fold axis as a special case.) As regards
the cubic groups, Ohis denoted m 3 m
(or 4/m 3
_
2/m), O is denoted 432, This
denoted m3 (or 2/m 3
_
), Tdis denoted
4
_
3 m, and T is denoted 23. In the Her-
mann–Mauguin system all the cubic
groups have 3 as the second number
because of the three-fold axis that oc-
curs in all cubic groups.
heroin (diacetylmorphine)A highly
addictive drug produced by acetylat-
ing *morphine. It is usually used as
the hydrochloride. In the UK it is a
class A drug but it can be prescribed
as a painkiller under the name dia-
morphine. The Marquis and Froedhe
tests are used to give an initial indica-
tion of heroin.
hertzSymbol Hz. The SI unit of
frequency equal to one cycle per
second. It is named after Heinrich
Hertz.
Hess’s law If reactants can be con-
verted into products by a series of re-
actions, the sum of the heats of these
reactions (with due regard to their
sign) is equal to the heat of reaction
for direct conversion from reactants
to products. More generally, the over-
all energy change in going from reac-
tants to products does not depend on
the route taken. The law can be used
to obtain thermodynamic data that
cannot be measured directly. For ex-
ample, the heat of formation of
ethane can be found by considering
the reactions:
2C(s) + 3H 2 (g) + 3½O 2 (g) →2CO 2 (g)
+ 3H 2 O(l)The heat of this reaction is 2∆HC+
3 ∆HH, where ∆HCand ∆HHare the
heats of combustion of carbon and
hydrogen respectively, which can be
measured. By Hess’s law, this is equal
to the sum of the energies for two
stages:
2C(s) + 3H 2 (g) →C 2 H 6 (g)(the heat of formation of ethane, ∆Hf)
and
C 2 H 6 (g) + 3½O 2 →2CO 2 (g) + 3H 2 O(l)
(the heat of combustion of ethane,
∆HE). As ∆HEcan be measured and as
∆Hf+ ∆HE= 2∆Hc+ 3∆HH
∆Hfcan be found. Another example
is the use of the *Born–Haber cycle
to obtain lattice energies. The law
wasÜrst put forward in 1840 by the
Swiss-born Russian chemist Germain
Henri Hess (1802–50). It is sometimes
called the law of constant heat sum-
mationand is a consequence of the
law of conservation of energy.hetero atom An odd atom in the
ring of a heterocyclic compound. For
instance, nitrogen is the hetero atom
in pyridine.heterocyclic See cyclic.heterogeneous Relating to two or
more phases, e.g. a heterogeneous
*catalyst. Compare homogeneous.heterolytic ÜssionThe breaking
of a bond in a compound in which
the two fragments are oppositely
charged ions. For example, HCl →H+
+ Cl–. Compare homolytic fission.
heteronuclear Denoting a mol-
ecule in which the atoms are of dif-
ferent elements.heteropolar bond See chemical
bond.heteropoly compound See clus-
ter compound.heteropolymer See polymer.Heusler alloysFerromagnetic al-
loys containing no ferromagnetic el-
ements. The original alloys contained
copper, manganese, and tin and were
Ürst made by Conrad Heusler (19th-
century mining engineer).
hexachlorobenzene A colourless
crystalline compound, C 6 Cl 6 ; m.p.269 hexachlorobenzene
h