Redox Reactions

Key Concept

In redox reactions (RR), the oxidation states of elements change.

Process	Electrons	Oxidation state	Substance
Oxidation	gives up	increases	reducing agent
Reduction	gains	decreases	oxidizing agent

• Oxidation: loss of electrons, the oxidation state increases. The reducing agent gives up electrons.
• Reduction: gain of electrons, the oxidation state decreases. The oxidizing agent gains electrons.

Coefficients are found by the electron balance method: the number of electrons given up equals the number of electrons gained.

Finding Oxidation States

• In a simple substance, the oxidation state is 0.
• In compounds, hydrogen is +1, −1 in hydrides.
• Oxygen is −2, −1 in peroxides.
• The sum of the oxidation states in a molecule is 0.
• Mn in KMnO₄: +7. S in H₂SO₄: +6. Cr in K₂Cr₂O₇: +6.

Oxidizing and Reducing Agents

• Strong oxidizing agents: KMnO₄, K₂Cr₂O₇, HNO₃, concentrated H₂SO₄, O₂, halogens.
• Reducing agents: metals, H₂, C, CO, H₂S.

Reduction of KMnO₄ depending on the medium:

• Mn²⁺ in acidic medium
• MnO₂ in neutral medium
• K₂MnO₄ (MnO₄²⁻) in alkaline medium

Types of Reactions

• Intermolecular.
• Intramolecular (decomposition of KMnO₄).
• Disproportionation (Cl₂ + H₂O).

Zn + 2HCl → ZnCl₂ + H₂↑: Zn is the reducing agent, H⁺ is the oxidizing agent.