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1. HYDROGEN
FOR XI STD CHEMISTRY
Cbse

2. INTRODUCTION
 Hydrogen, chemical element that exists as a gas at
room temperature. When hydrogen gas burns in
air, it forms water. French chemist Antoine
Lavoisier named hydrogen from the Greek words
for ―water former.‖
 Hydrogen has the smallest atoms of any element.
A hydrogen atom contains one proton, and only
one electron . The proton is the center, or
nucleus, of the hydrogen atom, and the electron
travels around the nucleus.
 Pure hydrogen exists as hydrogen gas, in which
pairs of hydrogen atoms bond together to make
molecules.

3. The hydrogen atom
consisting the proton
in the centre or the
nucleus of the hydro-
gen atom and the ele-
ctron travelling aroun-
d the nucleus.

4. POSITION IN THE PERIODIC TABLE
 Hydrogen is the first element in the periodic table
of the elements and is represented by the symbol
H.
 Hydrogen, with only one proton, is the simplest
element. It is usually placed in Period 1 and
Group 1 of the periodic table.
 Hydrogen can combine chemically with almost
every other element and forms more compounds
than does any other element. These compounds
include water, minerals, and hydrocarbons—
compounds made of hydrogen and carbon—such
as petroleum and natural gas.

5. Position of hydrogen in the periodic table

6. OCCURRENCE OF DIHYDROGEN
 Hydrogen is the tenth most common element on
Earth. Because it is so light, though, hydrogen
accounts for less than 1 percent of Earth's total mass.
It is usually found in compounds. Pure hydrogen gas
rarely occurs in nature, although volcanoes and some
oil wells release small amounts of hydrogen gas.
 Hydrogen is in nearly every compound in the human
body. For example, it is in keratin, the main protein
that forms our hair and skin, and in the enzymes that
digest food in our intestines. Hydrogen is in the
molecules in food that provide energy:
fats, proteins, and carbohydrates.

7. Presence of hydrogen in volcanoes and
in our food particles

8. Cont….
 Hydrogen accounts for about 73 percent of the
observed mass of the universe and is the most
common element in the universe.
 Hydrogen atoms were the first atoms to form in
the early universe and that the atoms of the other
elements formed later from the hydrogen atoms.
 About 90 percent of the atoms in the universe are
hydrogen, about 9 percent are helium, and all the
other elements account for less than 1 percent.

9. Cont….
 Common Molecules:
Many common
molecules contain
hydrogen. In these
molecules, butane
contains ten hydrogen
atoms, ammonia
contains three hydrogen
atoms, and water
contains two hydrogen
atoms.

10. INTERIOR OF THE SUN
 The Sun‘s energy is
produced in the core
through nuclear fusion
of hydrogen atoms into
helium. Gases in the
core are about 150
times as dense as water
and reach temperatures
as high as 16 million
degrees C (29 million
degrees F).

11. ISOTOPES OF HYDROGEN
 Atoms of an element that
have different numbers of
neutrons in their nuclei are
called isotopes of that
element.
 Isotopes all usually share the
same chemical behavior, but
have different masses.
 The isotopes of hydrogen are
protium, deuterium, and
tritium. Hydrogen always
has one proton in its nucleus.

12. PREPARATION OF DIHYDROGEN
 Laboratory preparation of dihydrogen:
1.It is usually prepared by the reaction of
granulated zinc with dilute hydrochloric acid.
The chemical equation for this reaction is the
following:
Zn + 2HCl → ZnCl2 + H2
2.It can also be prepared by the reaction of zinc
with aqueous alkali. The chemical equation for
this reaction is the following:
Zn +2NaOH Na2ZnO2 + H2
(Sodium zincate)

13. Dihydrogen in 3D

14. Cont….
 Commercial production of dihydrogen:
1. Electrolysis of acidified water using platinum
electrodes gives hydrogen.
2 H2O electrolysis 2H2 + O2
This chemical equation shows that two water
molecules (with electricity), form two molecules
of hydrogen gas and one molecule of oxygen gas.
2.High purity (>99.95%) dihydrogen is obtained by
electrolysing warm aqueous barium hydroxide
solution between nickel electrodes.

