Molecular Hydrogen

By Author: Pierce Brosnan
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Introduction to Molecular Hydrogen

Molecular Hydrogen is a colourless, odourless, tasteless, combustible gas. It is lighter than air and insoluble in water. Molecular Hydrogen is the most abundant element in the universe (70% of the total mass of the universe) and is the principal element in the solar atmosphere. The giant planets Jupiter and Saturn 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.

Preparation of Molecular Hydrogen

In the laboratory, molecular hydrogen is usually prepared by the reaction of granulated zinc with dilute hydrochloric acid.

Zn + 2H+ Zn2+ + H2

Commercial Production of Molecular Hydrogen

(i) Electrolysis of acidified water using platinum electrodes gives hydrogen.

2H2O ...
... (l) 2H2(g)+O2 (g)

(ii) High purity (>99.95%) Molecular Hydrogen is obtained by electrolysing warm aqueous barium hydroxide solution between nickel electrodes.

(iii) Reaction of steam on hydrocarbons or coke at high temperatures in the presence of catalyst yields hydrogen

CH4(g)+ H2O (g) CO (g) +3H2 (g)

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'.

The production of Molecular Hydrogen can be increased by reacting carbon monoxide of syngas mixtures with steam in the presence of iron chromate as catalyst at 673k.

CO (g) + H2O (g) CO2 (g)+ H2 (g)

This is called water-gas shift reaction.

Carbon dioxide is removed by scrubbing with sodium arsenite solution.

Chemical Properties

The H–H bond dissociation enthalpy of Molecular Hydrogen (435.88 kJ mol–1) is the highest for a single bond between two atoms of any elements. This property is made use of in the atomic hydrogen torch which generates a temperature of ~4000K and is ideal for welding of high melting metals. Though Molecular Hydrogen is rather inactive at room temperature because of very high negative dissociation enthalpy, it combines with almost all the elements.

Hydrogen and chlorine reacts explosively to yield hydrogen chloride in presence of sparks as H2 + Cl2 2HCl.

It reacts with dioxygen at 300°C to form water as 2H2 + O2 2H2O.

It reacts with ammonia by the equation 3H2 + N2 2NH3.

Hydrogen reduces the oxides of most metals and many metallic salts to the metals at elevated temperatures and pressures. For example, hydrogen gas and ferrous oxide react, yielding metallic iron and water, H2 + FeO Fe + H2O.

It reacts with many organic compounds in the presence of catalysts to give useful hydrogenated products of commercial importance. For example: (i) Hydrogenation of vegetable oils using nickel as catalyst gives edible fats (margarine and vanaspati ghee). (ii) Hydroformylation of olefins yields aldehydes which further undergo reduction to give alcohols.

H2 +CO +RCH =CH2 RCH2CH2CHO.

H2 +RCH2CH2CHO RCH2CH2CH2OH.

Uses of Molecular Hydrogen

The largest single use of Molecular Hydrogen is in the synthesis of ammonia which is used in the manufacture of nitric acid and nitrogenous fertilizers.

Molecular Hydrogen is used in the manufacture of vanaspati fat by the hydrogenation of polyunsaturated Vegetable oils like soyabean, cotton seeds etc.

It is used for the preparation of hydrogen chloride, a highly useful chemical.

In space programmes, it is used as a rocket fuel. In fact, it has promising potential for use as a non-polluting fuel of the near future (Hydrogen Economy).

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