Redox Reactions: Understanding Oxidation Numbers

This section focuses on understanding oxidation numbers, which are crucial for identifying redox reactions.

What is an Oxidation Number?

An oxidation number is a way of representing the hypothetical charge an atom would have if all bonds were completely ionic. It's a useful concept for tracking electron transfer in chemical reactions.

It's a bookkeeping tool, not necessarily the actual charge on an atom in a covalent compound.

Rules for Assigning Oxidation Numbers

1. In a neutral atom, the oxidation number is 0.
2. In a monatomic ion (single atom ion), the oxidation number is equal to the ion's charge (e.g., Na⁺ is +1, Cl^- is -1).
3. Oxygen usually has an oxidation number of -2 in compounds.
4. Hydrogen usually has an oxidation number of +1 in compounds.
5. Metals usually have a negative oxidation number (e.g., -1, -2).
6. Non-metals usually have a positive oxidation number.
7. In monoxide compounds (e.g., CO), carbon has an oxidation number of +2 and oxygen has an oxidation number of -2.
8. In fluorides (e.g., KF), fluorine always has an oxidation number of -1.

Using Roman Numerals

When the oxidation number of an element is more than two, we use a Roman numeral to indicate it. For example:

• +3 is represented as +III
• +4 is represented as +IV
• +5 is represented as +V
• +6 is represented as +VI
• +7 is represented as +VII

Examples of Assigning Oxidation Numbers

Compound	Element	Oxidation Number	Explanation
H2O	Hydrogen (H)	+1	Hydrogen is usually +1.
H2O2	Hydrogen (H)	+1	Hydrogen is usually +1.
KMnO4	Potassium (K)	+1	Metals are usually +1.
KMnO4	Manganese (Mn)	+7	Oxygen is usually -2. $x(-2) = -8$. Therefore, Mn must be +7 to balance the charge.
CO2	Carbon (C)	+4	Oxygen is usually -2. $x(-2) = -8$. Therefore, C must be +4 to balance the charge.
FeCl3	Iron (Fe)	+3	The chloride ion (Cl-) has a charge of -1. To balance the charge of the compound, Fe must have a +3 oxidation number.

Redox Reactions and Electron Transfer

Redox reactions involve the transfer of electrons. The element that loses electrons is oxidized (oxidation number increases), and the element that gains electrons is reduced (oxidation number decreases).

For example, in the reaction: $Zn(s) + Cu²⁺(aq) \rightarrow Zn²⁺(aq) + Cu(s)$

Zinc (Zn) is oxidized (loses 2 electrons, oxidation number increases from 0 to +2). Copper (Cu²⁺) is reduced (gains 2 electrons, oxidation number decreases from +2 to 0).

Practice

Determine the oxidation number of the underlined element in the following compounds:

• K₂SO₄: Potassium (K)
• HCl: Chlorine (Cl)
• Al₂O₃: Aluminum (Al)
• Na₂O: Sodium (Na)

Answers:

• K₂SO₄: K = +1
• HCl: Cl = -1
• Al₂O₃: Al = +3
• Na₂O: Na = +1