5 Hydrogen
One of the most readily observed reactions in chemistry is the
familiar production of bubbles of a colourless gas when certain
metals (for example, iron, zinc) react with dilute acids. Cavendish
investigated these reactions rather more than 200 years ago, and
found the gas evolved to be the same in each case; the gas, later
named hydrogen, was much lighter than air and when burned in air
produced water.
Hydrogen in the combined state, mainly as water, hydrocarbons
and other organic compounds, constitutes about 11 % of the earth's
crust by weight*. Hydrogen gas is not very reactive; it reacts
spontaneously with very electropositive elements (some ot the
metals of Groups I and II) and with the very electronegative element
fluorine; with other elements, reactions usually require a catalyst—
heat or light—and even then may be incomplete. If hydrogen gas
is passed through a solution containing a strongly oxidising ion,
for example manganate(VII) (permanganate)MnO 4 or iron(III).
Fe(Jaq), reduction does not take place unless a catalyst is present, and
even then it is often slow and incomplete, despite the fact that for
the redox system H 3 O+ + e~ -> jH?(g) + H 2 O, £^ = OV, i.e.
hydrogen is a mild reducing agent. This absence of reactivity does
not usually arise because the hydrogen molecule is energetically
stable, but rather because it is kinetically stable (p. 64); almost any
process in which the hydrogen molecule is to participate must
involve the breaking of the H—H bond, which is relatively strong
(p. 72), This kinetic stability can be removed by a catalyst (for
example heat, light, a metal surface) which breaks up the hydrogen
- Large-scale methods of producing hydrogen are considered in a later chapter
fp. 180). "
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