Redox Reactions and Half-Equations

This lesson covers oxidation states, identifying redox reactions, writing and balancing half-equations, combining half-equations, and redox titrations.

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Oxidation States

Oxidation states (oxidation numbers) are assigned using these rules:

1. Elements in their uncombined state: 0 (e.g. Fe, O₂, S₈)
2. Simple monatomic ions: equal to the charge (e.g. Na⁺ = +1, Cl⁻ = −1)
3. Oxygen is usually −2 (except in peroxides: −1, and in OF₂: +2)
4. Hydrogen is usually +1 (except in metal hydrides: −1)
5. Fluorine is always −1
6. The sum of oxidation states in a neutral compound = 0
7. The sum of oxidation states in a polyatomic ion = charge of the ion

Example 1: Find the oxidation state of Mn in KMnO₄.

K = +1, each O = −2, total = 0 +1 + Mn + 4(−2) = 0 → Mn = +7

Example 2: Find the oxidation state of S in SO₄²⁻.

S + 4(−2) = −2 → S = +6

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Redox Definitions

Oxidation is the loss of electrons (increase in oxidation state). Reduction is the gain of electrons (decrease in oxidation state). Mnemonic: OIL RIG — Oxidation Is Loss, Reduction Is Gain.

An oxidising agent (oxidant) causes oxidation by accepting electrons — it is itself reduced. A reducing agent (reductant) causes reduction by donating electrons — it is itself oxidised.

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Writing Half-Equations

Steps for writing half-equations in acidic solution:

1. Write the species being oxidised or reduced on each side.
2. Balance atoms other than O and H.
3. Balance O by adding H₂O.
4. Balance H by adding H⁺.
5. Balance charge by adding electrons (e⁻).

Example 3: Write the half-equation for the reduction of MnO₄⁻ to Mn²⁺ in acidic solution.

Step 1: MnO₄⁻ → Mn²⁺ Step 2: Mn is already balanced. Step 3: Add 4H₂O to balance O: MnO₄⁻ → Mn²⁺ + 4H₂O Step 4: Add 8H⁺ to balance H: MnO₄⁻ + 8H⁺ → Mn²⁺ + 4H₂O Step 5: Left charge = −1 + 8 = +7. Right charge = +2. Add 5e⁻ to the left:

MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O

Mn goes from +7 to +2 (gain of 5 electrons) — this is reduction.

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Combining Half-Equations

To combine two half-equations into a full redox equation: