🎓 Prepared by students from Boğaziçi University

What are Oxidation-Reduction Reactions?

Redox reactions involve the transfer of electrons between atoms or molecules. An atom loses electrons (oxidation); another gains them (reduction). Both happen together.

Short answer

Oxidation is losing electrons; reduction is gaining electrons. In redox reactions, a reducing agent (donates e⁻) and oxidizing agent (accepts e⁻) exchange electrons. Every redox has both.

Redox Process

1
Oxidation
Atom loses electrons
↓
2
Electrons flow
e⁻ → from one atom to another
↓
3
Reduction
Atom gains electrons
↓
4
Net result
Charge redistribution

Try it: interactive calculator

Oxidation state

2dimensionless

= 6-4

Number of protons6 protonsNumber of electrons4 electrons

Step-by-step worked examples

In Fe + O₂ → Fe₂O₃, identify what is oxidized and reduced.

Fe: 0 → +3 (loses 3e⁻) = oxidized
O: 0 → −2 (gains 2e⁻) = reduced
Fe is the reducing agent; O₂ is the oxidizing agent.

Assign oxidation states: H₂SO₄.

H: +1 (always in compounds)
S: +6 (must balance: +1×2 + S + (−2)×4 = 0)
O: −2 (usually)
Check: +2 + 6 − 8 = 0 ✓

Combustion: C + O₂ → CO₂. Redox?

C: 0 → +4 (oxidized)
O: 0 → −2 (reduced)
Redox; C is reducer, O₂ is oxidizer.

Flashcards

Quick quiz

Q1.In Zn + Cu²⁺ → Zn²⁺ + Cu, Zn is…

Correct answer: B. Zn loses electrons (0 → +2) — oxidized.

Q2.Oxidizing agent is the species that…

Correct answer: B. Oxidizing agent accepts e⁻ (is reduced).

Q3.Oxidation state of O in CO?

Correct answer: C. O is usually −2; C must be +2 to balance.

Q4.In combustion, fuel is…

Correct answer: A. Fuel loses e⁻ to O₂ — oxidized (is the reducer).

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Common mistakes

✗

Oxidation means adding oxygen. — Correct: Oxidation = losing e⁻, even if no oxygen involved.

✗

Reduction means removing oxygen. — Correct: Reduction = gaining e⁻, regardless of oxygen.

✗

Oxidation state = charge on ion. — Correct: Oxidation state applies to all atoms; charge only to ions.

✗

Only metals can be oxidized. — Correct: Any element or compound can be oxidized or reduced.

FAQ

Difference between oxidation and combustion?

Combustion is rapid oxidation (e⁻ loss to O₂) at high temp, releasing heat.

Can an atom be oxidized and reduced at once?

No, but in disproportionation a single element both oxidizes and reduces.

Why 'reduction' if atoms lose oxygen?

Historical: ore reduction (removing O to get metal) became 'reduction'. Now means gaining e⁻.

Real-world redox examples?

Combustion, rusting (Fe oxidation), respiration (glucose oxidation), batteries (electrode redox).