15. Commercial production of dihydrogen

16. Cont….
3. It is obtained as a byproduct in the manufacture of
sodium hydroxide & chlorine by the electrolysis of
brine solutions .
The reactions that takes place are:
At anode : 2Cl- Cl2 +2e-
At cathode: 2H2O + 2e- H2 + 2OH-
The overall reaction is
2Na+ + 2Cl- +2H2O
Cl2 + H2 + 2Na+ + 2OH-

17. Cont….
4. Reaction of steam on hydrocarbons at high
temperature in the presence of catalyst yields
hydrogen. e.g.,
CH4 + H2O 1270K Ni CO + 3H2
 The mixture of CO & H2 is called water gas.
 It is used for synthesis of methanol & a number of
hydrocarbons, therefore it is called synthesis gas or
‘syngas’.
 The production of dihydrogen can be increased by
reacting carbon monoxide with steam in the presence
iron chromate as catalyst.
CO + H2O 673K CO2 + H2
Catalyst
 This is called water-gas shift reaction. Carbon
dioxide is removed by scrubbing with sodium arsenite
solution.

18. PHYSICAL PROPERTIES OF H2
 Dihydrogen is a :
 Colourless ,
 Odourless
 Tasteless
 Combustible gas
 Lighter than air
 Insoluble in water
 It‘s melting point – 18.73 K
& boiling point – 23.67 K

19. CHEMICAL PROPERTIES OF H2
 Hydrogen gas does not usually react with other
chemicals at room temperature, because the bond
between the hydrogen atoms is very strong and can
only be broken with a large amount of energy.
 Since its orbital is incomplete with 1s1 electronic
configuration, it does combine with almost all the
elements .
It accomplishes reactions by:
1.loss of one e- to give H+
2.gain of an e- to form H-
3.sharing electrons to form a single covalent bond.

20. CHEMISTRY OF DIHYDROGEN
 Reaction with halogens:
It reacts with halogens, X2 to give hydrogen
halides, HX,
H2+X2 2HX (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.
2H2 + O2 catalyst or heating 2H2O ; H = -285.9
kJ mol-1

21. Cont….
 Reaction with dinitrogen:
With dintrogen it forms ammonia.
3H2 +N2 673K,200atm 2NH3; H=-92.6 kJ mol-1
This is the method for the manufacture of ammonia by Haber
process.
Haber Process:
German chemist and Nobel laureate Fritz Haber developed an
economical method of producing ammonia from air and
seawater. In his process, nitrogen is separated from the other
components of air through distillization. Hydrogen is obtained
from seawater by passing an electric current through the
water. The nitrogen and hydrogen are combined to form
ammonia (NH3).

22. Cont….
 Reaction with metals:
Hydrogen also forms ionic bonds with some
metals, at a high temperature, creating a
compound called a hydride.
H2 +2M 2MH
Where M is an alkali metal (e.g.
lithium, sodium, potassium, rubidium, cesium, an
d francium.)
 Reactions with metal ions & metal oxides:
It reduces some metal ions in aqueous solution &
oxides of metals (less active than iron ) into
corresponding metals.
H2+Pd 2+ Pd + 2H+
yH +M O xM + yH O

23. Cont….
 Reactions with organic compounds:
1.Hydrogenation of vegetable oils using nickel as
catalyst gives edible fats. (margarine & vanaspati
ghee).
2.Hydroformylation of olefins yields aldehydes
which further undergo reduction to give alcohols.
H2+CO+RCH=CH2 RCH2CH2CHO
H2 +RCH2CH2CHO RCH2CH2CH2OH

24. USES OF DIHYDROGEN
 The largest use of dihydrogen is in the synthesis
of ammonia which is used in the manufacture of
nitric acid & nitrogenous fertilizers.
 Dihydrogen is used in the manufacture of
vanaspati fat.
 It is used in the manufacture of bulk organic
chemicals, particularly methanol.
CO + 2H2 catalyst cobalt CH3OH
 It is widely used for the manufacture of metal
hydrides.
 It is used for the preparation of hydrogen
chloride, a highly useful chemical.

