Hydrogen bonds are not purely elec-
trostatic and can be shown to have
some covalent character.
hydrogen bromide A colourless
gas, HBr; m.p. –88.5°C; b.p. –67°C. It
can be made by direct combination
of the elements using a platinum cat-
alyst. It is a strong acid dissociating
extensively in solution (hydrobromic
acid).
hydrogencarbonate (bicarbonate)
A salt of *carbonic acid in which one
hydrogen atom has been replaced; it
thus contains the hydrogencarbonate
ion HCO 3 –.
hydrogen carrier (hydrogen accep-
tor)A molecule that accepts hydro-
gen atoms or ions, becoming reduced
in the process (see oxidation–
reduction). The electron transport
chain, whose function is to generate
energy in the form of ATP during res-
piration, involves a series of hydro-
gen carriers, including NAD and
*FAD, which pass on the hydrogen
(derived from the breakdown of glu-
cose) to the next carrier in the chain.
hydrogen chloride A colourless
fuming gas, HCl; m.p. –114.8°C; b.p.
–85°C. It can be prepared in the labo-
ratory by heating sodium chloride
with concentrated sulphuric acid
(hence the former name spirits of
salt). Industrially it is made directly
from the elements at high tempera-
ture and used in the manufacture of
PVC and other chloro compounds. It
is a strong acid and dissociates fully
in solution (hydrochloric acid).
hydrogen cyanide (hydrocyanic
acid; prussic acid)A colourless liquid
or gas, HCN, with a characteristic
odour of almonds; r.d. 0.699 (liquid at
22 °C); m.p. –14°C; b.p. 26°C. It is an
extremely poisonous substance
formed by the action of acids on
metal cyanides. Industrially, it is made
by catalytic oxidation of ammonia
and methane with air and is used in
producing acrylate plastics. Hydrogen
cyanide is a weak acid (Ka= 2.1 × 10 –9
mol dm–3). With organic carbonyl
compounds it forms *cyanohydrins.hydrogen electrode See hydro-
gen half cell.hydrogen ÛuorideA colourless
liquid, HF; r.d. 0.99; m.p. –83°C; b.p.
19.5°C. It can be made by the action
of sulphuric acid on calciumÛuoride.
The compound is an extremely corro-
siveÛuorinating agent, which attacks
glass. It is unlike the other hydrogen
halides in being a liquid (a result of
*hydrogen bond formation). It is also
a weaker acid than the others be-
cause the small size of theÛuorine
atom means that the H–F bond is
shorter and stronger. Solutions of hy-
drogenÛuoride in water are known
as hydroÛuoric acid.hydrogen half cell (hydrogen elec-
trode) A type of *half cell in which a
metal foil is immersed in a solution
of hydrogen ions and hydrogen gas is
bubbled over the foil. The standard
hydrogen electrode, used in measur-
ing standard *electrode potentials,
uses a platinum foil with a 1.0 M so-
lution of hydrogen ions, the gas at 1
atmosphere pressure, and a tempera-
ture of 25°C. It is written Pt(s)|H 2 (g),
H+(aq), the effective reaction being
H 2 →2H++ 2e.hydrogenic Describing an atom or
ion that has only one electron; for ex-
ample, H, He+, Li2+, C5+. Hydrogenic
atoms (or ions) do not involve elec-
tron–electron interactions and are
easier to treat theoretically.hydrogen iodide A colourless gas,
HI; m.p. –51°C; b.p. –35.38°C. It can
be made by direct combination of
the elements using a platinum cata-
lyst. It is a strong acid dissociating ex-
tensively in solution (hydroiodic acid277 hydrogen iodide
h