5. Types of Redox Reactions

In all reactions, presented in sub-chapter 3, oxidizing and reducing atoms are parts of different substances. Such type of redox reactions is the most extended and called Intermolecular Oxidation-Reduction.

Compounds with the maximum oxidation number of a given element can play the role of oxidizing agents in redox reactions. The oxidation number can only be lower in this case. Conversely, compounds with the minimum oxidation number can only be reducing' agents. Here the oxidation number of an element can only grow. If an element is in an intermediate oxidation state, however, its atoms can, depending on the conditions prevailing, either take on or give up electrons. In the first case, the oxidation number of the element will be lower, and in the second one it will grow. Consequently, compounds containing elements in intermediate oxidation states have oxidation-reduction duality - they are capable of entering into-reactions with either oxidizing or reducing agents.

For example, Nitrogen forms compounds in which its oxidation number changes from -3 (Ammonia and Ammonium salts) to +5 (Nitric acid and its salts). The Nitrogen in Ammonia can only be a reducing agent, and that in Nitric acid - only an oxidizing agent. Nitrous acid HNO₂ and its salts, in which the oxidation number of Nitrogen is +3 however, enter into reactions with both strong oxidizing and strong reducing agents. In the first case, it is oxidized to Nitric acid, and in the second, it is usually reduced to Nitrogen (II) Oxide NO. We can exemplify the oxidation-reduction duality of Nitrous acid and its salts by the reactions:

5 KNO₂ + 2 KMnO₄ + 3 H₂SO₄ = 5 KNO₃ + 2 MnSO₄ + K₂SO₃ + 3 H₂O -

N⁺³ is a reducing agent;

2 HNO₂ + H₂S = 2 NO + S + 2H₂O -

N⁺³ is an oxidizing agent.

In addition to Nitrous acid HNO₂, Sulfur S, Iodine I₂, Hydrogen Peroxide H₂O₂ and a number of other substances have oxidation-reduction duality.

Substances containing element in intermediate oxidation state often have another characteristic property. In definite conditions one part of the element is oxidized and the other part is reduced. This process is known as autoxidation-autoreduction. For instance, when Chlorine reacts with water, the mixture of Hydrochloric and Hypochlorous acids is produced:

Cl₂ + H₂O = HCl + HClO.

Here the Chlorine undergoes both oxidation and reduction:

Cl2 + 2 = 2 Cl-	1 (reduction);
Cl2 - 2 = 2Cl+	1 (oxidation).

Autoxidation-autoreduction is also called disproportionation.

Some compounds in definite conditions (usually when heated) experience intramolecular oxidation-reduction. In this process, one constituent part of the substance is an oxidizing agent, and the other is a reducing one. Examples of intramolecular oxidation-reduction are many processes of thermal dissociation. For instance, when Potassium Chlorate decomposes by heating:

2 KClO₃ 2 KCl + 3 O₂

the Oxygen is oxidized (its oxidation number grows from -2 to 0), and the Chlorine is reduced (its oxidation number diminishes from +5 to -1).

Another example is the decomposition of Ammonium Nitrite em­ployed in the laboratory to obtain pure Nitrogen:

NH₄NO₂ = N₂ + 2H₂O.

Here the ion NH₃ is oxidized and the ion NO₂^- is reduced to free Nitrogen.