Oxidation States

Skills to develop

• Assign an oxidation state (or oxidation number) to an atom in a molecule or ion.
• Describe oxidation and reduction reaction in terms of oxidation state change.

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, and balancing redox chemical reactions. You are learning the skill to assign oxidation states (or oxidation numbers) to a variety of compounds and 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, 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, 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:
   • +1 for alkali metals - Li, Na, K, Rb, Cs;
   • +2 for alkaline earth metals - Be, Mg, Ca, Sr, Ba;
   • -1 for halogens except when they form compounds with oxygen or one another;

These rules are wordy because we have to point out the special cases such as H₂O₂ and Na₂O₂. Rule 3. deals with hydride. Other than these, you may simply remember the oxidation states for H and O are +1 and -2 respectively in a coumpound, and oxidation of other elements can be asigned by making the algebraic sum of the oxidation states equal to the net charge on the molecule or ion. 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.

Element	Oxidation state	Compound or ion
H	+1	H+
Group 1	+1	H2O
	0	H2
	-1	NaAlH4
Cl	-1	Cl-
Group 7	0	Cl2
	+1	ClO-
	+3	ClO2-
	+4	ClO2
	+5	ClO3-
	+7	ClO4-
N	-3	NH3
Group 5	-2	N2H4
	-1	NH2OH
	0	N2
	+1	N2O
	+2	NO
	+3	NO2-
	+4	NO2
	+5	NO3-

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.

Element	Oxidation state	Compound or ion
Fe	+2	Fe2+	Fe = Fe2+ + 2 e-
	+3	Fe3+	Fe2+ = Fe3++ e-
Zn	0	Zn	Zn is reducing agent
	+2	Zn2+
O	-1	H2O2	H2O2 = O2 + H2O
	0	O2
	-2	H2O
Cr	+6	Cr2O72-
	+3	Cr3+	Cr2O72- + 6 e = 2 Cr3+
	+6	CrO42-
Mn	+7	MnO4-
	+6	MnO42-
	+4	MnO2	MnO4- + 3 e = MnO2
	+2	Mn2+	MnO4- + 5 e = Mn2+
C	+3	H2C2O4
	+4	CO2
	+4	CO32-
	+2	CO

Oxidation and Reduction in Terms of Oxidation State

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),
MnO₄^- + H⁺ -> Mn²⁺ + H₂O the oxidation of Mn goes from +7 to +2. Thus, Mn is reduced. In the following reaction (unbalanced), H₂C₂O₄ + O₂ -> CO₂ + H₂O the element C is oxidized, because its oxidation state changes from +3 to +4 in the reaction.

Confidence Building Questions

• What is the oxidation state of Mo in MoO₄^2-?
  Skill:
  Note that (+6) + 4*(-2) = -2, the charge of the ion MoO₄^2-. An oxidation state is a signed number, in this case +6.
• What is the oxidation state of S in SO₃^2-?
  Discussion:
  Even though S is in group VIA: O, S, Se, Te, Bi, the sulfur in sulfite ions has an oxidation number +4.
• What is the oxidation state of As in AsO₄^3-?
  Discussion:
  This is the normal oxidation state of group VA: N, P, As, Sb, Bi.
• What is the oxidation state of Br in BrO₃^-?
  Discussion:
  Br has many oxidation states, e.g. +7 in BrO₄^-.
• What is the oxidation state of As in As₂S₃?
  Discussion:
  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.
• What is the oxidation state of Cr in Cr(OH)₃?
  Discussion:
  There are three OH^- groups in the compound. Chromium hydroxide, Cr(OH)₃, is a greenish gelatinous solid.
• In the reaction, P₄(s) + 5O₂(g) = P₄O₁₀(s) What is element reduced?
  Discussion:
  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.