Preparation of H2

Preparation of  H₂

There are a number of methods for preparing dihydrogen from metals and metal hydrides.

Laboratory Preparation of Dihydrogen

1.    It is usually prepared by the reaction of granulated zinc with dilute hydrochloric acid.

       Zn  +  2H⁺   -   Zn²⁺  +  H₂

2.   It can also be prepared by the reaction of zinc with aqueous alkali.

     Zn  +  2NaOH   →   Na₂ZnO₂  +  H₂

                                     Sodium Zincate

Commercial production of Dihydrogen

The commonly used processes are outlined below:
(1) Electrolysis of acidified water using platinum electrodes gives hydrogen.

(2) High purity (>99.95%) dihydrogen is obtained by electrolysis warm aqueous barium hydroxide          solution between nickel electrodes.

(3) It is obtained as a byproduct in the manufacture of sodiumhydroxide and chlorine by the electrolysis of brine solution. During electrolysis, the reactions that take place are:

            At anode: 2Cl^- (aq)   -   Cl₂ (g)  +  2e^-

            At cathode: 2H₂O (I)  +  2e   -   H₂ (g)  +  2OH^- (aq)

The overall reaction is

(4) Reaction of Steam on hydrocarbons or coke at high temperatures in the presence of catalyst yields hydrogen.

The mixture of CO and H₂ is called water gas. As this mixture of CO and H₂ is used for the synthesis of methanol and a number of hydrocarbons, it is also called synthesis gas or ‘syngas’. Nowadays “syngas’ is produced from sewage, saw-dust, scrap wood, newspapers etc. the process of producing ‘syngas’ from coal is called ‘coal gasification’.

The production of digydrogen can be increased by reacting carbon monoxide of syngas mixtures with steam in the presence of iron chromate as catalyst.

This is called water-gas shift reaction. Carbon dioxide is removed by scrubbing with sodium arsenite solution.

Presently ~77% of the industrial dihydrogen is produced from petro-chemicals, 18% from coal, 4% from electrolysis of acqueous solutions and 1% from other sources.