graphite. The process was patented
in 1896 by the US inventor Edward
Goodrich Acheson (1856–1931).
achiral Describing a molecule that
does not contain a *chirality element.
acid 1. A type of compound that
contains hydrogen and dissociates in
water to produce positive hydrogen
ions. The reaction, for an acid HX, is
commonly written:
HX ˆ H
+ + X
–
In fact, the hydrogen ion (the proton)
is solvated, and the complete reaction is:
HX + H 2 O ˆ H 3 O
+ + X
–
The ion H 3 O
+ is the oxonium ion (or
hydroxonium ion or hydronium ion).
This deÜnition of acids comes from
the Arrhenius theory. Such acids tend
to be corrosive substances with a
sharp taste, which turn litmus red
and give colour changes with other
*indicators. They are referred to as
protonic acids and are classiÜed into
strong acids, which are almost completely dissociated in water (e.g. sulphuric acid and hydrochloric acid),
and weak acids, which are only partially dissociated (e.g. ethanoic acid
and hydrogen sulphide). The strength
of an acid depends on the extent to
which it dissociates, and is measured
by its *dissociation constant. See also
base.
2. In the Lowry–Brønsted theory of
acids and bases (1923), the deÜnition
was extended to one in which an
acid is a proton donor (a Brønsted
acid), and a base is a proton acceptor
(a Brønsted base). For example, in
HCN + H 2 O ˆ H 3 O
+ + CN
–
the HCN is an acid, in that it donates
a proton to H 2 O. The H 2 O is acting as
a base in accepting a proton. Similarly, in the reverse reaction H 3 O
+ is
an acid and CN
– a base. In such reactions, two species related by loss or
gain of a proton are said to be conjugate. Thus, in the reaction above
HCN is the conjugate acid of the base
CN
–
, and CN
– is the conjugate base of
the acid HCN. Similarly, H 3 O
+ is the
conjugate acid of the base H 2 O. An
equilibrium, such as that above, is a
competition for protons between an
acid and its conjugate base. A strong
acid has a weak conjugate base, and
vice versa. Under this deÜnition
water can act as both acid and base.
Thus in
NH 3 + H 2 O ˆ NH 4
+ + OH
–
the H 2 O is the conjugate acid of OH
– .
The deÜnition also extends the idea
of acid–base reaction to solvents
other than water. For instance, liquid
ammonia, like water, has a high dielectric constant and is a good ionizing solvent. Equilibria of the type
NH 3 + Na
+
Cl
–
ˆ Na
+ NH 2
– + HCl
can be studied, in which NH 3 and
HCl are acids and NH 2
– and Cl
– are
their conjugate bases.
3. A further extension of the idea of
acids and bases was made in the
Lewis theory (G. N. Lewis, 1923). In
this, a Lewis acid is a compound or
atom that can accept a pair of electrons and a Lewis base is one that
can donate an electron pair. This
deÜnition encompasses ‘traditional’
acid–base reactions. In
HCl + NaOH → NaCl + H 2 O
the reaction is essentially
H
+ + :OH
–
→ H:OH
i.e. donation of an electron pair by
OH
–
. But it also includes reactions
that do not involve ions, e.g.
H 3 N: + BCl 3 → H 3 NBCl 3
in which NH 3 is the base (donor) and
BCl 3 the acid (acceptor). The Lewis
theory establishes a relationship between acid–base reactions and *oxiachiral
6
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in 1896 by the US inventor Edward
Goodrich Acheson (1856–1931).
achiral Describing a molecule that
does not contain a *chirality element.
acid 1. A type of compound that
contains hydrogen and dissociates in
water to produce positive hydrogen
ions. The reaction, for an acid HX, is
commonly written:
HX ˆ H
+ + X
–
In fact, the hydrogen ion (the proton)
is solvated, and the complete reaction is:
HX + H 2 O ˆ H 3 O
+ + X
–
The ion H 3 O
+ is the oxonium ion (or
hydroxonium ion or hydronium ion).
This deÜnition of acids comes from
the Arrhenius theory. Such acids tend
to be corrosive substances with a
sharp taste, which turn litmus red
and give colour changes with other
*indicators. They are referred to as
protonic acids and are classiÜed into
strong acids, which are almost completely dissociated in water (e.g. sulphuric acid and hydrochloric acid),
and weak acids, which are only partially dissociated (e.g. ethanoic acid
and hydrogen sulphide). The strength
of an acid depends on the extent to
which it dissociates, and is measured
by its *dissociation constant. See also
base.
2. In the Lowry–Brønsted theory of
acids and bases (1923), the deÜnition
was extended to one in which an
acid is a proton donor (a Brønsted
acid), and a base is a proton acceptor
(a Brønsted base). For example, in
HCN + H 2 O ˆ H 3 O
+ + CN
–
the HCN is an acid, in that it donates
a proton to H 2 O. The H 2 O is acting as
a base in accepting a proton. Similarly, in the reverse reaction H 3 O
+ is
an acid and CN
– a base. In such reactions, two species related by loss or
gain of a proton are said to be conjugate. Thus, in the reaction above
HCN is the conjugate acid of the base
CN
–
, and CN
– is the conjugate base of
the acid HCN. Similarly, H 3 O
+ is the
conjugate acid of the base H 2 O. An
equilibrium, such as that above, is a
competition for protons between an
acid and its conjugate base. A strong
acid has a weak conjugate base, and
vice versa. Under this deÜnition
water can act as both acid and base.
Thus in
NH 3 + H 2 O ˆ NH 4
+ + OH
–
the H 2 O is the conjugate acid of OH
– .
The deÜnition also extends the idea
of acid–base reaction to solvents
other than water. For instance, liquid
ammonia, like water, has a high dielectric constant and is a good ionizing solvent. Equilibria of the type
NH 3 + Na
+
Cl
–
ˆ Na
+ NH 2
– + HCl
can be studied, in which NH 3 and
HCl are acids and NH 2
– and Cl
– are
their conjugate bases.
3. A further extension of the idea of
acids and bases was made in the
Lewis theory (G. N. Lewis, 1923). In
this, a Lewis acid is a compound or
atom that can accept a pair of electrons and a Lewis base is one that
can donate an electron pair. This
deÜnition encompasses ‘traditional’
acid–base reactions. In
HCl + NaOH → NaCl + H 2 O
the reaction is essentially
H
+ + :OH
–
→ H:OH
i.e. donation of an electron pair by
OH
–
. But it also includes reactions
that do not involve ions, e.g.
H 3 N: + BCl 3 → H 3 NBCl 3
in which NH 3 is the base (donor) and
BCl 3 the acid (acceptor). The Lewis
theory establishes a relationship between acid–base reactions and *oxiachiral
6
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