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Hydrogen
(1) Position of hydrogen in the periodic table
Hydrogen is the first element in the periodic
table. Hydrogen is placed in no specific group due to its
property of giving electron (When
H is formed) and
also losing electron (When
H is formed).
(i) Hydrogen is placed in group I (Alkali metals)
as,
(a) It has one electron in its (Outer) shell-
1
1 s like
other alkali metals which have (inert gas)
1
ns
configuration.
(b) It forms monovalent
H ion like
Li , Na
(c) Its valency is also 1.
(d) Its oxide( )
2
HO is stable as Li ONaO
2 2
(e) It is a good reducing agent (In atomic as well
as molecular state) like Na , Li
(ii) Hydrogen also resembles halogens (Group VII
A) as,
(a) It is also diatomic( )
2
H like
2 2
F , Cl
(b) It also forms anion
H like
F , Cl by gain
of one electron.
(c)
H has stable inert gas ( He )configuration as
4 2 6
CH , CH like halogens
4 2 2
CCl , SFCl etc.
(d) H is one electron short of duplet (Stable
configuration) like F , Cl ,which are also one electron
deficient than octet,
2 5
F 2 s 2 p ;
2 5
Cl 3 s 3 p.
(e) ( IE ) of ( 1312 )
1
H kJ mol is of the same order as
that of halogens.
(iii) ( IE ) of H is very high in comparison with
alkali metals. Also size of
H is very small compared to
that of alkali metal ion. H forms stable hydride only
with strongly electropositive metals due to smaller
value of its electron affinity( 72. 8 )
1
kJ mol.
(iv) In view of the anomalous behaviour of
hydrogen, it is difficult to assign any definite position
to it in the periodic table. Hence it is customary to
place it in group I (Along with alkali metals) as well as
in group VII (Along with halogens).
(2) Discovery and occurrence : It was discovered
by Henry Cavendish in 1766. Its name hydrogen was
proposed by Lavoisier. Hydrogen is the 9
th
most
abundant element in the earth’s crust.
Hydrogen exists in diatomic state but in
triatomicstate it is called as Hyzone. Systematic name
of water is oxidane.
(3) Preparation of Dihydrogen : Dihydrogen can
be prepared by the following methods,
(i) By action of water with metals
(a) Active metals like Na , K react at room
temperature
2 2
2 M 2 HO 2 MOH H [ M = Na , K etc.]
(b) Less active metals like Ca , Zn , Mg , Al liberate
hydrogen only on heating.
2 2 3 2
2 Al 3 HO AlO 3 H
(c) Metals like Fe , Ni , Co , Sn can react only when
steam is passed over red hot metals.
Hydrogen and Its compounds
Chapter
2
Ferrosoferric oxide
2 3 4
3 Fe 4 HO (steam) FeO 4 H
(ii) By the action of water on alkali and alkaline
earth metals hydrides
2 2
NaH HO NaOH H
2 2 2 2
CaH 2 HO Ca ( OH ) 2 H
(iii) By reaction of metals like Zn, Sn, Al with
alkalies (NaOH or KOH)
2
sod. zincate
2 2
Zn 2 NaOH NaZnO H
2
Sod. meta-aluminate
2 2
Al 2 NaOH HO 2 NaAlO 2 H
2 2 3 2
Silicon
Si 2 NaOH 2 HO NaSiO 3 H
2
Sod. stannite
2 2
Tin
Sn 2 NaOH NaSnO H
(iv) By action of metal with acids : All active
metals which lie above hydrogen in electrochemical
series, can displace hydrogen gas from dilute mineral
acids like HCl ,
2 4
HSO
2 2
Fe 2 HCl FeCl H
(v) By the electrolysis of acidified water
At anode
2
At cathode
2
/Electroly sis
2
H O H O
H
(vi) Laboratory method : In laboratory, it is
obtained by action of granulated zinc with dilute
2 4
HSO
2 4 4 2
Zn dil. HSO ZnSO H
It must be noted that
(a) Pure zinc is not used for the preparation of
2
H as rate of reaction of pure Zn with dil.
