9.4 Properties of Dihydrogen
9.4.1 Physical Properties
Dihydrogen is a colourless, odourless, tasteless, combustible gas. It is lighter than air and insoluble in water. Its other physical properties alongwith those of deuterium are given in Table 9.1.
9.4.2 Chemical Properties
The chemical behaviour of dihydrogen (and for that matter any molecule) 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. What inferences would you draw from this fact ? It is because of this factor that the dissociation of dihydrogen into its atoms is only ~0.081% around 2000K which increases to 95.5% at 5000K. Also, it is relatively inert at room temperature due to the high H–H bond enthalpy. Thus, the atomic hydrogen is produced at a high temperature in an electric arc or under ultraviolet radiations. Since its orbital is incomplete with 1s1 electronic configuration, it does combine with almost all the elements. It accomplishes reactions by (i) loss of the only electron to
give H+, (ii) gain of an electron to form H–, and (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.
Reaction with dioxygen: It reacts with dioxygen to form water. The reaction is highly exothermic.
2H2(g) + O2 (g) 
2H2O(l);
= –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 process.
NEETprep Audio Note:
H2(g) +2M(g) → 2MH(s);
where M is an alkali metal
Reactions with metal ions and metal oxides: It reduces some metal ions in aqueous solution and oxides of metals (less active than iron) into corresponding metals.
Reactions with organic compounds: 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.
Problem 9.1
Comment on the reactions of dihydrogen with (i) chlorine, (ii) sodium, and (iii) copper(II) oxide NEETprep Audio Note:
Solution
(i) Dihydrogen reduces chlorine into chloride (Cl–) ion and itself gets oxidised to H+ ion by chlorine to form hydrogen chloride. An electron pair is shared between H and Cl leading to the formation of a covalent molecule.
(ii) Dihydrogen is reduced by sodium to form NaH. An electron is transferred from Na to H leading to the formation of an ionic compound, Na+H–.
(iii) Dihydrogen reduces copper(II) oxide to copper in zero oxidation state and itself gets oxidised to H2O, which is a covalent molecule.
9.4.3 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 (Section 9.5)
• It is used for the preparation of hydrogen chloride, a highly useful chemical.
• In metallurgical processes, it is used to reduce heavy metal oxides to metals.
• 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.