284 CHEMISTRY
UNIT 9
After studying this unit, you will be
able to
••
present informed opinions on the
position of hydrogen in the
periodic table;
••
identify the modes of occurrence
and preparation of dihydrogen on
a small and commercial scale;
describe isotopes of hydrogen;
••
••
explain how different elements
combine with hydrogen to form
ionic, molecular and non-
stoichiometric compounds;
••
••
describe how an understanding of
its properties can lead to the
production of useful substances,
and new technologies;
••
••
understand the structure of water
and use the knowledge for
explaining physical and chemical
properties;
••
••
explain how environmental water
quality depends on a variety of
dissolved substances; difference
between 'hard' and 'soft' water and
learn about water softening;
••
••
acquire the knowledge about
heavy water and its importance;
••
••
understand the structure of
hydrogen peroxide, learn its
preparatory methods and
properties leading to the
manufacture of useful chemicals
and cleaning of environment;
••
••
understand and use certain terms
e.g., electron-deficient, electron-
precise, electron-rich, hydrogen
economy, hydrogenation etc.
HYDROGEN
Hydrogen, the most abundant element in the universe and the
third most abundant on the surface of the globe, is being
visualised as the major future source of energy.
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 (H
2
) molecule and is called
dihydrogen. It forms more compounds than any other
element. Do you know that the global concern related to
energy can be overcome to a great extent by the use of
hydrogen as a source of energy? In fact, hydrogen is of
great industrial importance as you will learn in this unit.
9.1 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. As you know by now
that the elements in the periodic table are arranged
according to their electronic configurations.
Hydrogen has electronic configuration 1s
1
. On one
hand, its electronic configuration is similar to the outer
electronic configuration (ns
1
) of alkali metals , which belong
to the first group of the periodic table. On the other hand,
like halogens (with ns
2
np
5
configuration belonging to the
seventeenth group of the periodic table), it is short by one
electron to the corresponding noble gas configuration,
helium (1s
2
). Hydrogen, therefore, has resemblance to
alkali metals, which lose one electron to form unipositive
ions, as well as with halogens, which gain one electron to
form uninegative ion. Like alkali metals, hydrogen forms
oxides, halides and sulphides. However, unlike alkali
metals, it has a very high ionization enthalpy and does not
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285HYDROGEN
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 (Unit 3).
9.2 DIHYDROGEN, H
2
9.2.1 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
Property Hydrogen Deuterium Tritium
Relative abundance (%) 99.985 0.0156 10
–15
Relative atomic mass (g mol
–1
) 1.008 2.014 3.016
Melting point / K 13.96 18.73 20.62
Boiling point/ K 20.39 23.67 25.0
Density / gL
–1
0.09 0.18 0.27
Enthalpy of fusion/kJ mol
–1
0.117 0.197 -
Enthalpy of vaporization/kJ mol
–1
0.904 1.226 -
Enthalpy of bond
dissociation/kJ mol
–1
at 298.2K 435.88 443.35 -
Internuclear distance/pm 74.14 74.14 -
Ionization enthalpy/kJ mol
–1
1312 - -
Electron gain enthalpy/kJ mol
–1
–73 - -
Covalent radius/pm 37 - -
Ionic radius(H
)/pm 208
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.
9.2.2 Isotopes of Hydrogen
Hydrogen has three isotopes: protium,
1
1
H,
deuterium,
2
1
H or D and tritium,
3
1
H or T. Can
you guess how these isotopes differ from each
other ? 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.
The predominant form is protium.
Terrestrial hydrogen contains 0.0156% of
deuterium mostly in the form of HD. The
tritium concentration is about one atom per
10
18
atoms of protium. Of these isotopes, only
tritium is radioactive and emits low energy
β
particles (t
½
, 12.33 years).
Table 9.1 Atomic and Physical Properties of Hydrogen
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286 CHEMISTRY
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 (Table
9.1). However, in physical properties these
isotopes differ considerably due to their large
mass differences.
9.3 PREPARATION OF DIHYDROGEN, H
2
There are a number of methods for preparing
dihydrogen from metals and metal hydrides.
9.3.1 Laboratory Preparation of
Dihydrogen
(i) It is usually prepared by the reaction of
granulated zinc with dilute hydrochloric
acid.
Zn + 2H
+
Zn
2+
+ H
2
(ii) It can also be prepared by the reaction of
zinc with aqueous alkali.
Zn + 2NaOH
Na
2
ZnO
2
+ H
2
Sodium zincate
9.3.2 Commercial Production of
Dihydrogen
The commonly used processes are outlined
below:
(i) Electrolysis of acidified water using
platinum electrodes gives hydrogen.
(
)
(
)
(
)
Electrolysis
2
22
Traces of acid / base
2H O l 2Hg Og
 +
(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)
Cl
2
(g) + 2e
at cathode: 2H
2
O (l) + 2e
H
2
(g) + 2OH
(aq)
The overall reaction is
2Na
+
(aq) + 2Cl
(aq) + 2H
2
O(l)
Cl
2
(g) + H
2
(g) + 2Na
+
(aq) + 2OH
(aq)
(iv) Reaction of steam on hydrocarbons or coke
at high temperatures in the presence of
catalyst yields hydrogen.
