Hydrolysis of Oxoanions Nonmetals in positive oxidation states do not exist in aqueous solution as cations but rather as hydroxides or oxides. These species often act as oxoacids that ionize to form oxoanions in solution. The general form of an oxoanion is MOxy-. Since these oxyanions have quite different properties, Z2/r ratios for nonmetallic cations do not provide accurate predictions of their chemical behavior. A different approach is needed to determine what these elements will do when dissolved in water. Hydrolysis of oxoanionsLike cations, oxoanions are hydrated in aqueous solution. In this case the electrostatic attraction is between the pairs of electrons on the oxygen atoms of the oxoanion and the partially positive hydrogen atoms of the water molecule. The hydration of oxoanions is an exothermic process. The hydration energy depends on the charge and size of the oxoanion. Hydration energies increase with increasing charge and decreasing anion size. As with cations, if the interaction between the anion and the hydrogen in the water is strong enough, the hydrogen can be removed from the water generating a resulting hydroxide ion in a basic solution.MOxy - + H2O [MO( x-1)OH](y-1)- + OH-The equilibrium constant for this reaction is the basic ionization constant, Kb. The basic ionization constants are tabulated as pKbs. Subsequent ionizations are listed as pKb1, pKb2, etc. The higher the pKb value, the lower the degree of ionization and the weaker the base of the oxoanion. When determining the acidity of metal cations, three variables are important: charge, size, and electronegativity of the metal cations. the cation. When determining the basicity of an anion, the size of the nonmetal atom (or high oxidation state metal atom) is not relevant. This central metal atom is significantly smaller than the multiple oxygen atoms in the anion. Therefore, different central atoms carrying the same number of oxygens will have very similar sizes. Effect of charge on basicity Increasing the charge on an anion increases its tendency to hydrolyze and form basic solutions. pKb values ​​for some oxoanions Anion pKb1 pKb2 pKb3H3AsO4 10.5 6.8 1.5H3PO4 11.88 6.8 2H2SeO3 11.43 7.4H4GeO4 5.41 1The table above shows that the pKb values ​​of an oxoanion decrease by 4-5 units for each additional negative charge on the 'anion. Therefore, increasing the negative charge substantially increases the basicity of the anion. Effect of Number of Oxygen Groups Because most nonmetals have more than one oxidation state, they can form oxoanions that differ in the number of oxygens bonded to the metal.