Acids, bases and ions in aqueous solution Chapter 7

7.1 Introduction Liquid water is approximately 55 molar H 2 O, 7.2 Properties of water structure Part of the structure of ordinary ice; it consists of a 3-dimensional network of hydrogen-bonded H 2 O 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. 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/dm 3, interactions between the solute molecules or ions are significant, and the effective and real concentrations are no longer equal. The relative activity, a i, of a component i is dimensionless  i is the activity coefficient of the solute, and m i 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 (pK a = 3.45).

oxoacid Examples of oxoacids include hypochlorous acid (HOCl), perchloric acid (HClO 4 ), nitric acid (HNO 3 ), sulfuric acid (H 2 SO 4 ) and phosphoric acid (H 3 PO 4 ).  oxoacids may be mono-, di- or polybasic;  not all the hydrogen atoms in an oxoacid are necessarily ionizable.

Dr. Said M. El-Kurdi11 It is not possible to isolate pure H 2 SO 3 - sulfurous acid

phosphinic acid has the formula H 3 PO 2, 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 (pK b = 0.2). Inorganic bases: nitrogen bases

7.5 The energetics of acid dissociation in aqueous solution Hydrogen halides

H 2 S, H 2 Se and H 2 Te 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 EO n (OH) m empirical methods for estimating K a Bell’s rule Which relates the first acid dissociation constant to the number of ‘hydrogen-free’ O atoms in an acid of formula EO n (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 the first hydration shell each H 2 O molecule acts as a Lewis base while the metal ion functions as a Lewis acid. the M  O 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 Al 2 O 3, Ga 2 O 3, In 2 O 3, GeO, GeO 2, SnO, SnO 2, PbO, PbO 2, As 2 O 3, Sb 2 O 3 and Bi 2 O 3 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:  sol G o

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, H 3 B  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(OH 2 ) 6  x F x ] (3  x)+ (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 + < Mg 2+ < Al 3+. 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 Ca 2+ > Sr 2+ > Ba 2+.

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 + ?