+
+  + +
1270K
n 2n 2 2 2
Ni
C H nH O nCO (2n 1)H
e.g.,
(
)
(
)
(
)
(
)
1270K
42 2
Ni
CH g H O g CO g 3H g
+  +
The mixture of CO and H
2
is called water
gas. As this mixture of CO and H
2
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'.
(
)
(
)
(
)
(
)
1270K
2 2
+  +
The production of dihydrogen can be
increased by reacting carbon monoxide of
syngas mixtures with steam in the presence of
iron chromate as catalyst.
(
)
(
)
(
)
(
)
673 K
2 22
catalyst
COg HOg COg Hg
+ → +
This is called water-gas shift reaction.
Carbon dioxide is removed by scrubbing with
sodium arsenite solution.
Presently ~77% of the industrial
dihydrogen is produced from petro-chemicals,
18% from coal, 4% from electrolysis of aqueous
solutions and 1% from other sources.
9.4 PROPERTIES OF DIHYDROGEN
9.4.1 Physical Properties
Dihydrogen is a colourless, odourless,
tasteless, combustible gas. It is lighter than
air and insoluble in water. Its other physical
properties alongwith those of deuterium are
given in Table 9.1.
9.4.2 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. What inferences would you
draw from this fact ? 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
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287HYDROGEN
high H–H bond enthalpy. Thus, the atomic
hydrogen is produced at a high temperature
in an electric arc or under ultraviolet
radiations. Since its orbital is incomplete with
1s
1
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, X
2
to give hydrogen halides, HX,
(
)
(
)
(
)
2 2
H g X g 2HX g (X F,Cl, Br,I)
+→ =
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.
2H
2
(g) + O
2
(g) 2H
2
O(l);
H
= –285.9 kJ mol
–1
Reaction with dinitrogen: With dinitrogen
it forms ammonia.
(
)
(
)
(
)
673 K ,200atm
22 3
Fe
1
3H g N g 2NH g ;
92.6 kJ mol
+ 
=−H
This is the method for the manufacture of
ammonia by the Haber process.
Reactions with metals: With many metals it
combines at a high temperature to yield the
corresponding hydrides (section 9.5)
H
2
(g) +2M(g) 2MH(s);
where M is an alkali metal
Reactions with metal ions and metal
oxides: It reduces some metal ions in aqueous
solution and oxides of metals (less active than
iron) into corresponding metals.
(
)
(
)
(
)
(
)
() () () ()
2
2
2 xy 2
H g Pd aq Pd s 2H aq
yH g M O s xM s yH O l
+ +
+ →+
+ →+
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 of vegetable oils using
nickel as catalyst gives edible fats
(margarine and vanaspati ghee)
(ii) Hydroformylation of olefins yields
aldehydes which further undergo
reduction to give alcohols.
2 2 22
H CO RCH CH RCH CH CHO
++ =
2 22 222
H RCH CH CHO RCH CH CH OH
+
Problem 9.1
Comment on the reactions of dihydrogen
with (i) chlorine, (ii) sodium, and (iii)
copper(II) oxide
Solution
(i) Dihydrogen reduces chlorine into
chloride (Cl
) ion and itself gets oxidised
to H
+
ion by chlorine to form hydrogen
chloride. An electron pair is shared
between H and Cl leading to the formation
of a covalent molecule.
(ii) Dihydrogen is reduced by sodium to
form NaH. An electron is transferred from
Na to H leading to the formation of an ionic
compound, Na
+
H
.
(iii) Dihydrogen reduces copper(II) oxide
to copper in zero oxidation state and itself
gets oxidised to H
2
O, which is a covalent
molecule.
9.4.3 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.
(
)
(
)
(
)
cobalt
23
catalyst
CO g 2H g CH OH l
+ 
It is widely used for the manufacture of
metal hydrides (Section 9.5)
It is used for the preparation of hydrogen
chloride, a highly useful chemical.
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288 CHEMISTRY
In metallurgical processes, it is used to
reduce heavy metal oxides to metals.
Atomic hydrogen and oxy-hydrogen
torches find use for cutting and welding
purposes. Atomic hydrogen atoms
(produced by dissociation of dihydrogen
with the help of an electric arc) are allowed
to recombine on the surface to be welded
to generate the temperature of 4000 K.
It is used as a rocket fuel in space research.
Dihydrogen is used in fuel cells for
generating electrical energy. It has many
advantages over the 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.
9.5 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 EH
x
(e.g., MgH
2
)
or
E
m
H
n
(e.g., B
2
H
6
).
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
9.5.1 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, significant covalent
character is found in the lighter metal hydrides
such as LiH, BeH
2
and MgH
2
. In fact BeH
2
and
MgH
2
are polymeric in structure. The ionic
hydrides are crystalline, non-volatile and non-
conducting in solid state. However, their melts
conduct electricity and on electrolysis liberate
dihydrogen gas at anode, which confirms the
existence of H
ion.
