Oxidation and Reduction (Redox Reactions)

Oxidation and reduction reactions, commonly known as redox reactions, are fundamental processes in chemistry where electrons are transferred between species. They are essential in industrial processes, biological systems, and electrochemical cells.

1. Definitions of Oxidation and Reduction

• Classic View (Oxygen/Hydrogen): Oxidation is the addition of oxygen or the removal of hydrogen. Reduction is the removal of oxygen or the addition of hydrogen.
• Electron Transfer View: This is the most comprehensive definition. Oxidation is the loss of electrons (OIL), while Reduction is the gain of electrons (RIG).
• Oxidation Number View: Oxidation is an increase in oxidation number, while reduction is a decrease in oxidation number.

2. Oxidation Numbers (Oxidation States)

The oxidation number is a formal charge assigned to an atom. Rules for assignment include:

• The oxidation state of an element in its free state (e.g., O2, Na, S8) is zero.
• For monoatomic ions, it equals the charge (e.g., Na+ is +1, Cl- is -1).
• Oxygen is usually -2 (except in peroxides like H2O2 where it is -1, and OF2 where it is +2).
• Hydrogen is +1 (except in metal hydrides like NaH where it is -1).
• The sum of oxidation numbers in a neutral compound is zero; in a polyatomic ion, it equals the ion's charge.

3. Oxidizing and Reducing Agents

• Oxidizing Agent (Oxidant): A substance that causes oxidation in another substance by accepting electrons. It is itself reduced.
• Reducing Agent (Reductant): A substance that causes reduction in another substance by donating electrons. It is itself oxidized.

4. IUPAC Nomenclature

IUPAC names for inorganic compounds use Roman numerals to indicate the oxidation state of the central element. For example:

• FeCl2: Iron(II) chloride
• FeCl3: Iron(III) chloride
• KMnO4: Potassium tetraoxomanganate(VII)
• K2Cr2O7: Potassium heptaoxodichromate(VI)

5. Tests for Oxidizing and Reducing Agents

• Oxidizing Agents: Acidified Potassium tetraoxomanganate(VII) (KMnO4) turns from purple to colorless in the presence of a reducing agent. Acidified Potassium heptaoxodichromate(VI) (K2Cr2O7) turns from orange to green.
• Reducing Agents: They can turn starch-iodide paper blue-black (by liberating iodine from KI) or decolorize acidified KMnO4.

6. Balancing Redox Equations

Redox equations are balanced by ensuring that the number of electrons lost equals the number of electrons gained. This can be done using the oxidation number method or the half-reaction method.