When an oxidation number is assigned to the element, it does not imply that the element in the compound acquires this as a charge, but rather that it is a convenient number to use for balancing chemical reactions. The guidelines for assigning oxidation states (numbers) are given below:
For your practice, we provide some examples below. Please study the following examples and derive the oxidation state for all elements. The oxidation numbers of the key element are given in case you need help.
The following are some common oxidants or reductants. Changes of oxidation states in redox reactions of the key elements are given in groups. Please justify the assigned oxidation state to your satisfaction as you read on, and assign the oxidation number to all element in the formulas.
|Fe||+2||Fe2+||Fe = Fe2+ + 2 e-|
|+3||Fe3+||Fe2+ = Fe3++ e-|
|Zn||0||Zn||Zn is reducing agent|
|O||-1||H2O2||H2O2 = O2 + H2O|
|+3||Cr3+||Cr2O72- + 6 e = 2 Cr3+|
|+4||MnO2||MnO4- + 3 e = MnO2|
|+2||Mn2+||MnO4- + 5 e = Mn2+|
In the chemistry of battery, you have learned that oxidation is the loss of electron. A loss of one electron raises the oxidation state by one. We now have another definition for oxidatin and reduction.
A raise of oxidation state is an oxidation whereas a lower of oxidation state is a reduction.
For example, in the reaction (unbalanced),
Note that (+6) + 4*(-2) = -2, the charge of the ion MoO42-. An oxidation state is a signed number, in this case +6.
Even though S is in group VIA: O, S, Se, Te, Bi, the sulfur in sulfite ions has an oxidation number +4.
This is the normal oxidation state of group VA: N, P, As, Sb, Bi.
Br has many oxidation states, e.g. +7 in BrO4-.
The oxidation for S is -2, since it is in the group VIA: O, S, Se, Te, Po. Super! S being in the same group as O, has an oxidation state of -2.
There are three OH- groups in the compound. Chromium hydroxide, Cr(OH)3, is a greenish gelatinous solid.
What are the oxidation states of O in O2 and P4O10? This equation represents the burning of phosphorus in air. P4O10 dissolves in water to form phosphoric acid.