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Lewis acids and bases
ALewis acid is an electron pair acceptor.
ALewis base is an electron pair donor.
Molecules or ions with lone pairs of electrons are capable of behaving as bases, and molecules or
ions which can accept a pair of electrons are capable of behaving as acids. For example, in the
reaction between H and H
Consequently, the reaction that occurs between H and H
H + H
In transition metal chemistry, all ligands are Lewis bases and all metal cations are Lewis acids. The
ligand donates a pair of electrons to the metal.
When a transition metal ions is placed in water, one of the two lone pairs of electrons on each of
the six water molecules is used to form a co-ordinate bond with the metal ion. The result is a
co-ordination compound or complex .
So strong is the bonding between the water molecules and the metal ions, that the hexaaqua ion
remains intact when solutions containing this ion are evaporated. Thus, solid transition metal salts
in the laboratory are often not simple salts but contain a complex, the metal hexaaqua ion.
Examples of `salts' containing the hexaaqua ion in the solid state
green contains [Fe(H
2 2 O pink contains [Co(H
pale violet contains [Fe(H
Whilst reactions of metal(II) salts with water do not appear to be
violent, enough energy is given out out on hydration to overcome the lattice energy of the solid so
that it dissolves; reaction with water breaks down the crystal lattice. When the charge on the
metal ion is increased to 3+, however, the hydration reaction is often very exothermic. Thus,
aluminium chloride dissolves in water violently with evolution of heat and fumes:
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(aq) + 3Cl-(aq)
Note that there is no precipitate of a metal hydroxide in this reaction; in general, when simply
added to water, metal(III) chlorides do not hydrolyse to form metal(III) hydroxides.
Metal ions exist in aqueous solution as 6-water or hexaaqua ions. The hexaaqua ion contains a
metal ion surrounded octahedrally by six oxygen atoms from six water molecules.…read more