2 4
HSO is
quite slow.
(b) Conc.
2 4
HSO is not used because then
2
SO gas
is evolved instead of
2
H
(vii) Preparation of pure hydrogen : It can be
obtained by
(a) The action of pure dil.
2 4
HSO on pure
magnesium ribbon.
2 4 4 2
Mg HSO MgSO H
(b) Hydrogen of high purity (> 99.95%) is
obtained by electrolysing warm aqueous barium
hydroxide between nickel electrodes.
(c) By the action of water on sodium hydride.
2 2
NaH HO NaOH H
(d) By the action of KOH (aq.) on aluminium.
2 2 2
2 Al 2 KOH 2 HO 2 KAlO 3 H
(viii) Commercial production of dihydrogen
(a) Bosch process : In this method, water gas is
mixed with twice its volume of steam and passed over
heated catalyst
2 3
FeO in the presence of a promoter
2 3
CrO
or
2
ThO
at 773 K when
2
CO
and
2
H
are
obtained.
2
CO is removed by dissolving it in water
under pressure (20- 25 atm ) and
2
H left undissolved is
collected.
Watergas
2
1270
2
C HO CO H
K
2 2
,
773
2 2
23 23
H CO HO CO H
FeO CrO
K
About 18% of the world’s production of
2
H is
obtained from coal.
(b) Lane’s process : By passing steam over
spongy iron at 773 - 1050 K.
2 3 4 2
3 Fe 4 HO FeO 4 H
The ferrosoferric oxide ( )
3 4
FeO so produced is
reduced back to iron with water. this reaction is known
as Vivification reactions
Fe O H Fe HO
3 4 2 2
3 4 2
Fe O 4 CO 3 Fe 4 CO
(c) By electrolysis of water : Electrolysis of
acidified water using platinum electrodes is used for
the bulk preparation of hydrogen.
(d) From hydrocarbons : Hydrocarbons (alkanes)
react with steam at high temperature to produce
carbon monoxide and hydrogen, e.g. ,
2
Cataly st
1270
4 2
CH g HOg COg H g
K
The mixture of CO and
2
H
so obtained can be
converted into hydrogen as in Bosch process. About
77% of the world’s production of
2
H
is obtained from
hydrocarbons.
(e) It is also produced as a by-product of the brine
electrolysis process for the manufacture of
2
Cl and
NaOH.
(4) Physical properties of dihydrogen : It is a
colourless, tasteless and odourless gas. It is slightly
soluble in water. It is highly combustible. The Physical
constants of atomic hydrogen are,
Atomic radius (pm) – 37
Ionic radius of
H ion (pm) – 210
Ionisation energy( )
1
kJ mol – 1312
ordinary hydrogen is passed through acidified 4
KMnO
(pink in colour), its colour is not discharged. On the
other hand, if zinc pieces are added to the same
solution, bubbles of hydrogen rise through the solution
and the colour is discharged due to the reduction on
4
KMnO by nascent hydrogen.
KMnO H HSO Noreaction
Molecular
4 2 2 4
Nascenthydrogen
Zn HSO ZnSO 2 [ H ]
2 4 4
KMnO HSO H KSO MnSO HO
4 2 4 2 4 4 2
2 3 10 [ ] 2 8
(3) Ortho and para hydrogen : A molecule of
dihydrogen contains two atoms. The nuclei of both the
atoms in each molecule of dihydrogen are spinning.
Depending upon the direction of the spin of the nuclei,
the hydrogen is of two types,
Fig. 17.
(i) Molecules of hydrogen in which the spins of
both the nuclei are in the same directions, called ortho
hydrogen.
(ii) Molecules of hydrogen in which the spins of
both the nuclei are in the opposite directions, called
para hydrogen.
