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Thursday, 23 December 2021

Hydrogen Class 11 Notes

 Hydrogen Class 11th Chemistry Notes

Hydrogen is the first element in the periodic table and is the lightest element. It exists as a diatomic molecule (H2) and is called dihydrogen. It has one proton and one electron and it has electronic configuration 1s1. This electronic configuration is responsible for dual nature of hydrogen i.e. hydrogen shows resemblance with alkali metals as well as halogens.

Resemblance of hydrogen with alkali metals

Like alkali metals –

·         Hydrogen exhibits electropositive character i.e. it has tendency to lose electron.

·         The normal valency of hydrogen is 1.

·         Hydrogen forms halides, oxides and sulphides.

·         Hydrogen acts as strong reducing agent.


Resemblance of hydrogen with halogens

Like halogens-

  • Hydrogen exhibits electronegative character by accepting one electron to complete its valance shell.
  • Hydrogen shows same order of ionisation energy (IE1) as that of halogens.  
  • Hydrogen (H2) is a gas in molecular state similar to F2 and Cl2.
  • Hydrogen shows -1 oxidation state in its compounds with more electropositive element.



As the hydrogen resembles both the alkali metals and the halogens and also differs from them. Also hydrogen exhibits some unique behavior therefore the position of hydrogen is uncertain and it is placed separately in periodic to table.


Hydrogen has three isotopes, protium (11H), deuterium (12H) and tritium (13H).

            i.            Protium or ordinary hydrogen (11H).

 It is the most abundant isotope of hydrogen (99.985%). Its nucleus has one proton and no neutron (mass no. = 1).

          ii.             Deuterium or heavy hydrogen (12H or D).

It is less abundant and is present in heavy water (D2O). Its nucleus has one proton and one neutron (mass no. = 2).

        iii.            Tritium (13H or T)

It is the rarest isotope of hydrogen and is radioactive in nature. Its nucleus has one proton and two neutrons (mass no. = 3).



Laboratory methods

1.      Action of dilute HCl or H2SO4 on granulated zinc.

Zn    +     2HCl          ZnCl2    +    H2  


2.      Action of dilute H2SO4 on magnesium ribbon.

Mg    +     H2SO4         MgSO4    +    H2  


3.      Action of water on sodium hydride.

NaH    +     H2O          NaOH    +    H2  


4.      By action of KOH of scrap aluminum or silicon. (Uyeno’s method)

2Al    +     2KOH    +     2H2O          2KAlO2    +    3H2  


Commercial method

1) From water gas (Bosch process)

Water gas (CO + H2) is mixed with the steam and the gaseous mixture is passed over heated catalytic mixture of ferric oxide (Fe2O3) and chromium oxide (Cr2O3) at 773K.

H2     +    CO    +    H2O      Fe2O3 + Cr2O3 (773K)     CO2   +   2H2

  Water gas         Steam

 2) From Steam (Lanes process)

When superheated steam is passed over iron filling heated to 1023 – 1073K hydrogen is formed.

3Fe  +   4H2O(steam)      (1023 – 1073)K  (Fe)   Fe2O4   +   4H2(g)  


3) Hydrocarbon steam process

Hydrogen is prepared by the action of steam of hydrocarbon at 1270K.

CH4   +   H2O       1270K     CO   +   3H2


4) By electrolysis of water

Electrolysis of acidified water using platinum electrodes gives dihydrogen. Here dihydrogen is librated at the cathode while dioxygen is librated at anode.



1) Dihydrogen reacts with halogens (X2) to give hydrogen halide (HX).

    H2   +   X2       2HX    ( X = F, Cl, Br, I )


2) Dihydrogen reacts with dioxygen to forms water. This reactions is highly exothermic.

    2H2   +   O2      catalyst or heating     2H2O . ΔH = -285.9 KJ.mol-1


3) With dinitrogen dihydrogen forms ammonia. This process is coiled Hober process of    

    manufacture of ammonia.         

