Redox Reactions: Understanding Oxidation Numbers

In chemistry, a redox reaction (reduction-oxidation reaction) involves the transfer of electrons between two species. This transfer results in a change in the oxidation states (or oxidation numbers) of the atoms involved.

What is Oxidation Number?

The oxidation number is a way of assigning a hypothetical charge to an atom in a molecule or ion. It's a useful concept for tracking electron transfer in redox reactions. It's based on a set of rules, which we'll explore below.

Rules for Assigning Oxidation Numbers

1. The oxidation number of an atom in its elemental form is 0 (e.g., Na, O₂, Cu).
2. The oxidation number of a monatomic ion is equal to its charge (e.g., Na⁺ is +1, Cl^- is -1).
3. Oxygen usually has an oxidation number of -2 (except in peroxides like H₂O₂ where it's -1, and when combined with fluorine, like OF₂ where it's +2).
4. Hydrogen usually has an oxidation number of +1 (except in metal hydrides like NaH where it's -1).
5. Metals usually have a negative oxidation number (e.g., in ions like Fe²⁺, Al³⁺).
6. Non-metals usually have a positive oxidation number (e.g., in ions like Cl^-, S^2-).
7. The sum of the oxidation numbers in a neutral compound is 0.
8. The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion.

Using Roman Numerals

We use Roman numerals to indicate the oxidation state of an element in a compound. For example:

• Fe²⁺ is written as Fe(II)
• SO₄^2- is written as SO₄(II)
• Cr₂O₇^2- is written as Cr₂O₇(II)

Examples of Assigning Oxidation Numbers

Identifying Oxidation and Reduction

In a redox reaction:

• Oxidation is the loss of electrons (increase in oxidation number).
• Reduction is the gain of electrons (decrease in oxidation number).

The element that is oxidized is the reducing agent, and the element that is reduced is the oxidizing agent.