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Oxidation States

Skills to develop

Oxidation State

Oxidation state is a number assigned to an element in a compound according to some rules. This number enable us to describe oxidation-reduction reactions. For the purpose of balancing oxidation-reduction reactions, You have to assign oxidation states (or oxidation numbers) to a variety of compounds or ions.

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:

  1. The oxidation state of any element such as Fe, H2, O2, P4 and S8, is zero (0).

  2. The oxidation state of oxygen in its compounds is -2, except for peroxides like H2O2, and Na2O2, in which the oxidation state for O is -1.

  3. The oxidation state of hydrogen is +1 in its compounds, except for metal hydrides, such as NaH, LiH, LiAlH4, etc., in which the oxidation state for H is -1.

  4. The oxidation states of other elements are then assigned to make the algebraic sum of the oxidation states equal to the net charge on the molecule or ion.

  5. The following elements usually have the same oxidation states in their compounds:

    Examples of oxidation state assignment

    ElementOxidation
    state
    Compound
    or ion
    H +1 H+
    +1 H2O
    Cl -1 Cl-
    0 Cl2
    +1 ClO-
    +3 ClO2-
    +4 ClO2
    +5 ClO3-
    +7 ClO4-
    N -3 NH3
    -2 N2H4
    -1 NH2OH
    0 N2
    +1 N2O
    +2 NO
    +3 NO2-
    +4 NO2
    +5 NO3-
    In the examples given in the table here, take a look at the compound or ion, and assign the oxidation state (or oxidation number) to all elements. The oxidation state of one element is given here as a guidance.

    The following are some common oxidants or reductants. Changes of oxidation states in redox reactions of the key elements are given in groups. You should justify the assigned oxidation state to your satisfaction as you read on.

    ElementOxidation
    state
    Compound
    or ion
    Fe +2 Fe2+
    +3 Fe3+
    Zn 0 Zn
    +2 Zn2+
    O -1 H2O2
    0 O2
    -2 H2O
    Cr +6 Cr2O72-
    +3 Cr3+
    +6 CrO42-
    Mn +7 MnO4-
    +6 MnO42-
    +4 MnO2
    +2 Mn2+
    C +3 H2C2O4
    +4 CO2
    +4 CO32-
    +2 CO

    Confidence Building Questions

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