25. Cont……
 In metallurgical processes, it is widely used to
reduce heavy metal oxides to metals.
 Atomic hydrogen & oxy-hydrogen torches
find use for cutting & welding purposes.
 It is used as a rocket fuel in space research.
 Dihydrogen is used in the fuel cells for
generating electrical energy. It has many
advantages over the conventional fossil fuels
& electric power.

26. Various uses of dihydrogen

29. HYDRIDES
 Dihydrogen also forms ionic bonds with some
metals, at a high temperature, creating a
compound called a hydride.
 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:
1.Ionic or saline or saltlike hydrides.
2.Covalent or molecular hydrides.
3.Metallic or non-stoichiometric hydrides.

30. Nearly all elements are able to form hydride
compound

31. IONIC OR SALINE HYDRIDES
 These are stoichiometric compounds of dihydrogen
formed with most of the s-block elements which are
highly electropositive in character.
 Covalent character is found in the lighter metal
hydrides (e.g. LiH, BeH2 & MgH2).
 The ionic hydrides are crystalline, non-volatile &
non-conducting in solid state.
 Their melts conduct electricity & on electrolysis
liberate dihydrogen gas at anode, which confirms the
existence of H-ion.
2H-(melt) anode H2+2e-
 Saline hydrides react violently with water producing
dihydrogen gas .
NaH + H2O NaOH + H2

32. COVALENT OR MOLECULAR HYDRIDE
 Dihydrogen forms molecular compounds with most
of the p-block elements. For e.g. CH4, NH3, H2O &
HF.
 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 number of electrons & bonds in their
Lewis structure into:
1.Electron-deficient
2.Electron-precise
3.Electron-rich hydrides.

33. ELECTRON- ELECTRON- ELECTRON-RICH
DEFICIENT PRECISE HYDRIDES HYDRIDES
HYDRIDES
Has few electrons Have the required Have excess
for Lewis structure. number of electrons electrons which are
for Lewis structure. present as lone pair.
Elements of group Elements of group Electrons of group
13 forms these 14 forms these 15-17 forms such
compounds. compounds. compounds.
For e.g. Diborane For e.g. CH4. For e.g.NH3-
(B2H6). has1lonepair, H2O-
has 2 lone pairs.
They act as Lewis They act as Lewis
acids i.e. electron bases i.e. electron
acceptor. donor.

34. METALLIC HYDRIDES
 These are formed by many d-block & f-block
elements.
 The metals of group 7,8 & 9 do not form hydride.
 These hydrides conduct heat & electricity though not
as efficiently as their parent metals do.
 Unlike saline hydrides, they are almost non-
stoichiometric, being deficient in hydrogen. For e.g.
LaH2.87 & YbH2.55.
 Law of constant composition does not hold good.
 The property of absorption of hydrogen on transition
metal is widely used in catalytic
reduction/hydrogenation reactions for the preparation
of large number of compounds.
 Some of the metals can accommodate a very large
volume of hydrogen & can be used as its storage
media.

35. Water
 A major part of all living organisms is made up of
water.
 Human body has about 65% & 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.

36. Different uses of water

38. Physical properties of water
 It is a colourless & tasteless liquid.
 The unusual properties of water in the condensed
phase (liquid & solid) are due to the presence of
extensive hydrogen bonding between water
molecules.
 Water has a higher specific heat, thermal
conductivity, surface tension, dipole moment &
dielectric constant when compared to other
liquids.
 It is an excellent solvent for transportation of ions
& molecules required for plant & animal
metabolism.
 Due to hydrogen bonding with polar
molecules, even covalent compounds like alcohol
& carbohydrates dissolve in water.