    3H2   +   N2     673K, Fe catalyst, 200atm     2NH3


4) Dihydrogen combines with many metals at high temperature to yield the corresponding                             hydrides                                                                                                                                                                                                                                                                                                 

   2Na  +  H2     2NaH    (sodium hydride)


5) It acts as reducing agent and thus reduces certain oxides of metals.

    ZnO   +   H2      Zn   +   H2O

    Fe2O4   +   4H2      3Fe   +   4H2O

6) Hydrogenation of vegetable oils using nickel as catalyst gives edible facts (margarine and Vanaspati ghee)

    Vegetable oil   +   H2      Finely divided Ni, 450K, 8-10atm    Solid fat



·         Dihydrogen is used in the preparation of ammonia by Haber’s process.

·         It is used in the hydrogenation of vegetable oils.

·         It is used for the manufacture of metal hydrides.

·         It is used as rocket fuel in space research.

·         Dihydrogen is used in fuel cell for generating electrical energy.

·         It is used in the atomic hydrogen torch and oxy hydrogen torches for cutting and welding.



The binary compounds of hydrogen with other elements are called hydrides. These hydrides have the formula EHx or EmHn (E = Element). These are classified into three types.

a) Ionic or saline hydrides

b) Covalent or molecular hydrides

c) Metallic or nonstoichiometric or interstitial hydrides


a) Ionic or saline hydrides

These are formed by the combination of hydrogen with metals which have low electro negativity values and are electropositive with respect to hydrogen. These includes elements of S block element. Ionic hydrides are prepared by the direct combination of metals.

e.g. LiH, KH, CaH2, MgH2 etc.


b) Covalent or molecular hydrides

These hydrides are formed by the combination of elements of comparatively higher electro –negativity as p-block element. The bonds formed are mostly covalent in character. Covalent hydrides are generally volatile.

e.g. H2O, CH4, NH3, HF etc.


c) Metallic or nonstoichiometric or interstitial hydrides

Metallic hydrides are formed by many d - block and f - block  elements. Elements of group 7, 8, 9 of d – block do not form hydrides and this is referred as hydride gap. Metallic hydrides are non – stoichiometric and show electric conductance. In these hydrides, hydrogen occupies interstices in the metal lattice producing distortion without any change in its type. Therefore, they are termed as interstitial hydrides.

e.g. SCH2, TiH2, VH, ZrH2



Water is essential to all forms of life. It is most common, abundant and easily obtainable of all chemical compounds. It is regarded as universal solvent. Water (H2O) is hydride of oxygen. In nature, water exist in three physical states, water (liquid), Ice (Solid) and water vapour (gas).


Structure of water

Water molecule is a bent molecule with bond angle of 104.5ᵒ and O-H bond length is 95.7pm.

Structure of water - Hydrogen class 11th chemistry notes for free download

Structure of Ice

Ice is the solid form of water. It has a highly ordered three dimensional hydrogen bonded structure. In ice each oxygen atom is surrounded tetrahedrally by four other hydrogen atoms at a distance of 276pm. The strength of hydrogen bonding in ice is more than liquid water. There is empty space in crystal of ice due to hydrogen bonding. This makes density of ice lower than liquid water, hence ice floats on water.

Structure of ice

Chemical properties of water

1) Dissociation of water

Water dissociates into its ion.

H2O   +    H2O             H3O+       +        OH-

                                 Hydronium ion


2) Amphoteric nature

Water has the ability to act as on acid as well as base. Such kind of behavior is known as amphoteric nature.

 H2O     +      NH3             OH-         +        NH4+

 H2O     +      HCl             H3O+       +        Cl-



3) Oxidizing and reducing nature

Water can act both as an oxidizing and reducing agent in its chemical reactions. With active metals water behaves as an oxidizing agent while with highly electronegative element, it acts as a reducing agent.

