Redox Reactions in Acidic and Basic Medium

Redox reactions often take place in either an acidic or a basic aqueous medium. The method for balancing these reactions depends on the medium and involves H⁺, OH^– and H2O to account for hydrogen and oxygen atoms.

Redox Reactions in Acidic Medium

In acidic medium use H⁺ and H2O when balancing. Steps (half-reaction method):

1. Separate into oxidation and reduction half-reactions.
2. Balance atoms other than H and O.
3. Balance O using H2O.
4. Balance H using H⁺.
5. Balance charge with electrons (e^–).
6. Multiply to equalize electrons and combine; cancel common species.

Example (Acidic Medium)

MnO₄⁻ + Fe²⁺ → Mn²⁺ + Fe³⁺
Step 1: Oxidation: Fe²⁺ → Fe³⁺ + e⁻
Step 2: Reduction: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
Final: 5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O

Redox Reactions in Basic Medium

In basic medium use OH^– and H2O. Follow acidic steps, then remove H⁺ by adding OH^– to both sides and convert to water.

Example (Basic Medium)

Cr(OH)₃ + ClO⁻ → CrO₄²⁻ + Cl⁻
Step 1: Oxidation: Cr(OH)₃ → CrO₄²⁻
Step 2: Reduction: ClO⁻ → Cl⁻
Balanced: 2Cr(OH)₃ + 3ClO⁻ + 4OH⁻ → 2CrO₄²⁻ + 3Cl⁻ + 5H₂O

Key Differences

Aspect	Acidic Medium	Basic Medium
Ions used	H+	OH–
Neutralization	With H2O only	H+ + OH– → H2O
pH range	< 7	> 7

Quiz: Redox in Acidic & Basic Medium

1. What ion is used to balance H atoms in an acidic medium? AnswerH⁺
2. What is the purpose of OH^– in redox balancing? AnswerTo neutralize H⁺ and convert acidic-balanced equations to basic form.
3. Which ion reacts with H⁺ to form H2O? AnswerOH^–
4. Is ClO^– an oxidizing or reducing agent in basic medium? AnswerOxidizing agent.
5. What is the product of MnO₄^– reduction in acidic solution? AnswerMn²⁺

MCQs

1. In basic medium, H⁺ ions are neutralized by:
   a) Cl⁻ b) SO₄²⁻ c) OH⁻ d) H₂Correct: c) OH⁻
2. Which ion is added to balance O in acidic medium?
   a) OH⁻ b) H₂O c) H⁺ d) O₂Correct: b) H₂O
3. Which is correct in basic redox balancing?
   a) Add H⁺ to both sides b) Subtract OH⁻ from products c) Add OH⁻ to cancel H⁺ d) Add O₂ gasCorrect: c) Add OH⁻ to cancel H⁺
4. In MnO₄⁻ → Mn²⁺ in acidic medium, Mn is:
   a) Oxidized b) Reduced c) No change d) DisproportionatedCorrect: b) Reduced
5. In acidic medium, Cl⁻ → Cl₂ is:
   a) Reduction b) Oxidation c) Substitution d) HydrolysisCorrect: b) Oxidation

• Oxidation and Reduction — CHEMASH
• Oxidation Number — CHEMASH
• LibreTexts Chemistry
• Britannica: Oxidation–reduction reaction

FAQs

How do I balance redox reactions in acidic medium?

Use the half-reaction method: balance non-H/O atoms, then O with H2O, H with H+, balance charge with electrons, combine and simplify.

How do I change an acidic-balanced equation to basic?

Add OH- equal to the number of H+ to both sides, form water, then cancel waters.

What is disproportionation?

When a single species undergoes both oxidation and reduction simultaneously (e.g., 2H2O2 → 2H2O + O2).

Published: Sep 29, 2025 • Category: Redox Reactions