Hydrolysis of Oxoanions :: essays research papers

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 oxo acids which ionize to form oxoanions in solution. The general form of an oxoanion is MOxy-. Since these oxoanions are quite different in their properties, Z2/r ratios for nonmetal cations do not give accurate predictions of their chemical behavior. A different approach is needed to determine what these elements will do when they are dissolved in water. Oxo Anion Hydrolysis Like cations, oxoanions are hydrated in aqueous solution. In this case the electrostatic attraction is between the electron pairs on the oxoanion oxygen atoms and the partially positive hydrogen atoms of the water molecule. The hydration of oxoanions is an exothermic process. The hydration energy is dependent 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 of the water is sufficiently strong, the hydrogen can be removed from the water generating a hydroxide ion resulting in a basic solution. MOxy- + H2O [MO(x-1)OH](y-1)- + OH- The equilibrium constant for this reaction is the base ionization constant, Kb. Base ionization constants are tabulated as pKbs. Successive ionizations are listed as pKb1, pKb2, etc. The larger the pKb value the lesser the degree of ionization and the weaker base the oxoanion is. When determining the acidities of metal cations, three variable are important -- charge, size and electronegativity of the cation. When determining the basicity of an oxo 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 bearing the same number of oxygens will be very close in size. Effect of Charge on Basicity Increasing charge on an anion increases its tendency to hydrolyze and form basic solutions. pKb Values For Some Oxoanions Anion pKb1 pKb2 pKb3 H3AsO4 10.5 6.8 1.5 H3PO4 11.88 6.8 2 H2SeO3 11.43 7.4 H4GeO4 5.41 1 The table above shows that the pKb values of an oxoanion decrease by 4-5 units for each additional negative charge on the anion. Thus, increasing the negative charge substantially increases the anion's basicity. Effect of Number of Oxygen Groups Since most nonmetals exhibit more than one oxidation state, they can form oxoanions that differ in the number of oxygens bonded to the metal.