What Are Redox Reactions?
In a redox reaction, one species loses electrons (oxidation) while another gains them (reduction). These paired processes transfer electrons, changing oxidation states.
Redox reactions involve simultaneous oxidation and reduction: one reactant donates electrons while another accepts them. Always coupled—if something is oxidized, something else is reduced.
- •Loses electrons
- •Oxidation state increases
- •Example: Fe → Fe²⁺ + 2e⁻
- •Gains electrons
- •Oxidation state decreases
- •Example: O₂ + 4e⁻ → 2O²⁻
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Step-by-step worked examples
Balance: Cu + O₂ → Cu₂O. Identify oxidation and reduction.
Cu: 0 → +1 (loses 1e⁻, oxidized) O₂: 0 → −2 (gains 2e⁻, reduced) Balanced: 4Cu + O₂ → 2Cu₂O Cu is reducing agent, O₂ is oxidizing agent.
In Zn + Cu²⁺ → Zn²⁺ + Cu, which species is oxidized?
Zn: 0 → +2 (loses 2e⁻, oxidized) Cu²⁺: +2 → 0 (gains 2e⁻, reduced) Zn is the reducing agent, Cu²⁺ is oxidizing.
In MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O, how many electrons transferred?
Mn: +7 → +2 (gains 5e⁻, reduced) Fe²⁺: +2 → +3 (each loses 1e⁻, 5 Fe × 1 = 5e⁻ oxidized) Total electrons transferred: 5e⁻.
Flashcards
Quick quiz
Q1.In 2H₂ + O₂ → 2H₂O, what is oxidized?
Q2.Which describes reduction?
Q3.Reducing agent is the…
Q4.In Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂, which oxidation state changes?
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Common mistakes
Oxidation must release oxygen. — Correct: Oxidation = electron loss; oxygen may or may not be involved.
Reduction means getting smaller. — Correct: Reduction = electron gain; oxidation state decreases.
One species can be oxidized alone. — Correct: Oxidation and reduction always occur together in redox.
All combustion is oxidation. — Correct: Combustion is a type of redox, but not all oxidation is combustion.
FAQ
What is a redox reaction?
Simultaneous oxidation (electron loss) and reduction (electron gain) in one reaction.
How do you identify which species is oxidized?
Check oxidation states: whichever one increases is oxidized.
Is photosynthesis a redox reaction?
Yes—H₂O is oxidized to O₂; CO₂ is reduced to glucose.
What happens to the total number of electrons?
Electrons lost by one species equal electrons gained by another.