(
)
(
)
anode
2
2Hmelt Hg 2e
→ +
Saline hydrides react violently with water
producing dihydrogen gas.
(
)
(
)
(
)
(
)
2 2
NaH s H O aq NaOH aq H g
+→ +
Lithium hydride is rather unreactive at
moderate temperatures with O
2
or Cl
2
. It is,
therefore, used in the synthesis of other useful
hydrides, e.g.,
8LiH + Al
2
Cl
6
2LiAlH
4
+ 6LiCl
2LiH + B
2
H
6
2LiBH
4
9.5.2 Covalent or Molecular Hydride
Dihydrogen forms molecular compounds with
most of the p-block elements. Most familiar
examples are CH
4
, NH
3
, H
2
O and HF. For
convenience hydrogen compounds of non-
metals 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, (ii) electron-precise,
and (iii) electron-rich hydrides.
An electron-deficient hydride, as the name
suggests, has too few electrons for writing its
conventional Lewis structure. Diborane (B
2
H
6
)
is an example. In fact all elements of group 13
will form electron-deficient compounds. What
do you expect from their behaviour? They act
as Lewis acids i.e., electron acceptors.
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., CH
4
)
which are tetrahedral in geometry.
Electron-rich hydrides have excess
electrons which are present as lone pairs.
Elements of group 15-17 form such
compounds. (NH
3
has 1- lone pair, H
2
O 2
and HF –3 lone pairs). What do you expect from
the behaviour of such compounds ? 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.
Problem 9.2
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.
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289HYDROGEN
Solution
On the basis of molecular masses of NH
3
,
H
2
O and HF, their boiling points are
expected to be lower than those of the
subsequent group member hydrides.
However, due to higher electronegativity
of N, O and F, the magnitude of hydrogen
bonding in their hydrides will be quite
appreciable. Hence, the boiling points
NH
3
, H
2
O and HF will be higher than the
hydrides of their subsequent group
members.
9.5.3 Metallic or Non-stoichiometric
(or Interstitial ) Hydrides
These are formed by many d-block and f-block
elements. However, the metals of group 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 non-
stoichiometric, being deficient in hydrogen. For
example, LaH
2.87
, YbH
2.55
, T iH
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 good.
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 lattice 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 large number of compounds.
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 high potential for
hydrogen storage and as a source of energy.
Problem 9.3
Can phosphorus with outer electronic
configuration 3s
2
3p
3
form PH
5
?
Solution
Although phosphorus exhibits +3 and +5
oxidation states, it cannot form PH
5
.
Besides some other considerations, high
a
H value of dihydrogen and
eg
H value
of hydrogen do not favour to exhibit the
highest oxidation state of P, and
consequently the formation of PH
5
.
9.6 WATER
A major part of all living organisms is made
up of water. 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. The estimated world water
supply is given in Table 9.2
Table 9.2 Estimated World Water Supply
Source % of Total
Oceans 97.33
Saline lakes and inland seas 0.008
Polar ice and glaciers 2.04
Ground water 0.61
Lakes 0.009
Soil moisture 0.005
Atmospheric water vapour 0.001
Rivers 0.0001
9.6.1 Physical Properties of Water
It is a colourless and tasteless liquid. Its
physical properties are given in Table 9.3 along
with the physical properties of heavy water.
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 high freezing point, high boiling point, high
heat of vaporisation and high heat of fusion in
comparison to H
2
S and H
2
Se. 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.
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290 CHEMISTRY
The high heat of vaporisation and heat
capacity are responsible for moderation of the
climate and body temperature of living beings.
It is an excellent solvent for 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.
9.6.2 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 9.1(a). It is a highly
Property H
2
O D
2
O
Molecular mass (g mol
–1
) 18.0151 20.0276
Melting point/K 273.0 276.8
Boiling point/K 373.0 374.4
Enthalpy of formation/kJ mol
–1
–285.9 –294.6
Enthalpy of vaporisation (373K)/kJ mol
–1
40.66 41.61
Enthalpy of fusion/kJ mol
–1
6.01 -
Temp of max. density/K 276.98 284.2
Density (298K)/g cm
–3
1.0000 1.1059
Viscosity/centipoise 0.8903 1.107
Dielectric constant/C
2
/N.m
2
78.39 78.06
Electrical conductivity (293K/ohm
–1
cm
–1
) 5.7×10
–8
-
Table 9.3 Physical Properties of H
2
O and D
2
O
polar molecule, (Fig 9.1(b)). Its orbital overlap
picture is shown in Fig. 9.1(c). 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. Density of ice is
less than that of water. Therefore, an ice cube
floats on water. In 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.
9.6.3 Structure of Ice
Ice has a highly ordered three dimensional
hydrogen bonded structure as shown in
Fig. 9.2. Examination of ice crystals with
Fig. 9.1 (a) The bent structure of water; (b) the
water molecule as a dipole and
(c) the orbital overlap picture in water
molecule.
Fig. 9.2 The structure of ice
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