The hydrolysis of cations in aqueous solution

We have looked only at acid solutions since relatively few simple cations M +(aq) can exist in appreciable concentrations at alkaline or even neutral pH values. Note that in Fig. 5.4, Mn(IV) is represented by Mn02(s) and not by Mn4+(aq), while the higher oxidation states are represented by anionic species. The relative stabilities of the oxidation states are strongly pH-dependent. Thus Mn2+(aq) is quite difficult to oxidise to Mn3+(aq) in acid solution. When a manganous salt is dissolved in water and the pH adjusted to about 10, Mn(OH)2 is precipitated. If allowed to stand in contact with air, this undergoes oxidation to an Mn(III) hydrated oxide usually (but erroneously) formulated as Mn(OH)3  

It is now necessary to consider the hydrolysis of ions in aqueous solution. 

The term hydrolysis implies a reaction in which the O-H bonds of water molecules are ruptured. Consider the following equilibria and associated equilibrium constants K which are set up in aqueous solution  

Most M3+(aq) and a number of M2+(aq) cations behave in much the same way as Fe3+(aq). Evidently the viabilities of simple aquo-cations are restricted by the possibilities for hydrolysis, as well as the redox processes discussed in Section 5.4. The simplest explanation for the hydrolysis of Fe3+(aq) acknowledges the considerable polarising power of Fe3+. When surrounded by six water molecules, the cation will tend to attract electron density from the O atoms. This makes the coordinated O atoms less attractive to protons than those in the bulk solvent, and encourages the transfer of protons from the coordination sphere. If the bonding in the complex [Fe(H20)6]3+ is regarded as covalent, with the formation of coordinate bonds, we can arrive at the same conclusion the delocalisation of the positive charge over the complex ion [Fe(H20)50H]2+ will tend to discourage recombination of the coordinated OH- ion with a proton. 

Where the polarising power of a cation is very great, no simple aquo-cation - or even no cationic species whatever - may be stable to hydrolysis, even at extremely acid pH. For example, let us contemplate the viability of B3+(aq). The hydration enthalpy of B3+ is estimated to be about -6000kJ mol-1. From this and the other relevant data given in the treatment of BF3(s) in Section 5.3, we can estimate AH° for the reaction  

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