Chapter 7
Acids, bases and ions in aqueous solution
7.1 Introduction
Liquid water is approximately 55 molar H2O,
7.2 Properties of water
structure
Part of the structure of ordinary
ice; it consists of a 3-dimensional
network of hydrogen-bonded
H2O molecules.
The variation in the value of the density of water between 283 and
373 K.
The self-ionization of water
If a pure liquid partially dissociates into ions, it is self ionizing.
Water as a Brønsted acid or base
A Brønsted acid can act as a proton donor, and a Brønsted
base can function as a proton acceptor.
Brønsted base
Brønsted acid
Activity
When the concentration of a solute is greater than about
0.1 mol/dm3, interactions between the solute molecules or
ions are significant, and the effective and real concentrations
are no longer equal.
The relative activity, ai, of a component i is dimensionless
i is the activity coefficient of the solute, and mi is the
molality
7.4 Some Brønsted acids and bases
Carboxylic acids: examples of mono-, di- and
polybasic acids
Inorganic acids
Each of the hydrogen halides is monobasic and for X = Cl, Br
and I, the equilibrium lies far to the right-hand side, making
these strong acids
Hydrogen fluoride, on the other hand, is a weak acid
(pKa = 3.45).
oxoacid
Examples of oxoacids include hypochlorous acid (HOCl),
perchloric acid (HClO4), nitric acid (HNO3), sulfuric acid
(H2SO4) and phosphoric acid (H3PO4).
 oxoacids may be mono-, di- or polybasic;
 not all the hydrogen atoms in an oxoacid are necessarily
ionizable.
It is not possible to isolate pure H2SO3 - sulfurous acid
Dr. Said M. El-Kurdi
11
phosphinic acid has the formula H3PO2, is monobasic
Inorganic bases: hydroxides
Many inorganic bases are hydroxides, and the term alkali is
commonly used.
The group 1 hydroxides NaOH, KOH, RbOH and CsOH are
strong bases, being essentially fully ionized in aqueous
solution; LiOH is weaker (pKb = 0.2).
Inorganic bases: nitrogen bases
7.5 The energetics of acid dissociation in aqueous solution
Hydrogen halides
H2S, H2Se and H2Te
the explanation of the trend in values is not simple
the decrease in the XH bond strength with the increasing atomic number
of X plays an important role
as group 16 is descended and X becomes more metallic, its hydride
becomes more acidic.
7.6 Trends within a series of oxoacids
EOn(OH)m
empirical methods for estimating Ka
Bell’s rule
Which relates the first acid dissociation constant to the
number of ‘hydrogen-free’ O atoms in an acid of formula
EOn(OH)m.
The increase in acid strength with increase in the number of
O atoms attached to atom E is generally attributed to the
greater possibility in the conjugate base of delocalization of
negative charge onto the O atoms.
7.7 Aquated cations: formation and
acidic properties
When a metal salt dissolves in water, the cation and anion are
hydrated.
ion–dipole interaction
Hexaaqua ion
each H2O molecule acts as a Lewis base
while the metal ion functions as a Lewis acid.
the MO interaction is essentially covalent
the first hydration shell
7.8 Amphoteric oxides and hydroxides
Periodic trends in amphoteric properties
some elements that lie next to the line ‘diagonal line’
(e.g. Si) are semi-metals
Be(OH)2 and BeO are amphoteric
Al2O3, Ga2O3, In2O3, GeO, GeO2, SnO, SnO2, PbO, PbO2,
As2O3, Sb2O3 and Bi2O3 are amphoteric.
7.9 Solubilities of ionic salts
Solubility and saturated solutions
The temperature-dependence of the solubilities in water
Sparingly soluble salts and solubility products
The energetics of the dissolution of an ionic salt:
solGo
7.11 Coordination complexes: an
introduction
Definitions and terminology
The word ligand is derived from the Latin verb ‘ligare’ meaning
‘to bind’.
In a coordination complex, a central atom or ion is
coordinated by one or more molecules or ions (ligands) which
act as Lewis bases, forming coordinate bonds with the central
atom or ion; the latter acts as a Lewis acid.
Atoms in the ligands that are directly bonded to the central
atom or ion are donor atoms.
In a complex:
 a line is used to denote the interaction between an
anionic ligand and the acceptor;
 an arrow is used to show the donation of an electron pair
from a neutral ligand to an acceptor.
When a Lewis base donates a pair of electrons to a Lewis
acid, a coordinate bond is formed and the resulting species
is an adduct. The centred dot in, for example, H3BTHF
indicates the formation of an adduct.
Investigating coordination complex
formation
7.12 Stability constants of
coordination complexes
Stepwise stability constants for the formation of
[Al(OH2)6xFx](3x)+ (x = 1–6).
The stabilities of complexes of the non-d-block metal ions
of a given charge normally decrease with increasing cation
size .
Thus, for a complex with a given ligand, L, the order of
stability is Ca2+ > Sr2+ > Ba2+.
For ions of similar size, the stability of a complex with a
specified ligand increases substantially as the ionic charge
increases, e.g. Li+ < Mg2+ < Al3+.
Hard and soft metal centres and ligands
The principle of hard and soft acids and bases (HSAB)
 hard, metal ions and ligands containing particular donor
atoms exhibit trends in stabilities as follows:
 soft, metal ions and ligands containing these donor
atoms are:
Pearson’s classification of hard and soft acids comes from a
consideration of a series of donor atoms placed in order of
electronegativity:
A hard acid is one that forms the most stable complexes
with ligands containing donor atoms from the left-hand end
of the series. The reverse is true for a soft acid.
Polarizability
The polarizability, of an atom is its ability to be distorted by an
electric field (such as that of a neighboring ion)
Fajan’s rules
 Small, highly charged cations have polarizing ability.
 Large, highly charged anions are easily polarized.
 Cations that do not have a noble-gas electron configuration
are easily polarized.
Which would be the more polarizable, an F ion or an I 
ion?
Which would be more polarizing, Na+ or Cs+?