39. STRUCTURE OF WATER
 In the gas phase water is a bent molecule with a bond angle
of 104.50 ,
and O-H bond length of 95.7 pm.
 It is a highly polar molecule .
 In the liquid phase water molecules are associated together
by hydrogen bonds.
 Density of water is more than that of ice.

40.  Hydrogen Bonding in Water:
Hydrogen bonds are chemical bonds that form between
molecules containing a hydrogen atom bonded to a
strongly electronegative atom . Because the
electronegative atom pulls the electron from the hydrogen
atom, the atoms form a very polar molecule, meaning one
end is negatively charged and the other end is positively
charged. Hydrogen bonds form between these molecules
because the negative ends of the molecules are attracted to
the positive ends of other molecules, and vice versa.
Hydrogen bonding makes water form a liquid at room
temperature.

41. 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 a
distance of 276pm.
 Hydrogen bonding gives ice a rather open type
structure with wide holes. These holes can
hold some other molecules of appropriate size
interstitially.

42. Structure of ice

43. Chemical properties of water
 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 Bronsted sense it acts an acid with NH3 and a
base with H2S.
H2O+ NH3 OH- + NH4+
H2O+ H2S H3O++ HS-
The auto-protolysis (self- ionization) of water takes
place as follows:
H2O +H2O H3O+ +OH-
acid-1 base-2 acid-2 base-1

44. REDOX REACTIONS INVOLVING WATER
 Water can be easily reduced to dihydrogen by highly
electropositive metals.
2H2O +2Na 2NaOH +H2
Thus ,it is a great source of dihydrogen.
 Water is oxidised to O2 during photosynthesis.
6CO2 +12H2O C6H12O6 + 6H2O +6O2
 With fluorine also it is oxidised toO2.
2F2 + 2H2O 4H+ + 4F- +O2

45. Hydrolysis reaction
 Due to high dielectric constant, it has a very
strong hydrating tendency. It dissolves many ionic
compounds. However, certain covalent& some
ionic compounds are hydrolysed in water.
P4O10 +6H2O 4H3PO4
SiCl4 +2H2O SiO2 + 4HCl
N3- + 3H2O NH3 +3OH-

46. A hydrolysis process generally involves water

47. HYDRATES FORMATION
 From aqueous solutions many salts can be
crystallised as hydrated salts. Such an association
of water is of different types viz.,
(i) coordinated water e.g.,
[Cr(H2O)6 ]3+ 3Cl-
(ii) interstitial water e.g.,
BaCl2.2H2O
(iii) hydrogen-bonded water e.g.,
[Cu(H2O)4]2+SO42-.H2O in CuSO4.5H2O

48.  The presence of calcium & magnesium salts in
the form of hydrogen carbonate, chloride &
sulphate in water makes water ‘hard’.
 Hard water does not give lather with soap.
 Water free from soluble salts of calcium &
magnesium is called soft water.
 It gives lather with soap easily.

49. Cont….
 Hard water forms scum/precipitate with soap.
 Soap containing sodium stearate( C17H35COONa) reacts
with hard water to precipitate out Ca/Mg stearate.
2C17H35COONa(aq)+M2+(aq) M(C17H35COO)2 +2Na+(aq);
M is Ca/Mg
 Hard water is harmful for boilers, because of deposition of
salts in the form of scale. This reduces the efficiency of the
boiler.
 The hardness of water is of two types:
 1.Temporary hardness.
 2.Permanent hardness.

50. TEMPORARY HARDNESS:
 Temporary hardness is due to the presence magnesium &
calcium hydrogen carbonates. It can be removed by:
1.Boiling: During boiling, the soluble Mg(HCO3)2 is
converted into insoluble Mg (OH)2 & Ca(HCO3)2 is
changed to insoluble CaCO3. It is because of 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. Filtrate thus obtained will
be soft water.
Mg(HCO3)2 heating Mg(OH)2 + 2CO2
Ca(HCO3)2 heating CaCO3 + H2O + CO2

51. Cont….
 2.Clark’s method: In this method calculated
amount of line is added to hard water. It
precipitates out calcium carbonate & magnesium
hydroxide which can be filtered off.
 Ca(HCO3)2+Ca(OH)2 2CaCO3 +2H2O
 Mg(HCO3)2+2Ca(OH)2 2CaCO3 +Mg(OH)2 +2H2O

52. Ways to reduce temporary hardness

53. PERMANENT HARDNESS:
 It is due to the presence of soluble salts of
magnesium & calcium in the form of chlorides &
sulphates in water.
 Permanent hardness is not removed by boiling. It
can be removed by the following methods:
 1.Treatment with washing soda (sodium
carbonate): Washing soda reacts with soluble
calcium & magnesium chlorides & sulphates in
hard water to form insoluble carbonates.
MCl2 + Na2CO3 MCO3 + 2NaCl (M=Mg, Ca)
MSO4+Na2CO3 MCO3 + Na2SO4

54. Cont….
2.Calgon’s method: Sodium hexametaphosphate
(Na6P6O18), commercially called ‗calgon‘, when added to hard
water ,the following reactions takes place.
Na6P6O18 2Na+Na4P6O182- (M=Mg, Ca)
M2++Na4P6O182- [Na2MP6O18]2-+2Na+
The complex anion keeps the Mg2+ & Ca2+ions in solution.
3.Ion-exchange method: This method is also called zeolite
/permutit process. Hydrated sodium aluminium silicate is zeolite.
Sodium aluminium silicate (NaAlSiO4) can be written as NaZ.
When this is added to hard water, exchange reactions take place.
2NaZ(s)+M2+(aq) MZ2(s)+2Na+(aq) (M=Mg, Ca)

55. Cont….
 Permutit/zeolite is said to be exhausted when all the
sodium in it is used up. It is generated for further use by
treating with an aqueous sodium chloride solution.
MZ2(s)+2NaCl(aq) 2NaZ(s)+MCl2(aq)
4.Synthetic resins method: This method is more efficient
than zeolite process. Cation exchange resins contain large
-organic molecule with-SO3H group & are water
insoluble. Ion exchange resin (RSO3H) is changed to
RNa by treating it with NaCl. The resin exchanges Na+
ions with Ca2+ & Mg2+ ions present in hard water to make
the water soft. Here R is resin anion.
2RNa+M2+ R2M+2Na+
 The resin can be regenerated by adding aqueous NaCl
solution.

56. Cont….
 Pure de-mineralised (de-ionised) water free from all
soluble mineral salts is obtained by passing water
successively through a cation exchange (in the H+
form) & an anion-exchange ( in the OH- form)
resins:
2RH+M2+ MR2+2H+
 In this cation exchange process, H+ exchanges for
Na+,Ca2+, Mg2+ & other cations present in water.
 This process results in proton release & thus makes
the water acidic.

57. Cont….
 In the anion exchange process:
RNH2 + H2O RNH3+.OH-
RNH3+.OH- + X- RNH3+.X- + OH-
OH- exchanges for anions like Cl-,HCO3-, SO42-
etc. present in water. OH- ions liberated neutralise
the H+ ions set free in the cation exchange.
H+ + OH- H2O
The exhausted cation & anion exchange resin
beds are regenerated by treatment with dilute acid
& alkali solutions respectively.

58. Methods for reducing permanent
hardness

59. HYDROGEN PEROXIDE:
 Hydrogen peroxide is an important chemical used in
pollution control treatment of domestic & industrial effluents.
PREPARATION:
 It can be prepared by the following methods:
1.Acidifying barium peroxide & removing excess water by
evaporation under reduced pressure gives hydrogen peroxide.
BaO2.8H2O+H2SO4 BaSO4+H2O2+8H2O
2.Preoxodisulphate, obtained by electrolytic oxidation of
acidified sulphate solutions at high current density, on
hydrolysis yields hydrogen peroxide.
2HSO4- electrolysis HO3SOOSO3H hydrolysis 2HSO4-
+2H++H2O2

60. Cont….
 This method is now used for the laboratory preparation of
D2O2.
K2S2O8+2D2O 2KDSO4+D2O2
3.Industially it is prepared by the auto-oxidation of 2-
alklylanthraquinols.
2-ethylanthraquinol H2O2+(oxidised product)
 In this case 1% H2O2 is formed. It is extracted with water
& 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.

