Hydrogen

Hydrogen

Hydrogen has the simplest atomic structure among all the elements around us in Nature. In atomic form it consists of only one proton and one electron. However, in elemental form it exists as a diatomic (H₂) molecule and is called dihydrogen. It forms more compounds than any other elements.

Position of Hydrogen in the Periodic Table

Hydrogen is the first element in the periodic table. However, its placement in the periodic table has been a subject of discussion in the past. As we all know that the elements in the periodic table are arranged according to their electronic configurations.

Hydrogen has electronic configuration 1s¹. On one hand, its electronic configuration is similar to the outer electronic configuration (ns¹) of alkali metals, which belong to the first group of the periodic table. On the other hand, like halogens (with ns²np⁵ configuration belonging to the seventeenth group of the periodic table), it is short by one electron to the corresponding noble gas configuration, helium (1s²). Hydrogen, therefore, has resemblance to alkali metals, which lose one electron to form unipositive ions, as well as with halogens, which gain one electron to form uninegative ion. Like alkali metals, hydrogen forms oxides, halides and sulphides. However, unlike alkali metals, it has a very high ionization enthalpy and does not possess metallic characteristics under normal conditions. 

In fact, in terms of ionization enthalpy, hydrogen resembles more with halogens, 🛆_iH of Li is 520 kJ mol^-1, F is 1680 kJ mol^-1 and that of H is 1312 kJ mol^-1. 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.

Dihydrogen, H₂

Dihydrogen is the most abundant element in the universe (70% of the total mass of the universe) and is the principle element in the solar atmosphere. The giant planets Jupiter and Satrun consist mostly of hydrogen. However, due to its light nature, it is much less abundant (0.15% by mass) in the earth’s atmosphere. Of course, in the combined form it constitutes 15.4% of the earth’s crust and the oceans. In the combined form besides in water, it occurs in plant and animal tissues, carbohydrates, proteins, hydrides including hydrocarbons and many other compounds.

Isotopes of Hydrogen

Hydrogen has three isotopes: Protium, ¹₁H, deuterium, ²₁H or D and tritium, ³₁H or T. These isotopes differ from one another in respect of the presence of neutrons. Oridinary hydrogen, protium, has no neutrons, deuterium (also known as heavy hydrogen) has one and tritium has two neutrons in the nucleus. In the year 1934, an American scientist, Harold C, Urey, got Noble Prize for separation hydrogen isotope of mass number 2 by physical methods.

The predominant form is protium. Terrestrial hydrogen contains 0.0156% of deuterium mostly in the form of HD. The tritium concentration is about one atom per 10¹⁸ atoms of protium. Of these isotopes, only tritium is radioactive and emits low energy β^- particles.

Preparation of Dihydrogen, 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.