- Colourless, tasteless, odourless gas with very low solubility in water.
- It is the lightest gas or molecule.
- Under high pressure and low temperatures, H2 can be liquefied.
Dihydrogen is chemically inert, as the bond dissociation energy for dihydrogen is quite high at 435.88 KJ/mol, which is the highest for any single bond between any two atoms. Thus, a temperature of over 5000 K needs to be applied to dissociate H2.
However, H2 is highly combustible and burns in air to give H2O
H2(g) + O2(g) H2O(l)
With Electropositive Metals
H2 forms hydrides (compounds in which the oxidation state of H is -1) with alkali and alkaline earth metals and other electropositive metals at high temperatures. These compounds can either be covalent or ionic in nature.
2 K + H2 2 KH, above 3000 C
Mg + H2 MgH2, above 3000 C
H2 reacts with N2 in presence of catalyst (finely divided Fe/Mo) at 673 K high pressure to give NH3. This process is known as Haber's process, which is employed in the commercial preparation of NH3.
N2(g) + 3 H2(g) 2 NH3(g)
With Halogens, X2, dihydrogen reacts under various conditions depending on the halogen to give, HX
H2 + F2 2 HF, dark
H2 + Cl2 2 HCl, sunlight
H2 + Br2 2 HBr, 673 K
H2 + I2 2 HI, 673 K, Pt catalyst
H2 as a reducing agent
H2 is a very good reducing agent and reduces metal oxides to metals.
CuO + H2 Cu + H2O
Fe3O4 + H2 3 Fe + 4 H2O
Note: H2 is capable of reducing only less electropositive metals such as transition metals and heavy metals.
However, alkali and alkaline earth metals and other electropositive metals cannot be reduced by this method.
Applications of H2
- Haber's process for the manufacture of NH3
- Bulk Production of Methanol and HCl
- Hydrogenation of unsaturated oils or unsaturated hydrocarbons