Wednesday, August 29, 2012


Quality Education At Every level

 Class :XII                         Chapter #02 

 Prepared by:

Lecturer. S.Fayyaz Hussain
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            Hydrogen is the simplest of all the known elements and is the first element of the periodic table. It is generally found in the combined state in the form of compound with oxygen, carbon, nitrogen & sulphur etc. free hydrogen occurs in small amounts in the atmosphere.

Position of Hydrogen in the Periodic Table:

            Elements are arranged in the periodic table on the basis of their electronic configuration and to some extent on the basis of their properties. Electronic configuration and properties of hydrogen partially resemble with the elements of group IA, group IVA and with group VII A, does not resemble completely with the members of any one of the groups.

Resemblance with Group IA:

1- Hydrogen atom has only one electron and has electronic configuration   
1S1. Alkali metals also contain one electron in their outermost S – orbital. e.g.               11Na = 1S22S22P63S1
2-Hydrogen can lose electron to form H+.
                        i.e.  H                         H+ + e-
Alkali metals can also lose electron from ns1 – orbital.
3-   Both hydrogen and alkali metals have strong tendency to combine with electronegative atoms such as halogens to form halides.
e.g. HCl  ;  NaCl    etc.                                                                                                           

4-   The compounds of hydrogen and alkali metals form positive ions in the aqueous medium & reduced at cathode in the electrolysis.
e.g.      H                           H+ + e-


1-                  Hydrogen is a gas exists in diatomic state like H2 while alkali metals exist in solid state.
2-                  Most of the hydrogen compounds are covalent while the compounds of alkali metals are ionic.
3-                  Hydrogen needs only one electron to complete its valence shell, whereas alkali metals need seven electrons to complete the outermost shells.
4-                  Ionisation potential of hydrogen i.e. 1356Kj/mole is greater than the ionisation potential of the alkali metals such as Li has 524Kj/mole

Resemblance With Group IV A:

1-                  The valence shell of hydrogen is half – filled like those of group IVA elements.
2-                  Hydrogen and carbon are generally covalently bonded with other elements by mutual sharing of electrons.
3-                  Some thermodynamic properties such as ionization potential and electron affinity of hydrogen are found similar to that of carbon and other members of group IVA.
4-                  Electronegativity of hydrogen i.e. 2.1 is nearer to carbon i.e. 2.4.


1-                  Hydrogen is a gas, whereas the members of group IV A solid under normal temperature and pressure.
2-                  Hydrogen is a monovalent element, whereas members of group IV A are tetravalent.
3-                  Valence shell of hydrogen consist of only ‘S’ orbital, whereas members of carbon family consists of ‘S’ and ‘P’ orbitals in valence shell.
4-                  Hydrogen does not form catenation while carbon family forms catenation.

Resemblance with Group VII A:

1-                  Hydrogen is a non – metal like halogens.
2-                  Hydrogen requires one electron like halogens to complete the outermost shell.
3-                  Hydrogen atom forms hydride ions (H-) by gain of an electron, just like hydrides. e.g.
H  +  e-                           H-
Cl +  e-                       Cl-
4-                  Hydrogen is a gas consisting of diatomic (H2) molecules like F2 and Cl2.


1-                  H- ion is not stable in water, whereas halides ions (e.g. Cl-) are stable, since H- immediately reacts with water. i.e.
H- + H2O                    H2  +  OH-
2-                  Hydrogen has only one electron in its valence shell, whereas halogens have seven electrons in their valence shell.
3-                  Electron affinity of hydrogen is also much less than halogens.
4-                  Hydrogen forms H+ ions in aqueous medium while halogens form stable halide ions.


            From the above discussion, we conclude that position of hydrogen in the periodic table has been a matter of controversy, and its exact position ---- still remains undecided. However generally it is placed separately at the top of periodic table.


Industrial Preparation Of Hydrogen

            In industry, hydrogen is manufactured from water, natural gas and ammonia.

1 From Water:
(a)   Steam & Methane:
When a mixture of steam and natural gas passed over a catalyst like Ni at high temperature, a mixture of hydrogen and carbon monoxide, is obtained known as WATER GAS.
                        CH4  +  H2O         Ni             3H2  +  CO
Water Gas

b)  Steam & Coal :
            When steam is passed over red hot coke at 1000oC, a mixture of hydrogen and carbon monoxide (water gas) is produced. i.e.
                        C  +  H2O          1000oC        H2  +  CO

 Water gas
            CO can be separated from H2 by the following methods.

i) Liquefaction:
            The mixture is cooled at –200oC when CO becomes liquid and it separates out from the gaseous hydrogen. The traces of CO are removed by passing hydrogen gas through a solution of NaOH, where CO reacts with sodium hydroxide to form sodiumformate liberating hydrogen which is collected.
            NaOH(aq)  +  CO  +  H2                                 HCOONa(aq)  +  H2 ­
The wet hydrogen is then passed over sulphuric acid to moisture and obtained dry and pure hydrogen gas.

ii) Bosch Process:
            The most suitable method for the separation of CO from water gas is by passing steam at 500oC over water gas in the presence. i.e.
                        CO  +  H2  +   H2O              FeO          CO2  +  H2
            Carbon dioxide is readily soluble in water under pressure and can be separated easily by dissolving it in water, whereas pure H2 bubbles out. The wet hydrogen is then passed over sulphuric acid to moisture and obtained dry and pure hydrogen gas.

