What Are Oxidation-Reduction (Redox) Reactions?
Redox reactions involve the transfer of electrons between chemical species. Oxidation is the loss of electrons; reduction is the gain of electrons. These processes always occur simultaneously, one substance oxidized and one reduced.
In redox reactions, one substance is oxidized (loses electrons) and another is reduced (gains electrons). The substance losing electrons is the reducing agent; the one gaining electrons is the oxidizing agent. Electron transfer always drives the reaction.
- •Loss of electrons
- •Increase in oxidation number
- •Substance is oxidized
- •Substance acts as reducing agent
- •Example: Fe → Fe³⁺ + 3e⁻
- •Gain of electrons
- •Decrease in oxidation number
- •Substance is reduced
- •Substance acts as oxidizing agent
- •Example: O₂ + 4e⁻ → 2O²⁻
Step-by-step worked examples
In Fe + O₂ → Fe₂O₃, identify oxidation and reduction.
Fe goes from 0 to +3 (loses 3 electrons) — oxidation. O₂ goes from 0 to −2 (gains 2 electrons) — reduction. Fe is the reducing agent; O₂ is the oxidizing agent.
Identify redox in: 2H₂ + O₂ → 2H₂O
H goes from 0 to +1 (loses electrons) — oxidation. O goes from 0 to −2 (gains electrons) — reduction. H₂ is oxidized; O₂ is reduced.
In Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂, which is oxidized?
Br⁻ goes from −1 to 0 (loses 1 electron each) — oxidation. Cl goes from 0 to −1 (gains 1 electron each) — reduction. Br⁻ is oxidized (reducing agent).
Flashcards
Quick quiz
Q1.In 2Na + Cl₂ → 2NaCl, what is oxidized?
Q2.Redox reactions always involve…?
Q3.The reducing agent…?
Q4.In Cu + 2Ag⁺ → Cu²⁺ + 2Ag, Ag⁺ is…?
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Common mistakes
Oxidation always involves oxygen. — Correct: Oxidation is electron loss; oxygen is just a common oxidizing agent.
A reducing agent is reduced. — Correct: A reducing agent is oxidized — it loses electrons to reduce something else.
Oxidation and reduction can occur separately. — Correct: They always occur together in the same reaction.
Oxidation number always equals charge. — Correct: Oxidation number tracks electron flow; it can differ from formal charge.
FAQ
What is the difference between oxidation and reduction?
Oxidation is electron loss (increase in oxidation number); reduction is electron gain (decrease in oxidation number).
What is a redox reaction?
A reaction where electrons are transferred from one substance (oxidized) to another (reduced).
Why are they called redox?
Redox = reduction-oxidation. Both must occur simultaneously in any redox reaction.
Is combustion always a redox reaction?
Yes — the fuel is oxidized and oxygen is reduced.