Chemical Properties

Why the H–H Bond Controls Everything

The chemical behaviour of dihydrogen is dominated by the strength of its H–H bond, which has the highest bond dissociation enthalpy of any single bond between two atoms of any element. This is why $H_2$ resists splitting into atoms except at very high temperature: only about 0.081% of molecules dissociate near 2000 K, rising to 95.5% by 5000 K. At room temperature it is correspondingly unreactive; atomic hydrogen can be generated deliberately using an electric arc or ultraviolet radiation.

Because its $1s^1$ orbital is incomplete, hydrogen can react in three distinct ways: losing its electron to form $H^+$, gaining an electron to form the hydride ion $H^-$, or sharing its electron to form a single covalent bond.

Reaction with halogens gives hydrogen halides:

Reactivity varies sharply across the series — the reaction with fluorine proceeds even in the dark, while iodine needs a catalyst to react at a useful rate.

Reaction with dioxygen is highly exothermic, forming water:

Reaction with dinitrogen gives ammonia — the basis of the industrial Haber process:

Reaction with metals, particularly alkali metals, at high temperature yields the corresponding hydrides (covered fully in section 9.5):

Reaction with metal ions and metal oxides. Dihydrogen can reduce some metal ions in solution and reduce oxides of metals less active than iron back to the free metal:

$yH_2(g) + M_xO_y(s) \rightarrow xM(s) + yH_2O(l)$ …