years to
come, for its usage as a common source of energy.
