Chemistry > Hydrogen

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  • Hydrogen is the first element in the periodic table.
  • Hydrogen has resemblance to alkali metals, which lose one electron to form uni-positive ions, as well as with halogens, which gain one electron to form uni-negative ion. Like alkali metals, hydrogen forms oxides, halides and sulphides. However, unlike alkali metals, it has a very high ionization enthalpy and does notpossess metallic characteristics under normal conditions.
  • 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.
  • Reason for separate place in the periodic table--Loss of the electron from hydrogen atomresults 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 aconsequence, H+ does not exist freely and is always associated with other atoms ormolecules. Thus, it is unique in behavior and is, therefore, best placed separately in the periodic table.
  • Hydrogen has three isotopes: protium - 1H1, deuterium -  2H1or D and tritium -  3H1 or T. These isotopes differ from one another in respect of the presence of neutrons.
  • Laboratory Preparation of Di-hydrogen
  • Commercial Production of Di-hydrogen
    • Electrolysis of acidified water using platinum electrodes gives hydrogen.
    • High purity (>99.95%) di-hydrogen is obtained by electrolyzing warm aqueousbarium hydroxide solution between nick elelectrodes.
    • It is obtained as a byproduct in the manufacture of sodium hydroxide and chlorine by the electrolysis of brine solution.

    • Reaction of steam on hydrocarbons or cokeat high temperatures in the presence of catalyst yields hydrogen.

      The mixture of CO and H2 is called water gas or syngas.

      The process of producing syngas from coal is called coal gasification.

      Water gas shift reaction-

  • Physical properties of Di-hydrogen

    Di-hydrogen is a colorless, odorless, tasteless, combustible gas. It is lighter than air and insoluble in water.

  • Chemical properties of Di-hydrogen
  1. Reaction with halogens

  2. Reaction with di-oxygen

  3. Reaction with di-nitrogen

     at 673 K, 200 atm and catalyst- Fe

  4. Reactions with metals

  5. Reactions with metal ions and metal oxides

    It reduces some metal ions in aqueous solution and oxides of metals into corresponding metals.

  • Uses of di-hydrogen
    • The largest single use of di-hydrogen is inthe synthesis of ammonia.
    • Di-hydrogen is used in the manufacture ofvanaspati fat by the hydrogenation of polyunsaturated vegetable oils.
    • It is used in the manufacture of bulkorganic chemicals

       Catalyst – Cobalt

    • It is widely used for the manufacture of metal hydrides.
    • It is used for the preparation of hydrogen chloride.
    • 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.
    • It is used as a rocket fuel in space research.
    • Di-hydrogen is used in fuel cells for generating electrical energy.
  • Hydrides
    • Di-hydrogen, under certain reaction conditions, combines with almost all elements, except noble gases, to form binary compounds, called hydrides.
    • The hydrides are classified into three categories

Ionic or Saline Hydrides

These are stoichiometric compounds of di-hydrogen formed with most of the s-block elements which are highly electropositive in character. The ionic hydrides are crystalline, non-volatile and non-conducting in solid state. However, their melts conduct electricity and on electrolysis liberate di-hydrogen gas at anode, which confirms the existence of H ion.

2H(melt) ---- >H2 g + 2e

Saline hydrides react violently with water producing di-hydrogen gas.

Covalent or Molecular Hydride

Di-hydrogen forms molecular compounds with most of the p-block elements. 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 has too few electrons for writing its conventional Lewis structure. Ex-Di-borane (B2H6)

Electron-precise compounds have the required number of electrons to write their conventional Lewis structures.

Electron-rich hydrides have excess electrons which are present as lone pairs. Elements of group 15-17 form 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.

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. 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.

  • 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. It is highly polar molecule. In the liquid phase water molecules are associated together by hydrogen bonds.

  • Structure of Ice

Ice has a highly ordered three dimensional hydrogen bonded structure. Each oxygen atom is surrounded tetrahedrally by four other oxygen atoms at a distance of 276 pm.

  • 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.

    • Redox Reactions Involving Water

       Water can be easily reduced to di-hydrogen by highly electropositive metals.

