Properties of dihydrogen

A. Physical properties: dihydrogen is a colourless, odourless, tasteless gas that burns with a pale blue flame; it is non-polar, essentially insoluble in water, and lighter than air.

B. Chemical properties: (i) Reaction with metals — dihydrogen combines with reactive metals (alkali metals, and alkaline earth metals such as calcium, strontium and barium) at high temperature to form ionic metal hydrides, e.g. 2Na(s) + H2(g) → 2NaH(s); in this respect dihydrogen behaves like the halogens, which likewise react with metals to form metal halides. (ii) Reaction with dioxygen — dihydrogen reacts with O2, either with a catalyst or on heating, in a highly exothermic reaction to form water: 2H2(g) + O2(g) →(catalyst/heat) 2H2O(l); ΔH = −235 kJ/mol. (iii) Reaction with halogens — dihydrogen inflames with fluorine even at −250°C in the dark, but needs a catalyst to react with iodine; the vigour of the reaction falls off as the halogen's atomic number increases: H2(g) + X2(g) → 2HX(g); here too dihydrogen resembles the alkali metals, which also combine with halogens to form halides. (iv) Reducing nature — dihydrogen reduces the oxides (and certain ions) of metals less reactive than iron down to the free metal at moderate temperature, for example CuO(s) + H2(g) → Cu(s) + H2O(l), Fe3O4(s) + 4H2(g) → 3Fe(s) + 4H2O(l), and Pd²⁺(aq) + H2(g) → Pd(s) + 2H⁺(aq). This reducing ability extends to unsaturated organic compounds too: hydrogenating an unsaturated vegetable oil over a nickel catalyst converts its C=C double bonds to saturated …