electrolytes conduct because of the
presence of ions. He also worked on
kinetics and proposed the *Arrhenius equation. He was the Ürst to predict the greenhouse effect resulting
from carbon dioxide. Arrhenius was
awarded the 1903 Nobel Prize for
chemistry.
Arrhenius equation An equation
of the form
k = Aexp(–E a /RT)
where k is the rate constant of a
given reaction and E a the *activation
energy. A is a constant for a given reaction, called the pre-exponential
factor (or A-factor). Often the equation is written in logarithmic form
lnk = lnA – E a /RT
A graph of lnk against 1/T is a straight
line with a gradient –E a /R and an intercept on the lnk axis of lnA. It is
named after Svante *Arrhenius.
Arrhenius theory See acid.
arsenate(III) See arsenic(iii) oxide.
arsenate(V) See arsenic(v) oxide.
arsenic Symbol As. A metalloid element of *group 15 (formerly VB) of
the periodic table; a.n. 33; r.a.m.
74.92; r.d. 5.7; sublimes at 613°C. It
has three allotropes – yellow, black,
and grey. The grey metallic form is
the stable and most common one.
Over 150 minerals contain arsenic
but the main sources are as impurities in sulphide ores and in the minerals orpiment (As 2 S 3 ) and realgar
(As 4 S 4 ). Ores are roasted in air to
form arsenic oxide and then reduced
by hydrogen or carbon to metallic arsenic. Arsenic compounds are used in
insecticides and as doping agents in
semiconductors. The element is included in some lead-based alloys to
promote hardening. Confusion can
arise because As 4 O 6 is often sold as
white arsenic. Arsenic compounds
are accumulative poisons. The element will react with halogens, concentrated oxidizing acids, and hot alkalis. Albertus Magnus is believed to
have been the Ürst to isolate the element in 1250.
A
• Information from the WebElements site
arsenic acid See arsenic(v) oxide.
arsenic(III) acid See arsenic(iii)
oxide.
arsenic hydride See arsine.
arsenic(III) oxide (arsenic trioxide;
arsenious oxide; white arsenic) A
white or colourless compound,
As 4 O 6 , existing in three solid forms.
The commonest has cubic or octahedral crystals (r.d. 3.87; sublimes at
193°C) and is soluble in water,
ethanol, and alkali solutions. It occurs naturally as arsenolite. A vitreous form can be prepared by slow
condensation of the vapour (r.d.
3.74); its solubility in cold water is
more than double that of the cubic
form. The third modiÜcation, which
occurs naturally as claudetite, has
monoclinic crystals (r.d. 4.15). Arsenic(III) oxide is obtained commercially as a byproduct from the
smelting of nonferrous sulphide ores;
it may be produced in the laboratory
by burning elemental arsenic in air.
The structure of the molecule is similar to that of P 4 O 6 , with a tetrahedral
arrangement of As atoms edge linked
by oxygen bridges. Arsenic(III) oxide
is acidic; its solutions were formerly
called arsenious acid (technically, arsenic(III) acid). It forms arsenate(III)
salts (formerly called arsenites). Arsenic(III) oxide is extremely toxic and
is used as a poison for vermin; trace
doses are used for a variety of medicinal purposes. It is also used for producing opalescent glasses and
enamels.
arsenic(V) oxide (arsenic oxide) A
Arrhenius equation
44
a
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presence of ions. He also worked on
kinetics and proposed the *Arrhenius equation. He was the Ürst to predict the greenhouse effect resulting
from carbon dioxide. Arrhenius was
awarded the 1903 Nobel Prize for
chemistry.
Arrhenius equation An equation
of the form
k = Aexp(–E a /RT)
where k is the rate constant of a
given reaction and E a the *activation
energy. A is a constant for a given reaction, called the pre-exponential
factor (or A-factor). Often the equation is written in logarithmic form
lnk = lnA – E a /RT
A graph of lnk against 1/T is a straight
line with a gradient –E a /R and an intercept on the lnk axis of lnA. It is
named after Svante *Arrhenius.
Arrhenius theory See acid.
arsenate(III) See arsenic(iii) oxide.
arsenate(V) See arsenic(v) oxide.
arsenic Symbol As. A metalloid element of *group 15 (formerly VB) of
the periodic table; a.n. 33; r.a.m.
74.92; r.d. 5.7; sublimes at 613°C. It
has three allotropes – yellow, black,
and grey. The grey metallic form is
the stable and most common one.
Over 150 minerals contain arsenic
but the main sources are as impurities in sulphide ores and in the minerals orpiment (As 2 S 3 ) and realgar
(As 4 S 4 ). Ores are roasted in air to
form arsenic oxide and then reduced
by hydrogen or carbon to metallic arsenic. Arsenic compounds are used in
insecticides and as doping agents in
semiconductors. The element is included in some lead-based alloys to
promote hardening. Confusion can
arise because As 4 O 6 is often sold as
white arsenic. Arsenic compounds
are accumulative poisons. The element will react with halogens, concentrated oxidizing acids, and hot alkalis. Albertus Magnus is believed to
have been the Ürst to isolate the element in 1250.
A
• Information from the WebElements site
arsenic acid See arsenic(v) oxide.
arsenic(III) acid See arsenic(iii)
oxide.
arsenic hydride See arsine.
arsenic(III) oxide (arsenic trioxide;
arsenious oxide; white arsenic) A
white or colourless compound,
As 4 O 6 , existing in three solid forms.
The commonest has cubic or octahedral crystals (r.d. 3.87; sublimes at
193°C) and is soluble in water,
ethanol, and alkali solutions. It occurs naturally as arsenolite. A vitreous form can be prepared by slow
condensation of the vapour (r.d.
3.74); its solubility in cold water is
more than double that of the cubic
form. The third modiÜcation, which
occurs naturally as claudetite, has
monoclinic crystals (r.d. 4.15). Arsenic(III) oxide is obtained commercially as a byproduct from the
smelting of nonferrous sulphide ores;
it may be produced in the laboratory
by burning elemental arsenic in air.
The structure of the molecule is similar to that of P 4 O 6 , with a tetrahedral
arrangement of As atoms edge linked
by oxygen bridges. Arsenic(III) oxide
is acidic; its solutions were formerly
called arsenious acid (technically, arsenic(III) acid). It forms arsenate(III)
salts (formerly called arsenites). Arsenic(III) oxide is extremely toxic and
is used as a poison for vermin; trace
doses are used for a variety of medicinal purposes. It is also used for producing opalescent glasses and
enamels.
arsenic(V) oxide (arsenic oxide) A
Arrhenius equation
44
a
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