Ordinary dihydrogen is an equilibrium mixture of
ortho and para hydrogen. Ortho hydrogen ⇌ Para
hydrogen. The amount of ortho and para hydrogen
varies with temperature as,
(a) At 0° K , hydrogen contains mainly para
hydrogen which is more stable.
(b) At the temperature of liquefaction of air, the
ratio of ortho and para hydrogen is 1:1.
(c) At the room temperature, the ratio of ortho to
para hydrogen is 3:1.
(d) Even at very high temperatures, the ratio of
ortho to para hydrogen can never be more than 3:1.
Thus, it has been possible to get pure para
hydrogen by cooling ordinary hydrogen gas to a very
low temperature (close to 20 K ) but it is never possible
to get a sample of hydrogen containing more than 75%
of ortho hydrogen. i.e., Pure ortho hydrogen can not be
obtained.
(4) Hydrides : Hydrogen forms binary hydrides of
the type
x
MH or
m n
MH with
(a) All main group elements except noble gases
and probably indium and thallium.
(b) All lanthanoids and actinoids.
(c) Transition metals ( Sc , Y , La , Ac , Tc , Zr , Hf and
to a lesser extent V , Nb , Ta , Cr , Cu and Zn ). In group 6
only Cr forms hydride ( CrH ).
Hydrides are classified into three main categories.
(i) Saline or ionic hydrides : Most of the s - block
metals form this type of hydrides. These are non-
volatile, non-conducting crystalline solids. However,
2
BeH and
2
MgH have covalent polymeric structure.
These ionic hydrides have rock-salt structure. Thermal
stability of 1
st
and 2
nd
group hydrides are in the order;
LiH > NaH > KH > RbH > CsH
2 2 2
CaH SrH BaH
2 2
BeH , MgH and LiH have significant covalent
character.
Electrolysis of solution of saline hydride in
molten alkali halide produces
2
H at anode. Saline
hydrides react explosively with water.
2 2
NaH s HOaq NaOHaq H g
The fire so produced cannot be extinguished by
2
CO as it gets reduced by the hot metal hydride. Only
sand is useful, as it is a solid.
Alkali metal hydrides are used for making
4 4
LiAlH , NaBH etc. Alkali metal hydrides are also used
for the removal of last traces of water from organic
compounds.
(ii) Metallic or interstitial hydrides : Elements
of groups 3, 4, 5 ( d - block) and f - block elements form
metallic hydrides. In group 6, only Cr forms hydride
( CrH ). Metals of group 7, 8, 9 do not form hydrides.
This region of periodic table from group 7 to group 9 is
known as hydride gap. Examples of hydrides of group 3
to 5 are, , , , , , , , , ,
2 2 3 2 3 2 2 2
ScH YH YH LaH LaH TiH ZrH HfH VH
VH , NbH , NbH , TaH
2 2
The f - block metals form hydrides of limiting
compositions of
2
MH and
3
MH. All these hydrides are
non-stoichiometric with variable composition e.g. ,
Para hydrogen
Ortho hydrogen
Nuclei
ZrH ( 1. 30 x 1. 75 )
x
TiH ( 1. 8 x 2. 0 )
x
Most of these hydrides are good conductors of
electricity in solid state.
Metallic hydrides can be used to store hydrogen
especially in cars working on fuel cells.
(iii) Molecular or covalent hydrides : Hydrogen
form molecular compounds with p - block elements ( B ,
C , N , O , F ; Si , P , S , Cl ; Ga , Ge , As , Sb , Br ; In , Sn , Sb , Te ,
I ; Tl , Pb , At ). common examples of such hydrides are
CH , NH , HO , HF
4 3 2
etc. The stability of these hydrides
decreases down the group. For example,
3 3 3 3 3
NH PH AsH SbH BiH. In a period the stability
increases with increasing electronegativity. For
example, CH NH HO HF 4 3 2
. Molecular hydrides
are classified as electron rich, electron precise and
electron deficient hydrides.
