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Study of the First Element : Hydrogen                                                                149








                                                                          3
                                                       2
                                      1
                      Name :   Protium ( H)    Deuterium ( H or D)   Tritium ( H or T)
                                      1
                                                                          1
                                                       1
                 Mass number :     1                  2                  3
           Nuclear composition  :
                     Protons       1                  1                  1
                    Neutrons       Nil                1                  2
                                                         –2
             Relative abundance  :   1            1.56 × 10            1 × 10 –17
                              Nonradioactive     Nonradioactive      Radioactive
                                 Fig. 6.1  Three isotopes of hydrogen
          Preparation of Hydrogen
          Hydrogen can be prepared by various methods:
            ƒ From water               ƒ  From acids            ƒ From alkalis
          Preparation of hydrogen from water
          Water contains about 11% (by mass) of hydrogen. Water is the most
          abundant and the cheapest source of hydrogen. Following are some of the
          methods used for preparing hydrogen from water:


          By electrolysis of water
          This process is suitable for places where cheap electric supply is available.
          Pure water is a poor conductor of electricity. It is made a good conductor of
          electricity by adding a small quantity of sulphuric acid or sodium hydroxide
          to it. Hydrogen obtained by this method is 99.9% pure.

                                                           +
                                                                            2–
                              H SO (aq)      o         2H (aq)     +    SO (aq)
                                2
                                                                            4
                                   4
                              sulphuric acid           hydrogen ion      sulphate ion
                      2–
              The SO  does not get involved in the electrode reaction. The ion which
                     4
                                                                –
                                                        2–
          gets oxidised at the anode in preference to SO  is OH  (coming from the
                                                        4
          ionisation of water).
                            +
              At cathode, H  ions get reduced to give hydrogen gas.
                                                       1
                                     –
          At cathode:   H +   +     e        o           H (g)]           ×  4
                                                       2  2
                                                                      +
          At anode:                H O       o       OH  –   +     H ]    ×  4
                                     2
                                      –
                                                                      –
                                   OH        o       OH      +      e ]   ×  4         ƵƌŝŶŐ ĞůĞĐƚƌŽůLJƐŝƐ ŽĨ ǁĂƚĞƌ͕
                                                                                       ŚLJĚƌŽŐĞŶ ĂŶĚ ŽdžLJŐĞŶ ŐĂƐĞƐ ĂƌĞ
                                   2OH       o       H O     +      O]    ×  2        ĞǀŽůǀĞĚ ŝŶ ƚŚĞ ǀŽůƵŵĞ ƌĂƟŽ ŽĨ Ϯ ͗ ϭ͘
                                                        2
                        O     +     O        o       O (g)
                                                       2
          Net reaction at anode: 2H O        o       4H +    +    O (g) +  4e  –
                                                                     2
                                     2
                                           electricity
          Net overall reaction:  2H O(l)  o 2H (g)         +    O (g)
                                                                     2
                                    2
                                                        2
                                 acidulated          hydrogen      oxygen
                                   water            (at cathode)      (at anode)
          By the action of metals on water
          Metals above hydrogen in the electrochemical series displace hydrogen
          from water under conditions depending upon their reactivity.
          Action of metals on cold water
          Elements like sodium, potassium, calcium, etc. displace hydrogen from
          water at room temperature. With alkali metals, the reaction is explosive.
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