Haber-Weiss reaction The Haber-Weiss cycle con-
sists of the following two reactions:

H 2 O 2 + OH.ÔH 2 O + O 2 – + H+and

H 2 O 2 + O 2 – ÔO 2 + OH–+ OH.

The second reaction achieved notoriety as a possi-
ble source of hydroxyl radicals. However, it has a negli-
gible rate constant. It is believed that iron(III)
complexes can catalyze this reaction: first Fe(III) is
reduced by superoxide, followed by oxidation by dihy-
drogen peroxide.
See alsoFENTON REACTION.

habituation Reaction to the repeated presentation of
the same stimulus that has no special significance and
that causes reduced attention to the stimulus. Short-
term habituation lasts for only minutes, while longer-
termhabituation could last for weeks.

Haeckelian recapitulation (embryonic recapitulation)
Aclade goes through, in its development, an ontoge-
netic stage that is present in the adults of its sister
group. Named for Ernst Haeckel (1834–1919), who
promoted the idea that the development of the human
embryo in the womb is a rerun (recapitulation) of the
steps in humanity’s rise from a primitive creature.
Haeckel’s views about evolution have been refuted by
most scientists.

half-life For a given reaction the half-life, t^1 / 2 , of a

reactant is the time required for its concentration to

reach a value that is the arithmetic mean of its initial

and final (equilibrium) value. For a reactant that is

entirely consumed, it is the time taken for the reactant

concentration to fall to one-half of its initial value. For

a first-order reaction, the half-life of the reactant can be

called the half-life of the reaction. In nuclear chemistry,

(radioactive) half-life is defined, for a simple radioac-

tive decay process, as the time required for the activity

to decrease to half its value by that process.

See alsoBIOLOGICAL HALF-LIFE;GEOLOGICAL TIME.

haloperoxidase A PEROXIDASE that catalyzes the

oxidative transformation of halides to XO–(X being

Cl, Br, or I) or to organic halogen compounds. Most

are HEME proteins, but some bromoperoxidases from

algae are vanadium-containing ENZYMEs.

halophyte A plant that has adapted to grow in salt-

rich soils or salt-rich air, e.g., glassworts (Salicornia vir-

ginica). Obligatory halophytes need salt, whereas

facultative halophytes can live in freshwater conditions

as well.

hansch analysis The investigation of the quantitative

relationship between the biological activity of a series

of compounds and their physicochemical substituent or

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