(a) Electron rich molecular hydrides : These
hydrides have one or more lone pairs of electrons
around the central more electronegative element. For
example
H O H
..
, H
H
H N
|
..
H F
(b) Electron precise molecular hydrides : Elements
of group 14 form such hydrides. The bond length
increases on going down the group. A common example
of electron precise molecular hydrides is 4
CH.
(c) Electron deficient molecular hydrides : These
hydrides have lesser number of electrons than that
required for writing the conventional Lewis structure.
A common example of such molecular hydride is
diborane,
2 6
BH
(d) Systematic names of molecular hydrides : The
systematic names of these hydrides are obtained from
the name of the element and the suffix – ane. For
example,
Phosphane
3
PH
oxidane
2
HO
ozane
3
NH
Isotopes of Hydrogen
Isotopes are the different forms of the same
element, which have the same atomic number but
different mass numbers.
Table 17.1 Isotopes of hydrogen
Name Symbo
l
Atomic
numbe
Mass
numbe
Relative
abundanc
Nature
radioactive
r r e or non-
radioactive
Protium
or
Hydroge
n
H
1
1
or
H
1 1 99.985% Non-
radioactive
Deuteriu
m
H
2
1
or
D
1 2 0.015% Non-
radioactive
Tritium
H
3
1
or
T
1 3 15
10
%
Radioactive
Table 17.2 Physical constants of H 2 , D 2 and T 2
Property H 2 D 2 T 2
Molecular mass 2.016 4.028 6.
Melting point ( K ) 13.8 18.7 20.
Boiling point ( K ) 20.4 23.9 25.
Heat of fusion(kJ mol )
Heat of vaporisation
(kJ mol )
0.994 1.126 1. 393
Bond energy(kJ mol )
Isotopic effect : In general chemical properties of
isotopes are same but quantiative differences are
noticed amongst them. For example, the reaction
between
2
H and
2
Cl is 13.4 times faster between
2
D
and
2
Cl under similar conditions. Such differences in
chemical properties, which are due to difference in the
mass numbers of isotopes is known as isotopic effect.
Water
Water is the oxide of hydrogen. It is an important
component of animal and vegetable matter. Water
constitutes about 65% of our body. It is the principal
constituent of earth’s surface.
(1) Structure : Due to the presence of lone pairs,
the geometry of water is
distorted and the H O H bond
angle is 104.5°, which is less
than the normal tetrahedral
angle (109.5°). The geometry of
the molecule is regarded as
angular or bent. In water, each
O H bond is polar because of the high
electronegativity of oxygen (3.5) in comparison to that
of hydrogen (2.1). The resultant dipole moment of
water molecule is 1.84D.
In ice , each oxygen atom is tetrahedrally
surrounded by four hydrogen atoms; two by covalent
bonds and two by hydrogen bonds. The resulting
structure of ice is open structure having a number of
vacant spaces. Therefore, the density of ice is less than
that of water and ice floats over water. It may be noted
Lone Pair
of
Electron
: O
H
104.5 H
o
Hard water does not produce lather because the
cations( and )
2 2
Ca Mg present in hard water react with
soap to form insoluble precipitates,
M C H COONa C H COO M Na
Sodiumstearatesoap MetalstearatePPt
From hardwater
( .)
17 35 2
( )
17 35
2
Where M = Ca or Mg
Therefore, no lather is produced until all the
calcium and magnesium ions are precipitated. This also
results into wastage of lot of soap.
(ii) Type of hardness of water : The hardness of
water is of two types,
(a) Temporary hardness : This is due to the
presence of bicarbonates of calcium and magnesium. It
is also called carbonate hardness.
(b) Permanent hardness : This is due to the
presence of chlorides and sulphates of calcium and
magnesium. It is also called non-carbonate hardness.
