Balancing by Half Reaction Ion Electron Method
Chapter: Redox Reactions | Topic: Balancing by Half Reaction Ion Electron Method
BALANCING BY HALF REACTION ION ELECTRON METHOD
Topic: Balancing by Half Reaction Ion Electron Method
Multiple Choice Questions
Q1. In the half-reaction method, the first step is to:
- Balance the entire equation at once
- Split the reaction into oxidation and reduction half-reactions
- Add H⁺ ions immediately
- Balance oxygen atoms first
Q2. When balancing half-reactions in acidic medium, H⁺ ions are added to:
- The side deficient in hydrogen
- The side rich in hydrogen
- Both sides equally
- Neither side
Q3. To balance oxygen atoms in a half-reaction in acidic medium, we add:
- OH⁻ ions
- H₂O molecules to the side deficient in oxygen
- O₂ molecules
- H₂O molecules to the side rich in oxygen
Q4. In basic medium, after balancing a half-reaction as if in acidic medium, we:
- Add H⁺ ions to both sides
- Add OH⁻ ions to both sides to neutralise H⁺
- Remove all water molecules
- Double all coefficients
Q5. The number of electrons lost in the oxidation half-reaction must:
- Be greater than the number gained in the reduction half-reaction
- Equal the number gained in the reduction half-reaction
- Be less than the number gained in the reduction half-reaction
- Be unrelated to the reduction half-reaction
Q6. In the half-reaction MnO₄⁻ → Mn²⁺ in acidic medium, the number of electrons involved is:
- 3
- 5
- 7
- 2
Short Answer Questions
Q7. Outline the steps involved in balancing redox reactions by the ion-electron (half-reaction) method in acidic medium.
Q8. Balance the following half-reaction in acidic medium:
Cr₂O₇²⁻ → Cr³⁺
Q9. Balance the following half-reaction in basic medium:
MnO₄⁻ → MnO₂
Q10. Explain why the half-reaction method is particularly useful for balancing redox reactions in aqueous solutions compared to the oxidation number method.
Q11. In the half-reaction SO₃²⁻ → SO₄²⁻ in acidic medium, how many electrons are transferred? Show the balanced half-reaction.
Q12. A student balances a half-reaction in acidic medium and obtains H⁺ ions on the product side. The reaction actually occurs in basic medium. What additional step must the student take?
Long Answer Questions
Q13. (a) Describe the step-by-step procedure for balancing redox reactions by the ion-electron (half-reaction) method in acidic medium.
(b) Balance the following reaction in acidic medium by the half-reaction method:
MnO₄⁻ + Fe²⁺ → Mn²⁺ + Fe³⁺
(c) Identify the oxidising agent and the reducing agent. How many moles of Fe²⁺ are oxidised by 1 mole of MnO₄⁻?
Q14. (a) Explain how to balance redox reactions in basic medium using the half-reaction method.
(b) Balance the following reaction in basic medium:
MnO₄⁻ + C₂O₄²⁻ → MnO₂ + CO₃²⁻
(c) Balance the following reaction in acidic medium:
Cr₂O₇²⁻ + H₂S → Cr³⁺ + S + H₂O
(d) A student claims that the half-reaction method is more versatile than the oxidation number method because it can handle complex ionic reactions more easily. Do you agree? Give reasons.
Q15. (a) Balance the following reaction in acidic medium by the half-reaction method:
As₂S₃ + NO₃⁻ → AsO₄³⁻ + SO₄²⁻ + NO
(b) Balance the following reaction in basic medium:
Cl₂ + OH⁻ → Cl⁻ + ClO⁻
(c) In the Indian context, the reaction of dichromate with ethanol is used in breathalysers to detect drunk driving:
Cr₂O₇²⁻ + C₂H₅OH → Cr³⁺ + CH₃COOH
Balance this reaction in acidic medium and explain how the colour change from orange to green indicates alcohol presence.
Numerical / Application-Based Problems
Q16. Balance the following redox reactions by the half-reaction method. Show all steps for each half-reaction:
(a) In acidic medium:
MnO₄⁻ + H₂O₂ → Mn²⁺ + O₂
(b) In acidic medium:
Cr₂O₇²⁻ + SO₃²⁻ → Cr³⁺ + SO₄²⁻
(c) In basic medium:
ClO₃⁻ + N₂H₄ → Cl⁻ + N₂
(d) In basic medium:
Al + NO₃⁻ → AlO₂⁻ + NH₃
For each reaction, identify:
(i) The oxidation half-reaction
(ii) The reduction half-reaction
(iii) The number of electrons transferred
Q17. A student performs redox titrations using the half-reaction method:
(a) A 25.0 mL sample of H₂O₂ solution is acidified and titrated against 0.02 M KMnO₄ solution. The reaction is:
MnO₄⁻ + H₂O₂ + H⁺ → Mn²⁺ + O₂ + H₂O
(i) Balance this reaction by the half-reaction method in acidic medium
(ii) If 20.0 mL of KMnO₄ is required, calculate the molarity of the H₂O₂ solution
(iii) Calculate the volume of O₂ gas produced at STP during this titration
(b) In another experiment, iodometric titration is performed:
I₂ + S₂O₃²⁻ → I⁻ + S₄O₆²⁻
(i) Balance this reaction by the half-reaction method
(ii) If 0.5 g of impure CuSO₄·5H₂O is dissolved and excess KI is added, the liberated I₂ requires 20.0 mL of 0.1 M Na₂S₂O₃. Calculate the percentage purity of the CuSO₄·5H₂O sample (M = 249.5 g/mol)
Q18. In Indian industry and environmental science, half-reaction balancing is essential:
(a) In the chlor-alkali process, chlorine is produced by electrolysis of brine:
2Cl⁻ + 2H₂O → Cl₂ + H₂ + 2OH⁻
(i) Split this into oxidation and reduction half-reactions
(ii) Balance each half-reaction in basic medium
(iii) Calculate the mass of Cl₂ produced when 1 mole of electrons passes through the cell
(b) In electroplating, chromium is deposited from chromic acid solution:
Cr₂O₇²⁻ + H⁺ + e⁻ → Cr³⁺ + H₂O (incomplete)
(i) Complete and balance the reduction half-reaction in acidic medium
(ii) Calculate how many moles of electrons are required to deposit 1 mole of Cr metal
(iii) If a current of 10 A flows for 2 hours, calculate the mass of chromium deposited (1 F = 96500 C/mol, M of Cr = 52 g/mol)
(c) In wastewater treatment, dichromate is used to measure chemical oxygen demand (COD):
Cr₂O₇²⁻ + organic matter + H⁺ → Cr³⁺ + CO₂ + H₂O
(i) Balance the half-reaction for the reduction of dichromate in acidic medium
(ii) If 1.5 g of glucose (C₆H₁₂O₆) is oxidised, calculate the moles of Cr₂O₇²⁻ required
(iii) Explain why this test is called 'chemical oxygen demand'