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 MO_x^y-. 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.

MO_x^y- + H₂O

[MO_(x-1)OH]^(y-1)- + OH^-

The equilibrium constant for this reaction is the base ionization constant, K_b.

Base ionization constants are tabulated as pK_bs. Successive ionizations are listed as pK_b1, pK_b2, etc. The larger the pK_b 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.

pK_b Values For Some Oxoanions

Anion	pK_b1	pK_b2	pK_b3
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 pK_b 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. The different oxoanions of a given nonmetal atom will differ substantially in their basicities. For example, chlorine forms four different oxoanions: ClO^-, ClO²^-, ClO₃^-, ClO₄^-. ClO^- has a pK_b of 6.5; ClO²^- 12.1 while the pK_b value of the other two oxoanions are such weak bases that their hydrolysis in not measureable. Adding additional oxygens decreases the basicity of the oxoanion. Each additional oxo group increases the pK_b value by about 5.7 units.

Effect of Electronegativity

As the electronegativity of the nonmetal atom decreases the basicity of the oxo anion increases. Comparing the pK_b values orClO^-, 6.50; BrO^-, 5.3; IO^-, 3.4 demonstrates this trend.

Reasonably accurate pK_b1 values can be obtained via the following equation where x is the number of oxo groups and y is the number of units of negative charge.

In Summary

higher pK_b values correspond to weaker basicity and lower pH
additional oxo groups weaken basicity and add to pK_b
additional units of negative charge increase basicity and subtract from pK_b

The table below lists calculated values of pK_b1 for the important simple oxo anions of the elements.

Table copied from Wulfsberg,G. Principles of Descriptive Chemistry; Brooks/Cole Publishing:Monterey CA, 1987; p.37.

Oxo anions can be placed into categories that describe the extent to which they hydrolyze. These categories are analogous to those used with cationic species. Nonbasic anions do not hydrolyze appreciably. Feebly basic anions produce small amounts of hydroxide ion which are not easily detected in the laboratory. The anions in these categories are typically salts of strong and fairly strong acids. Moderately basic anions give distinctly basic solutions which can be detected using test papers. The salts of weak acids which you studied in general chemistry fit into this category. Very strongly basic anions hydrolyze nearly completely in water.

Predicting Basicities

A rough extimate of the basicity of an oxo anion can be made by inspecting its chemical formula.

Note from the equation that adding an additional negative charge has twice the effect on the pK_b as addition of an oxo group. In other words, the addition of a negative decreasesthe pK_b value twice as much as adding an oxo group will raise it. This observation can be used to categorize the oxo anion's basicity. To do this you will remove the charges and oxo groups that counteract each other and use the resulting "formula" to determine the base category of the oxo anion. In general, anions containing a excess of oxogroups will not be very basic; anions with an excess of negative charge will be strongly basic.

Classification	Resulting Formula
Nonbasic	contains oxo groups & no charge
Feebly basic	no charge & no oxo groups
Moderately basic	no oxo groups & a charge of -1/2 or -1
Very strongly basic	no oxo groups & a charge more negative an -1

Examples:
Consider an oxo anion of formula MO₄^-. For this formula the effect of the unit of negative charge will cancel out the effect of two of the oxo groups. If you remove the negative charge and two oxo groups the resulting formula is "MO₂". This anion falls into the category of nonbasic. According to the table of pK_b values above, oxo anions of formula MO₄^- have high pK_b values and do fall into the nonbasic category.

Consider an oxo anion of formula MO₃^4-. The three oxo groups cancel 1.5 unit of negative charge. The resulting formula becomes "M^-2.5". This oxo anion will be very strongly basic.

PROBLEMS

Classify the basicity of the following oxo anions and describe their reactions with water.

Use the CRC to determine if the categories you have predicted are accurate. (HINT: pK_b values are rarely tabulated. You will have to look up the dissociation constants for the conjugate acids of these species and determine pK_b from there.)

NO₂^-
CrO₄^2-
IO^-

Use this method on the various oxo anions that you used in the laboratory exercise. Do your predictions agree with the experimental results?

Formulas of Oxo Anions

A wide variety of oxo anions exist. The number of oxo groups present in an oxo anion is dependent on the nature of the central atom M.

Size of Central Atom

The p-Block Elements:
There are two common oxidation numbers for elements in the p-block of the periodic table

the group number
the group number minus two

Picture assembling an oxo anion of a given element from a "cation" of charge equal to its oxidation number plus the requisite number of oxide ions. When the cation has an oxidation number equal to the group number minus 2, there will be two valence electrons occupying space around the central atom which are nonbonding. These nonbonding electrons require at least as much space as an oxo group.

Oxoanions in which the central atom has an oxidation number 2 less than the group number will possess at least one less oxo group than those in which the central atom has an oxidation number equal to the group number.

You can see this trend by comparing the oxoanions in the two orange boxes on the Table above.

The d-Block Elements:
Oxidation numbers are not as predictible among the d-block elements. For these elements nonbonding valence electrons do not take up the space that would otherwise be occupied by an oxo group. (why?)

The term total coordination number is used to indicate the total number of atoms and unshared p-electron pairs around a central atom.

In general, the number of oxo groups is not dependent on the oxidation number

Summary of Trends in Numbers of Oxo Groups in Oxo Anions

The oxo anions having the smallest central atoms are those of the Period 2 p-block. These can have a maximum coordination number of 3 and will accomodate either three oxo groups or two oxo groups and one unshared pair of p electrons.
Central atoms of the p-block of Periods 3 & 4 and those of the d-block of Periods 4 & 5 have larger radii and can have a maximum coordination number of 4. If the valence orbitals are 3p, ed, 4p or 4 d, the central atom can accommodate four oxo groups and unshared p-orbital electrons.
Central atoms of the p-block of Periods 5 and the p- and d-block of Period 6, have a maximum total coordination number of 6. If the valence orbitals on the central atom are 5p, 5d, 5f, 6p, or 6d, the central can accomodate 4-6 oxo groups and unshared p-orbital pairs.

The other important feature in the formula of an oxo anion is its charge. To calculate the charge, add the group number for the central atom plus the number of oxo anions at -2 charge.

PROBLEMS

Predict the formulas of the oxo anions formed by the following elements, each with the specified oxidation number:

N (III)
P(V)
As(III)
Se(IV)
I(VII)
I(V)
Mn(VI)