(iii) Softening of water : The process of the
removal of hardness from water is called softening of
water.
(a) Removal of temporary hardness : It can be
removed by the following methods,
By boiling : During boiling, the bicarbonates of
Ca and Mg decompose into insoluble carbonates and
give. 2
CO The insoluble carbonates can be removed by
filtration.
Ca HCO CaCO CO HO
PPt
Heat
Cal bicarbonate
2 2
.
3
.
3 2
Mg HCO MgCO CO HO
PPt
Heat
Mag bicarbonate
2 2
.
3
.
3 2
Clark’s method : This process is used on a
commercial scale. In this process, calculated amount of
lime 2
Ca ( OH ) is added to temporary hard water.
Ca HCO CaOH CaCO HO
2
Insoluble
3
Lime
2
Soluble
3 2
( ) ( ) 2 2
Mg HCO CaOH MgCO CaCO HO
2
(Insoluble )
3 3
Lime
2
Soluble
3 2
(b) Removal of permanent hardness : Permanent
hardness can be removed by the following methods,
By washing soda method : In this method, water
is treated with a calculated amount of washing soda
2 3
NaCO which converts the chlorides and sulphates of
Ca and Mg into their respective carbonates which get
precipitated.
CaCl NaCO CaCO NaCl
ppt
.
2 2 3 3
4 2 3 3 2 4
MgSO NaCO MgCO NaSO
ppt.
Permutit method : This is a modern method
employed for the softening of hard water. hydrated
sodium aluminium silicate (. )
2 2 2 8 2
Na AlSiO xH O
is called
permutit. These complex salts are also known as
zeolites.
The permutit as loosely packed in a big tank over
a layer of coarse sand. Hard water is introduced into
the tank from the top. Water reaches the bottom of the
tank and then slowly rises through the permutit layer
in the tank. The cations present in hard water are
exchanged for sodium ions. Therefore this method is
also called ion exchange method.
Na Z Ca CaZ 2 Na
zeolite
Cal
water)
(Fromhard
2
zeolite
Sodium
2
Na Z Mg MgZ 2 Na
zeolite
Magnesium
water)
(Fromhard
2
zeolite
Sodium
2
where Z AlSiO xHO
2 2 8 2
Hydrogen peroxide
Hydrogen peroxide ( )
2 2
HO
was discovered by
French chemist Thenard.
(1) Preparation : It is prepared by
(i) Laboratory method : In laboratory,
2 2
HO is
prepared by Merck’s process. It is prepared by adding
calculated amounts of sodium peroxide to ice cold
dilute (20%) solution of
2 4
HSO
2 2 2 4 2 4 2 2
Na O H SO NaSO HO
(ii) By the action of sulphuric acid or phosphoric
acid on hydrated barium peroxide BaO HO
2 2
(a) BaO HO HSO BaSO HO HO
2 2 2 4 4 2 2 2
It must be noted that anhydrous barium peroxide
does not react readily with sulphuric acid (because a
coating of insoluble barium sulphate is formed on its
surface which stops further action of the acid).
Therefore, hydrated barium peroxide, BaO HO
2 2
. 8 must
be used.
(b)
2 3 4 3 42 2 2
3 BaO 2 HPO Ba ( PO ) 3 HO
3 42 2 4 4 3 4
Ba ( PO ) 3 HSO 3 BaSO 2 HPO
Phosphoric acid is preferred to
2 4
HSO because
soluble impurities like barium persulphate (from
2 2 2 4
BaO. 8 HO HSO ) tends to decompose
2 2
HO while
3 4
HPO
acts as preservative (negative catalyst) for
2 2
HO.
(iii) Industrial method : On a commercial scale,
2 2
HO
can be prepared by the electrolysis of 50%
2 4
HSO solution. In a cell, peroxy disulphuric acid is
formed at the anode.
2
acid
Peroxy disulphuric
2 2 8 Elecroly sis
2 4
H SO HSO aq H g
This is drawn off from the cell and hydrolysed
with water to give 2 2
H O.