(2) Steam & Methanol:
            When a mixture of steam and vapours of methanol is heated at 250oC, a mixture of CO2 and H2 is obtained.  i.e.
                        CH3OH  +  H2O            250oC         CO2  +  H2

(3) From Natural Gas:
            When methane is heated in the absence of oxygen, it decomposes into its elements.   i.e.
                    CH4          700oC          C  +  H2­
The hydrogen gas formed is passed through filter to separate from the carbon.

(4) From Ammonia:
            When ammonia gas is heated at 1000oC in the presence of a catalyst, it decomposes into its elements, i.e.
                        2NH3           1000oC           N2  +  3H2
            The mixture of nitrogen and hydrogen is cooled to –196oC, when nitrogen becomes liquid and free hydrogen is obtained.

(5) Electrolysis Of Water :
            The purest but expensive hydrogen is obtained by passing the electricity through water containing a small quantity of an electrolyte (acid, base of salt). In this process, water decomposes into hydrogen and oxygen
                     2H2O       electricity                   2H2  +  O2

            Ordinary hydrogen exist as diatomic molecules (i.e. H2), in which two atoms linked together by a strong covalent bond. These molecules are very stable and dissociate into atom at very high energy (104  Kcal/mole). The product obtained as a result of dissociation of molecular hydrogen is known as atomic hydrogen. These atoms have very short life and quickly combine together to form molecules again. That’s why atomic hydrogen is also called NASCENT HYDROGEN.

(a)                Atomic hydrogen is prepared by thermal decomposition of ordinary hydrogen at very high temperature.
H2            5000oC             2H

(b)               Atomic hydrogen can also be prepared by electrical dissociation of ordinary hydrogen under reduced pressure.
H2          electric discharge           2H
0.1    mm pressure

            Atomic hydrogen is much more reactive than molecular hydrogen. When hydrogen atoms combine to form molecular hydrogen 453 Kj /mole energy is liberated.    i.e.
            H  +  H                          H2 ,     ΔH  =  -436.8 Kj/mole    OR -104 Kcal/mole

            Thus, atomic hydrogen is much more energetic than that molecular hydrogen. Many reactions which do not take place by ordinary hydrogen immediately occur by atomic hydrogen, some important reactions of atomic hydrogen are given below:
                        P          +  3H                           PH3     
                        O2          +  2H                           H2O2
                        S          +  2H                           H2S
                        Cl2       +  2H                           2HCl
                        CuO    +  2H                           Cu  +  H2O
                        AgCl   +  H                             Ag  +  HCl

             Atomic hydrogen is used to produce “atomic hydrogen torch” to attain a temperature of 4000oC – 5000oC, which is employed in welding purposes.
            It is based on the fact that when a jet of hydrogen is passed through an electric arc set up between the tungsten rods, atomic hydrogen is produced which at once recombines to give a flame having a temperature between 4000 – 5000oC.  i. e.
                        H  +  H                                   H2  +  104  K.Cal/mole

            The compounds formed between two elements are called “BINARY COMPOUNDS”. Hydrogen reacts with some elements to form binary compounds called “HYDRIDES”.
         The Variety of hydrides has been obtained depending upon the nature of the elements with which hydrogen combines. The hydrides are classified as:-
1)      Ionic Hydrides
2)      Covalent Hydrides
3)      Complex Hydrides
4)      Metallic Hydrides
5)      Polymeric Hydrides
6)      Border Line Hydrides

                        These types of hydrides are formed by the metal of group IA and group IIA except Be & Mg. In these hydrides metal atoms form positive ion by losing valence electrons and hydrogen form negative ion by gaining that electron in order to complete its shell. They have general formulae MH in case of group IA and MH2 in case of metals of group IIA. They are also called SALT – LIKE or SALINE hydrides.

