Ox O Anions Of Formula Mo 4 example essay topic

Hydrolysis of OxoanionsNonmetals in positive oxidation states do not exist in aqueous solution as cations but rather as hydroxides or oxides. These species often act as ox o acids which ionize to form in solution. The general form of an is MOxy-. Since these are quite different in their properties, Z 2/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, are hydrated in aqueous solution.

In this case the electrostatic attraction is between the electron pairs on the oxygen atoms and the partially positive hydrogen atoms of the water molecule. The hydration of is an exothermic process. The hydration energy is dependent on the charge and size of the. 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- + H 2 O [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 pubs. Successive ionization's are listed as p Kb 1, p Kb 2, etc. The larger the p Kb value the lesser the degree of ionization and the weaker base the is. When determining the acidifies of metal cations, three variable are important -- charge, size and electro negativity of the cation. When determining the basicity of an ox o 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 oxygen's 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. p Kb Values For Some OxoanionsAnion p Kb 1 p Kb 2 p Kb 3 H 3 AsO 4 10.5 6.8 1.5 H 3 PO 4 11.88 6.8 2 H 2 SeO 3 11.43 7.4 H 4 GeO 4 5.41 1 The table above shows that the p Kb values of an 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 that differ in the number of oxygen's bonded to the metal. The different of a given nonmetal atom will differ substantially in their. For example, chlorine forms four different: ClO-, ClO 2-, ClO 3-, ClO 4-.

ClO- has a p Kb of 6.5; ClO 2- 12.1 while the p Kb value of the other two are such weak bases that their hydrolysis in not measure able. Adding additional oxygen's decreases the basicity of the. Each additional ox o group increases the p Kb value by about 5.7 units. Effect of ElectronegativityAs the electro negativity of the nonmetal atom decreases the basicity of the ox o anion increases.

Comparing the p Kb values or ClO-, 6.50; BrO-, 5.3; IO-, 3.4 demonstrates this trend. Reasonably accurate p Kb 1 values can be obtained via the following equation where x is the number of ox o groups and y is the number of units of negative charge. In Summary higher p Kb values correspond to weaker basicity and lower pH additional ox o groups weaken basicity and add to p Kb additional units of negative charge increase basicity and subtract from p Kb The table below lists calculated values of p Kb 1 for the important simple ox o 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 BasicitiesA rough estimate of the basicity of an ox o 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 p Kb as addition of an ox o group.

In other words, the addition of a negative decreases the p Kb value twice as much as adding an ox o group will raise it. This observation can be used to categorize the ox o anion's basicity. To do this you will remove the charges and ox o groups that counteract each other and use the resulting 'formula' to determine the base category of the ox o anion. In general, anions containing a excess of will not be very basic; anions with an excess of negative charge will be strongly basic. Classification Resulting Formula Nonbasic contains ox o groups & no charge Feebly basic no charge & no ox o groups Moderately basic no ox o groups & a charge of -1/2 or -1 Very strongly basic no ox o groups & a charge more negative an -1 Examples: Consider an ox o anion of formula MO 4-.

For this formula the effect of the unit of negative charge will cancel out the effect of two of the ox o groups. If you remove the negative charge and two ox o groups the resulting formula is 'MO 2'. This anion falls into the category of nonbasic. According to the table of p Kb values above, ox o anions of formula MO 4- have high p Kb values and do fall into the nonbasic category. Consider an ox o anion of formula MO 34-. The three ox o groups cancel 1.5 unit of negative charge.

The resulting formula becomes 'M-2.5'. This ox o anion will be very strongly basic. PROBLEMS Classify the basicity of the following ox o anions and describe their reactions with water. Use the CRC to determine if the categories you have predicted are accurate. (HINT: p Kb values are rarely tabulated. You will have to look up the dissociation constants for the conjugate acids of these species and determine p Kb from there.) NO 2- CrO 42- IO- Use this method on the various ox o anions that you used in the laboratory exercise.

Do your predictions agree with the experimental results? Formulas of Oxo Anions wide variety of ox o anions exist. The number of ox o groups present in an ox o 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 ox o 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 non bonding.

These non bonding electrons require at least as much space as an ox o group. Oxo anions in which the central atom has an oxidation number 2 less than the group number will possess at least one less ox o 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 in the two orange boxes on the Table above. The d-Block Elements: Oxidation numbers are not as predictable among the d-block elements. For these elements non bonding valence electrons do not take up the space that would otherwise be occupied by an ox o 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 ox o groups is not dependent on the oxidation number Summary of Trends in Numbers of Oxo Groups in Oxo Anions The ox o 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 accommodate either three ox o groups or two ox o 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 3 p, ed, 4 p or 4d, the central atom can accommodate four ox o 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 5 p, 5d, 5 f, 6 p, or 6d, the central can accommodate 4-6 ox o groups and unshared p-orbital pairs. The other important feature in the formula of an ox o anion is its charge.

To calculate the charge, add the group number for the central atom plus the number of ox o anions at -2 charge.