fold axis as a special case.) As regards
the cubic groups, O h is denoted m3m
(or 4/m 3
_
2/m), O is denoted 432, T h is
denoted m3 (or 2/m 3
_
), T d is denoted
4
_
3m, and T is denoted 23. In the Hermann–Mauguin system all the cubic
groups have 3 as the second number
because of the three-fold axis that occurs in all cubic groups.
heroin (diacetylmorphine) A highly
addictive drug produced by acetylating *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 diamorphine. The Marquis and Froedhe
tests are used to give an initial indication of heroin.
hertz Symbol 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 converted into products by a series of reactions, 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 overall energy change in going from reactants to products does not depend on
the route taken. The law can be used
to obtain thermodynamic data that
cannot be measured directly. For example, 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∆H C +
3∆H H , where ∆H C and ∆H H are 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, ∆H f )
and
C 2 H 6 (g) + 3½O 2 → 2CO 2 (g) + 3H 2 O(l)
(the heat of combustion of ethane,
∆H E ). As ∆H E can be measured and as
∆H f + ∆H E = 2∆H c + 3∆H H
∆H f can be found. Another example
is the use of the *Born–Haber cycle
to obtain lattice energies. The law
was Ürst put forward in 1840 by the
Swiss-born Russian chemist Germain
Henri Hess (1802–50). It is sometimes
called the law of constant heat summation and 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 Üssion The 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 molecule in which the atoms are of different elements.
heteropolar bond See chemical
bond.
heteropoly compound See cluster compound.
heteropolymer See polymer.
Heusler alloys Ferromagnetic alloys containing no ferromagnetic elements. The original alloys contained
copper, manganese, and tin and were
Ürst made by Conrad Heusler (19thcentury mining engineer).
hexachlorobenzene A colourless
crystalline compound, C 6 Cl 6 ; m.p.
269
hexachlorobenzene
h
www.AzShimi.ir www.AzShimi.com
the cubic groups, O h is denoted m3m
(or 4/m 3
_
2/m), O is denoted 432, T h is
denoted m3 (or 2/m 3
_
), T d is denoted
4
_
3m, and T is denoted 23. In the Hermann–Mauguin system all the cubic
groups have 3 as the second number
because of the three-fold axis that occurs in all cubic groups.
heroin (diacetylmorphine) A highly
addictive drug produced by acetylating *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 diamorphine. The Marquis and Froedhe
tests are used to give an initial indication of heroin.
hertz Symbol 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 converted into products by a series of reactions, 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 overall energy change in going from reactants to products does not depend on
the route taken. The law can be used
to obtain thermodynamic data that
cannot be measured directly. For example, 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∆H C +
3∆H H , where ∆H C and ∆H H are 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, ∆H f )
and
C 2 H 6 (g) + 3½O 2 → 2CO 2 (g) + 3H 2 O(l)
(the heat of combustion of ethane,
∆H E ). As ∆H E can be measured and as
∆H f + ∆H E = 2∆H c + 3∆H H
∆H f can be found. Another example
is the use of the *Born–Haber cycle
to obtain lattice energies. The law
was Ürst put forward in 1840 by the
Swiss-born Russian chemist Germain
Henri Hess (1802–50). It is sometimes
called the law of constant heat summation and 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 Üssion The 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 molecule in which the atoms are of different elements.
heteropolar bond See chemical
bond.
heteropoly compound See cluster compound.
heteropolymer See polymer.
Heusler alloys Ferromagnetic alloys containing no ferromagnetic elements. The original alloys contained
copper, manganese, and tin and were
Ürst made by Conrad Heusler (19thcentury mining engineer).
hexachlorobenzene A colourless
crystalline compound, C 6 Cl 6 ; m.p.
269
hexachlorobenzene
h
www.AzShimi.ir www.AzShimi.com