2 2 8 2 2 4 2 2
H S O 2 HO 2 HSO HO The resulting
solution is distilled under reduced pressure when 2 2
HO
gets distilled while
2 4
HSO
with high boiling point,
remains undistilled.
(iv) By redox process : Industrially
2 2
HO is
prepared by the auto-oxidation of 2-alkylanthraquinols.
The process involves a cycle of reactions. The net
reaction is the catalytic union of 2
H and
2
O to give
2 2
HO.
The
2 2
HO formed (about 1%) is extracted with
water and concentrated.
(2) Physical properties
(i) Pure hydrogen peroxide is a pale blue syrupy
liquid.
(ii) It freezes at – 0.5° C and has a density of 1.
in pure state.
(iii) Hydrogen peroxide is diamagnetic.
(iv) It is more highly associated via hydrogen
bonding than water.
(v) Although it is a better polar solvent than HO
2
However, it can’t be used as such because of strong
autooxidation ability.
(vi) Dipole moment of
2 2
HO is 2.1 D.
(3) Chemical properties
(i) Decomposition : Pure
2 2
H O is an unstable
liquid and decomposes into water and 2
O either upon
standing or upon heating,
2 H O 2 HO O ; H 196. 0 kJ
2 2 2 2
(ii) Oxidising nature : It is a powerful oxidising
agent. It acts as an oxidising agent in neutral, acidic or
in alkaline medium. e.g.
2 2 2
2 KI HO 2 KOH I [In
neutral medium]
FeSO HSO HO Fe SO HO
4 2 4 2 2 2 43 2
2 ( ) 2 [In
acidic medium]
MnSO HO NaOH MnO NaSO HO
4 2 2 2 2 4 2
2 2
[In alkaline medium]
(iii) Reducing nature :
2 2
H O
has tendency to take
up oxygen from strong oxidising agents and thus, acts
as a reducing agent,
agent
From oxidising
2 2 2 2
H O O HO O
. It can act
as a reducing agent in acidic, basic or even neutral
medium.
In acidic medium,
H O 2 H O 2 e
2 2 2
In alkaline medium,
H O 2 OH 2 HO O 2 e
2 2 2 2
(iv) Bleaching action :
2 2
HO acts as a bleaching
agent due to the release of nascent oxygen.
H O HO O
2 2 2
Thus, the bleaching action of
2 2
H O is due to
oxidation. It oxidises the colouring matter to a
colourless product, Colouring matter + O Colour less
matter.
2 2
HO is used to bleach delicate materials like
ivory, silk, wool, leather etc.
(v) Acidic nature : Anhydrous hydrogen peroxide
is acidic in character
12
( 1. 55 10
a
K at 298 K ). its
dissociation in aqueous solution may be given as
2 2 2 3 2
HO HO HO HO
It forms two types of salts
NaOH HO NaHO HO
2
(Acidicsalt)
Sod. hy droperoxide
2 2 2
NaOH HO NaO HO
2
(Normalsalt)
Sod. peroxide
2 2 2 2
(vi) Addition reactions : Hydrogen peroxide is
capable of adding itself to ethylenic linkage.
Ethy lenegly col
2
2
|
2 2
Ethy lene
2
2
||
CH OH
CHOH
HO
CH
CH
(4) Structure of H 2
O
2
: Hydrogen peroxide is non-
linear, non-planar molecule. It has a open book
structure. The O O linkage is called peroxy linkage.
The structure is shown below.
H
O
O
H
(94.
)°
pm
pm
(111.5)
°
O
In gas
phase
H
O
O
H
(101.
)°
pm
pm
(90.2)°
O
In solid phase (
K )
OH
2 - Ethylanthraquinol
C 2
H 5
OH
O 2
C 2
H 5
O
O
2 -
Ethylanthraquinone
H 2
/ Pd
