Properties and Uses of Dihydrogen

Physical Properties:

·        Dihydrogen is a colourless, odourless, tasteless, highly combustible and non-toxic gas.

·        It is lighter than air and insoluble in water.

Image result for hydrogen crystal structure

Hydrogen crystal structure

·        Its melting temperature is -259.2 degrees Celsius.

·        Its boiling temperature is -252.76 degrees Celsius.

·        In this interval, hydrogen is a colourless liquid without any smell.

·         At high pressure, it forms snow-like crystals of solid hydrogen.

Chemical Properties:

            The chemical behaviour of dihydrogen is determined to a large extent by bond dissociation enthalpy. The H–H bond dissociation enthalpy is the highest for a single bond between two atoms of any element.

It accomplishes reactions by,

        i.            Loss of the only electron to give H+

     ii.            Gain of an electron to form H–

   iii.            Sharing electrons to form a single covalent bond

The chemistry of dihydrogen can be illustrated by the following reactions:

Reaction with Halogens:

·        It reacts with halogens (X2) to give hydrogen halides (HX).

·        While the reaction with fluorine occurs even in the dark, with iodine it requires a catalyst.

·        The reaction is highly exothermic.

H2 (g) + X2 (g) → 2HX (g)                   (X = F, Cl, Br, I)

Reaction with Dioxygen:

·        It reacts with dioxygen to form water.

·        The reaction is highly exothermic.

2H2 (g) + O2 (g)   2H2O (l)          (H = -285.9 KJ mol-1)

Reaction with Dinitrogen:

·        With dinitrogen it forms ammonia.

·        This is the method for the manufacture of ammonia by the haber’s process.

3H2 (g) + N2 (g)  2NH3 (g)                    (H = -92.6 KJ mol-1)

Reaction with Metals:

·        With many metals it combines wih at high temperature to yield the corresponding hydrides.

·        Here M is an alkali metal.

   H2 (g) + 2M (g)  2MH (s)

Reaction with Metal Ions and Metal Oxides:

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

H2 (g) + Pd2 (aq)

Pd (s) + 2H+ (aq)

yH2 (g) + MxOy (s)

xM (s) + yH2O (l)

Reaction with Organic Compounds:

It reacts with many organic compounds in the presence of catalysts to give useful hydrogenated products of commercial importance.

H2 + CO + RCH = CH2

RCH2 CH2CHO

H2 RCH2 CH2 CHO

RCH2 CH2 CH2OH

Uses of Dihydrogen:

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

·        Dihydrogen is used in the manufacture of vanaspati fat by the hydrogenation of polyunsaturated vegetable oils like soyabean, cotton seeds etc.

·        It is used in the manufacture of bulk organic chemicals particularly methanol.

·        It is widely used for the manufacture of metal hydrides.

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

·        Atomic hydrogen and oxy-hydrogen torches find use for cutting and welding purposes.

·        Atomic hydrogen atoms (produced by dissociation of dihydrogen with the help of an electric arc) are allowed to recombine on the surface to be welded to generate the temperature of 4000 K.

·        It is used as a rocket fuel in space research.

·        Dihydrogen is used in fuel cells for generating electrical energy.

·        It has many advantages over the conventional fossil fuels and electric power.

·        It does not produce any pollution and releases greater energy per unit mass of fuel in comparison to gasoline and other fuels.