Periodic trends in aqua acid strength

Key points: The strengths of aqua acids typically increase with increasing positive charge of the central metal ion and with decreasing ionic radius; exceptions are commonly due to the effects of covalent bonding.

The strengths of aqua acids typically increase with increasing positive charge of the central metal ion and with decreasing ionic radius. This variation can be rationalized to some extent in terms of an ionic model, in which the metal cation is represented by a sphere of radius r⁺ carrying z positive charges. Because protons are more easily removed from the vicinity of cations of high charge and small radius, the model predicts that the acidity should increase with increasing z and with decreasing r⁺.

The validity of the ionic model of acid strengths can be judged from Fig. 4.5. Aqua ions of elements that form ionic solids (principally those from the s block) have pKa values that are quite well described by the ionic model. Several d-block ions (such as Fe₂ and Cr₃) lie reasonably near the same straight line, but many ions (particularly those with low pKa, corresponding to high acid strength) deviate markedly from it. This deviation indicates that the metal ions repel the departing proton more strongly than is predicted by the ionic model. This enhanced repulsion can be rationalized by supposing that the positive charge of the cation is not confined to the central ion but is delocalized over the ligands and hence is closer to the departing proton. The delocalization is equivalent to attributing covalence to the element–oxygen bond. Indeed, the correlation is worst for ions that are disposed to form covalent bonds. For the later d- and the p-block metal ions (such as Cu₂ and Sn₂, respectively), the strengths of the aqua acids are much greater than the ionic model predicts. For these species, covalent bonding is more important than ionic bonding and the ionic model is unrealistic. The overlap between metal orbitals and the orbitals of an oxygen ligand increases from left to right across a period. It also increases down a group, so aqua ions of heavier d-block metals tend to be stronger acids.

Figure 4.5 The correlation between acidity constant and the electrostatic parameter of aqua ions.

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Liquid ammonia

Polyoxo compound formation

Amphoterism

Acidic and basic oxides

Extrinsic semiconductors

Intrinsic semiconductors

Insulators

The Fermi level

Densities of states

Band formation by orbital overlap

The conductivities of inorganic solids

Nonstoichiometry