2Na    +    2H2O        2NaOH    +    H2

        Oxidizing agent

2F2    +    2H2O        4HF    +    O2

        Reducing agent


4) Formation of Hydrates

Water has strong ability to form compounds with some metal salts known as hydrates. This hydrates can be classified into three types:

i) Coordinated water


ii) Interstitial water


iii) Hydrogen – bonded water

[Cu(H2O)4]2+SO42-.H2O     in




Heavy water is deuterium oxide (D2O). It was discovered by Urey in 1932. The reaction of D2O are slightly slower than H2O. Ordinary water contains one part of heavy water in 6000 part of it. It is used as a moderator in nuclear reactors. It is used as tracer compound in determining the mechanism of many organic reactions.



Natural water contains dissolved salts. Depending upon its behavior towards soap solution, water may be classified as hard water and soft water.

a) Soft water : 

Water which produces lather with soap solution readily is called soft water. For example : distilled water, rain water, and demineralized  water.


b) Hard water : 

Water which does not produces lather with soap solution readily is called hard water. For example : sea water, river water, well water and tap water.

The hardness of water is due to the presence of the bicarbonates, chlorides and sulphates of calcium and magnesium.


Isotopic varieties of water

Ordinary water contains 18 different kinds of water. Such as variety is possible due to the different isotopic forms of hydrogen and oxygen. Hydrogen has three isotopes H(protium), D(Deutrium), T(tritium) and oxygen also has three isotopes 16O, 17O, 18O. These isotopes of hydrogen and oxygen combine to give 18 different kinds of water from which H2O is most abundant.




Hydrogen peroxide was discovered by Thenard in 1818. It is an important chemical used in pollution control treatment of domestic and industrial effluents.


Preparation of H2O2

1) In laboratory H2O2 is prepared by acidifying barium peroxide and removing excess water by evaporation under reduced pressure.

BaO2.8H2O   +   H2SO4       BaSO4   +    H2O2   +    8H2O


2) By the action of dilute acid on sodium peroxide (Merck’s method)

Na2O2   +   H2SO4       NaSO4   +    H2O2


3) By bubbling CO2 through a paste of BaO2

BaO2   +    H2O   +   CO2       BaCO3   +    H2O2


4) By the action of phosphoric acid on BaO2

3BaO2    +   2H3PO4       Ba3(PO4)2   +    3H2O2  


5) Industrially H2O2 is prepared by the auto – oxidation of 2 – Ethyl anthraquinol.

      2-ethyl anthraquinol     O2 (air) (H2/Pd   2-ethyl anthraquinol    +   H2O2


Structure of  H2O2

It is non–planar open book (Skew) structure. The bond length and bond angle are slightly different in gas and solid phase due to hydrogen bonding. Structure of  H2O2 HYDROGEN PROXIDE.


Chemical Properties of H2O2

1) It decomposes rapidly on heating in presence of finely divided metals, such as Ca, Fe, Cu, Au, Ag, Pt, MnO2, Carbon, dust, light, etc.

H2O2   +   H2O2        2H2O    +    O2   (ΔH = -196KJ)


2) It acts as an oxidizing as well as reducing agent in both acidic and alkaline or basic medium. Some example are given below

i) Oxidizing action in acidic medium

PbS   +   4H2O2        PbSO4   +   4H2O

ii) Reducing action in acidic medium

HOCl    +   H2O2        H3O+   +    Cl-    +   O2

iii) Oxidizing action in alkaline medium

Mn2+   +   H2O2      2Fe3+   +    2OH-

iv) Reducing action in alkaline medium

Cl2   +   H2O2   +    2OH-      2Cl-   +    2H2O    +    O2


3) Hydrogen peroxide acts as a bleaching agents due to the release of nascent oxygen.

H2O2          H2O     +     O


4) Hydrogen peroxide undergoes addition reaction with alkenes to form glycols.

Hydrogen peroxide undergoes addition reaction with alkenes to form glycols.

Uses of H2O2

·         It is widely used in environmental chemistry.

·         It is used as a antiseptic and is sold in the market as perhydrol.

·         It is used in daily life as a hair bleach and as mild disinfectant.

·         It is used as a bleaching agent and to manufacture chemicals like sodium perborate and percarbonate.


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