Properties of H2 (Dihydrogen)

Properties of H₂  (Dihydrogen)

Dihydrogen is a colorless, odorless, tasteless, combustible gas. It is lighter than air and insoluble in water.

Chemical properties

The chemical behavior 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. 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 200K which increases to 95.5% at 500K. 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,

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

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.

Reaction with dinitrogen: with dinitrogen it forms ammonia.

This is the method for the manufacture of ammonia by the Haber process.

Reactions with metals: With many metals it combines at high a temperature to yield the corresponding hydrides.

H₂ (g)  +  2M (g)   →  2MH(s);   where M is an alkali meal.

Reaction 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.

H₂ (g) +  Pd²⁺ (aq)   →  Pd(s)  +  2H⁺ (aq)

yH₂ (g) + M_xO_y (s)   →    xM(s)  +  yH₂O (1)

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

•  Hydrogenation of vegetable oils using nickel as catalyst gives edible fat (margarine and vanaspati ghee)
•  Hydroformylation of olefins yields aldehydes which further undergo reduction to give alcohols.

H₂  +  CO  +  RCH  =  CH₂   →   RCH₂CH₂CHO

           H₂  +  RCH₂CH₂CHO   →   RCH₂CH₂CH₂OH