4. Most Important Oxidizing and Reducing Agents

What substances can display the properties of oxidizing agents, and what - of reducing agents? We have already mentioned that an oxidizing agent contains an element whose oxidation number de­creases, while a reducing agent contains an element whose oxidation number grows in the course of a reaction. Consequently, oxidizing agents will include first of all compounds with the higher, and reducing agents will include compounds with the lower oxidation numbers featuring a given element.

Metals display only a positive oxidation state in their compounds, and their minimum oxidation number is zero. In other words, they have the minimum oxidation number only in the free state. Indeed, all free metals, although to a different extent, are capable of exhibiting only reducing properties. The reducing agents used in practice include Aluminium, Magnesium, Sodium, Potassium and Zinc. If a metal can have several oxidation numbers, those of its compounds in which it displays the lowest of them are also reducing agents, as a rule. Examples are the compounds of Iron (II), Tin (II), Chromium (II) and Copper (I).

Those compounds of metals can be oxidizing agents in which the oxidation number is high and either is equal to the number of the group, which the metal belongs to or is close to it. Practical use has been found, in particular, by an Ammonia solution of Silver oxide, an Ammonia solution of Copper (II) Sulfate, Mercury (II) Chloride, Lead (IV) Oxide, Iron (III) Chloride, Potassium Chromate and Dichromate (K₂CrO₄ and K₂Cr₂O₇), Potassium Permanganate KMnO₄, and Manganese (IV) oxide MnO₂.

Non-metals exhibit both positive and negative oxidation states. It is natural that compounds containing non-metals in their higher positive oxidation states can be oxidizing agents, and compounds in which a non-metal displays a negative oxidation state can be reducing agents.

The most important reducing agents are Hydrogen H₂, Carbon C and Carbon (II) Oxide CO.

Non-metals of the upper part of groups VI and VII of the Periodic Table are strong oxidizing agents. The strong oxidizing properties of these substances are explained by the high electronegativity of their atoms. Fluorine F₂ has the strongest oxidizing properties, but in practice Oxygen O₂, Chlorine Cl₂ and Bromine Br₂ are used most frequently as oxidizing agents.

The compounds used as oxidizing agents also include acids. Hydrochloric HCl, Sulfuric H₂SO₄ and nitric HNO₃ acids have the greatest practical signifi­cance. The oxidizing element in Hydrochloric acid is Hydrogen H⁺; in Nitric acid it is Nitrogen N⁵⁺, in dilute Sulfuric acid - Hydrogen H⁺, and in the concentrated acid - Sulfur S⁺⁶. Hence, the equation of reduction with Hydrochloric and dilute Sulfuric and a few other acids (H₃PO₄, CH₃COOH, HClO₄) has the form:

2H⁺ + 2 → H₂.

Nitric acid, depending on its concentration, temperature, and the nature of the reducing agent, can be reduced to different oxidation numbers of the Nitrogen. One of the usual products of its reduction is Nitrogen (II) Oxide NO:

NO₃^_ + 4H⁺ + 3 = NO + 2 H₂O.

Various products may also be formed in reduction with concentrated Sulfuric acid. One of them is Sulfur (IV) Oxide:

SO₄^2- + 4H⁺ + 2 = SO₂ + 2H₂O.

Other compounds of non-metals used as oxidizing agents are Hydrogen Peroxide H₂O₂, the salts of acids in which the acid-forming element exhibits a high oxidation number - Chlorates (KClO₃), Perchlorates (KClO₄).

Possible oxidation numbers of some chemical elements are presented in Appendix 10.