Acids, bases and ions in aqueous solution

Chapter 7

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.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 Brønstedbase can function as a proton acceptor.

A Brønsted acid can act as a proton donor, and a Brønstedbase can function as a proton acceptor.

Brønsted base

Brønsted acid

Activity

When the concentration of a solute is greater than about0.1 mol/dm3, interactions between the solute molecules orions are significant, and the effective and real concentrationsare 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.

Dr. Said M. El-Kurdi 11

It is not possible to isolate pure H2SO3 - sulfurous acid

phosphinic acid has the formula H3PO2, is monobasic

Inorganic bases: hydroxides

Many inorganic bases are hydroxides, and the term alkali iscommonly 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 simplethe decrease in the XH 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.

as group 16 is descended and X becomes more metallic, its hydride becomes more acidic.

7.6 Trends within a series of oxoacidsEOn(OH)m

empirical methods for estimating Ka

Bell’s ruleWhich 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 andacidic properties

When a metal salt dissolves in water, the cation and anion are hydrated.

ion–dipole interaction

Hexaaqua ion

the first hydration shell

each H2O molecule acts as a Lewis basewhile the metal ion functions as a Lewis acid.

the MO interaction is essentially covalent

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: anintroduction

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.

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, H3BTHF indicates the formation of an adduct.

Investigating coordination complexformation

7.12 Stability constants of coordination complexes

Stepwise stability constants for the formation of[Al(OH2)6xFx](3x)+ (x = 1–6).

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+.

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+.

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+.

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)

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.

Fajan’s rules

Which would be the more polarizable, an F ion or an I ion?

Which would be more polarizing, Na+ or Cs+?