       Photosynthesis reaction

    • Hydrolysis Reaction

      Due to high dielectric constant, it has a very strong hydrating tendency. It dissolves many ionic compounds. However, certain covalent and some ionic compounds are hydrolyzed in water.

    • Hydrates Formation

       From aqueous solutions many salts can be crystalized as hydrated salts. Such an association of water is of different types viz. coordinated water (ii) interstitial water ex-BaCl2 .2H2O (iii) hydrogen-bonded water

  • Hard water and soft water

    Hard water does not give lather with soap. Water free from soluble salts of calcium and magnesium is called Soft water. It gives lather with soap easily.

     M-Calcium or Magnesium

    Hard water can be classified into temporary hard water and permanent hard water.

  • Methods to remove temporary hardness
    • Boiling-

    • Clarke's method

  • Methods to remove permanent hardness
    • Treatment with washing soda

    • Calgon's method

      Sodium hexametaphosphate commercially called'calgon', when added to hard water, the following reactions take place.

      M2+ + Na4 P6 O18 2- -----> [Na2MP6O18]2- + 2Na+

    • Ion-exchange method

      2NaZ + M2 ----- > MZ2 + 2Na+

      MZ2 + 2NaCl ------ > 2NaZ + MCl2

    • Synthetic resins method-

      Hard water is softened by using synthetic cation exchangers. This method is more efficient than zeolite process. Cation exchange resins contain large organic molecule with - SO3Hgroup and are water insoluble. Ion exchange resin (RSO3H) is changed to RNa by treating itwith NaCl. The resin exchanges Na+ ions with Ca2+ and Mg2+ ion present in hard water to make the water soft.          

  • Hydrogen Peroxide preparation 
    • Acidifying barium peroxide and removing excess water by evaporation under reduced pressure gives hydrogen peroxide.

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

    • Peroxodisulphate, obtained by electrolytic oxidation of acidified sulphate solutions at high current density, on hydrolysis yields hydrogen peroxide.

      2HSO4- ----- >HO3SOOSO3H ------ > 2HSO4- + 2H+ + H2O2

    • Industrially it is prepared by the auto-oxidationof 2-alklylanthraquinols

      2 ethylanthraquinol< ======= > H202  Catalyst for oxidation – O2

                                                                                         Rduction-H2/Pd

  • Physical properties of Hydrogen Peroxide

    H2O2 is miscible with water in all proportions and forms a hydrate H2O2.H2O (mp 221K). A 30% solution of H2O2 is marketedas '100 volume' hydrogen peroxide. It means thatone millilitre of 30% H2O2 solution will give 100 Vof oxygen at STP. Commercially, it is marketedas 10 V, which means it contains 3% H2O2.

  • Structure

  • Chemical properties

    It acts as an oxidizing as well as reducing agent in both acidic and alkaline media.

  • Storage

    H2O2 decomposes slowly on exposure to light.

    H2O2 ----- > 2H2O + O2

    It is, therefore, stored in wax-lined glass or plastic vessels in dark. Urea can be added as a stabilizer. It is kept away from dust because dust can induce explosive decomposition of the compound.

  • Heavy water

    It is extensively used as a moderator in nuclear reactors and 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.

    It is used for the preparation of other deuterium compounds.

           SO3 + D2O ---- > D2SO4

           CaC2 + 2D2O ---- > C2D2 + Ca(OD)2

 

Sample Examples

 

Question

How many hydrogen-bonded watermolecule(s) are associated inCuSO4.5H2O?

Solution

Only one water molecule, which is outside the brackets (coordination sphere), is hydrogen-bonded. The other four molecules of water are coordinated.

 

Question

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 oxidized 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 H2O, which is a covalent molecule.

 

Question

Calculate the strength of 10 volume solution of hydrogen peroxide.

Solution

10    volume solution of H2O2 means that1L of this H2O2 will give 10 L of oxygen at STP

2H2O2 O2 + H2O

2*34 g      22.4 L at STP

22.4 L of O2at STP is produced from H2O2 = 68 g

10 L of O2 at STP is produced fromH2O2= 30.36 g

Therefore, strength of H2O2in 10 volume H2O2 = 30.36 g/L

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