            Ionic hydrides are prepared by the direct combination of metals with hydrogen at high temperature.  e.g.
                        2Na  +  H2           200oC          2NaH
                        Ca    +  H2           200oC              CaH2

1.      Ionic hydrides are crystalline white solid.
2.      They have high melting and boiling points.
3.      They can conduct electricity in the molten state.
4.      They are insoluble in organic solvents but soluble in water with which they react and produce hydrogen.   e.g.
NaH   +  H2O                                                  NaOH  +  H2­
or         CaH2  +  2H2O                                    Ca (OH)2 + H2­
5.      They also produce hydrogen on reaction with acid and alcohols. e.g.
NaH  +  C2H5OH                                  C2H5ONa + H2­
            &         CaH2 +  2HCl                                       CaCl2  +  2H2­

                        These hydrides are formed by the combination of non – metals of groups III A to group VII A with hydrogen, they are very common and contain hydrogen linked to other atoms through covalent bonds.

            They are prepared by the direct action of non – metals with hydrogen or by any other suitable indirect method. e.g.
                        H2        +  Cl2               sunlight           2HCl
                        2H2      +  O2                ignition            2H2O
                        H2        +  S                  450oC              H2S
                        CaC2    +  2H2O                                   Ca(OH)2  +  C2H2
                        Al4C3   +  6H2O                                   2Al2O3 + 3CH4
                        Mg3N2+  6H2O                                    3Mg(OH)2 +  2NH3
                        Ca3P2  +  6H2O                                   3Ca(OH)2 + 2PH3

1)      Covalent hydrides are either gases or volatile liquids.
2)      They have low melting and boiling points.
3)      Non – polar hydrides do not conduct electricity, while strongly polar hydrides conduct electricity in aqueous solution.
4)      Hydrides of group III A & IV A are neutral, hydrides of group V A are basic while those of group VI A and VII A are generally basic.

            Members of group III A do not form stable binary compounds except boron. However, hydrides of these elements combine with alkali metal hydrides to form compound of complex nature possessing all the three type of chemical bonds. These compounds are called COMPLEX COMPOUND. They have general formulae ABH4, where A is a alkali metal, B is a member of group III A. e.g.
                        NaH  +  BH3                                         NaBH4
                        LiH   +  AlH3                                                    LiAlH4
1-                  They are solids and are powerful reducing agent.
2-                  They have tetrahedral structures.
3-                  They are soluble in water and produce hydrogen.   e.g.
NaAlH4  +  4H2O                             Al(OH)3  +  NaOH  +  4H2­

                        Metallic hydrides are compounds of hydrogen with transition metals formed by the adsorption of hydrogen in the spaces or interstices of the transition metal structure, so they are also called INTERSTITIAL HYDRIDES.
            Due to the physical adsorption of hydrogen and on the surface of the metals, no stoichiometric ration between hydrogen and the metal atoms exists. No true chemical bond is formed between hydrogen and the metal atoms.

            They are non – stoichiometric in nature and their properties are not very different.
            These hydrides on heating are released hydrogen in atomic state. For this reason, transition metals such as Pt, Ni etc. are used as catalysts in hydrogenation reactions.

            Be & Mg belongs to group IIA. Normal formulas of their hydrides are Be H2 & MgH2, but these hydrides polymerise to attain formulae (BeH2)n and (MgH2)n. therefore these hydrides are known as polymeric hydrides in which Be & Mg are held together by hydrogen bridge as shown below;


            Their chemical behaviour is between ionic and covalent hydrides.

                        These hydrides are formed by metals of group IB, IIB and few metals of group III A such as indium & thallium e.g. CuH2, ZnH2 & TlH2. These hydrides are intermediate between metallic hydrides and covalent hydrides.

“Isotopes are different form of atoms of an element having same atomic number but different mass numbers”.
“The atoms of the same element which have the same no. of protons but different no. of neutrons are called ISOTOPES of that element”.
            There are three types of isotopes of hydrogen, namely;
1-                  Ordinary hydrogen or Protium.
2-                  Heavy hydrogen or Deuterium.
3-                  Tritium.

                        Protium of ordinary hydrogen contains one proton in the nucleus and one electron in its K – shell. There is no neutron in it. It is symbolized as: H.
            Atomic no.                  =  1
            Atomic mass               =  1
            No. of Proton  =  1
            No. of Electron           =  1
            No. of Neutron           =  0

            The major portions of ordinary hydrogen consist of this isotope          (~ 99.98 y.).
                        Deuterium is present in very small amounts (0.0156 %) mixed with hydrogen. its symbol is D or H2. It contains one proton & one neutron in the nucleus and one Electron in the K – shell.

Atomic no.                  =  1
            Atomic mass   =  2
            No. of Proton  =  1
            No. of Electron           =  1
            No. of Neutron           =  1

                        Tritium is found only in trace amount in the upper atmosphere. It is present to the extent of one atom in 107 atoms of ordinary hydrogen. its symbol is T or ,H3, indicating the presence of one proton and two neutrons in the nucleus and one electron in the first energy shell.
            Atomic no.                  =  1
            Atomic mass   =  3
            No. of Protons            =  1
            No. of electrons          =  1
            No. of Neutrons          =  2

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