61. Preparation of hydrogen peroxide

62. PHYSICAL PROPERTIES:
 The pure state H2O2 is an almost
colourless liquid
 Meting point - 272.4K.
 Boiling point - 423K
 Vapour pressure (298K) – 1.9mmHg.
 H2O2 is miscible with water in all proportions &
forms a hydrate H2O2.H2O.
 A 30% solution of H2O2 is marketed as ‗100V‘
hydrogen peroxide. It means that 1ml of 30%
H2O2 solution will give 100V of oxygen at STP.
 Hydrogen peroxide has a non-planar structure.

63. CHEMICAL PROPERTIES:
 It acts as an oxidising as well as reducing
agent in both acidic & alkaline media.
1.Oxidising action in acidic medium:
2Fe2+ +2H+ +H2O2 2Fe3+ +2H2O
PbS +4H2O2 PbSO4+4H2O
2.Reducing action in acidic medium:
2MnO4- +6H+ +5H2O2 2Mn2+ +8H2O+5O2
HOCl +H2O2 H3O+ +Cl- +O2

64. Cont….
3.Oxidising action in basic medium:
2Fe2+ +H2O2 2Fe3+ +2OH-
Mn2+ +H2O2 Mn4+ +2OH-
4.Reducing action in basic medium:
I2+H2O2+2OH- 2I-+2H2O+O2
2MnO4-+3H2O2 2MnO2+3O2+2H2O+2OH-

65. STORAGE
 H2O2 decomposes slowly on exposure to light.
2H2O2 2H2O+O2
 In the presence of metal surfaces or traces of
alkali, the above reaction is catalysed. It is,
therefore stored in wax-lined glass or plastic
vessels in dark.
 It is kept away from dust because dust can induce
explosive decomposition of the compound.

66. USES:
 It is used as hair bleach & as a mild disinfectant.
As an antiseptic it is sold in the market as
perhydrol.
 It is used to manufacture chemicals like sodium
perborate & per-carbonate, which are used in high
quality detergents.
 It is used in the synthesis of
hydroquinone, tartaric acid & certain food
products & pharmaceuticals etc.
 It is employed in the industries as bleaching agent
for textiles, paper pulp, leather, oils, fats etc.
 It is also used in environmental chemistry.

68. HEAVY WATER,D2O
 It is extensively used as a moderator in nuclear
reactors & 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.
 PHYSICAL PROPERTIES:
 Molecular mass: 20.0276 g/mol.
 Melting point: 276.8K.
 Boiling point: 374.4K.

69. Cont…..
 The bottom ice cubes were made with heavy
water, which is water that uses deuterium
hydrogen (nucleus with an extra neutron) not
regular hydrogen which has no neutron.

70. Production of Hard water

71. USES:
 It is used for the preparation of other deuterium
compounds. For e.g.
CaC2 + 2D2O C2D2 + Ca(OD)2
SO3 + D2O D2SO4
Al4C3 + 12D2O 3CD4 + 4Al(OD)3

72. DIHYDROGEN AS A FUEL:
 Dihydrogen releases large quantities of heat on
combustion.
 Dihydrogen can release more energy than petrol's.
 HYDROGEN ECONOMY: The basic principle
of hydrogen economy is the transportation &
storage of energy in the form of liquid or gaseous
dihydrogen.
 Energy is transmitted in the form of dihydrogen &
not as electric power.
 It is also use in fuel cell for generation of electric
power.

74. Thank you