Questions on Balancing Redox Reactions
Multiple-choice questions on Balancing Redox Reactions using the following methods:
• Oxidation Number Change Method
• Half-Reaction Method
• Balancing in Basic Solution
Balancing Redox Reactions: 30 Multiple-Choice Questions
General Concepts
1. What is the main principle in balancing redox reactions?
A) Equal numbers of atoms only
B) Mass conservation only
C) Equal total charges and atoms
D) Energy balance
E) Same number of moles on each side
2. In redox balancing, electrons lost in oxidation must:
A) Be discarded
B) Equal electrons gained in reduction
C) Be ignored
D) Be more than gained
E) Be fewer than gained
3. The oxidation number of an element is used to:
A) Measure acidity
B) Determine temperature
C) Track electron transfer
D) Identify isotopes
E) Calculate molar mass
4. The half-reaction method involves:
A) Only oxidation half
B) Only reduction half
C) Both oxidation and reduction reactions separately
D) Energy calculations
E) Mass-to-mass conversions
5. Which step is not part of the half-reaction method?
A) Balance atoms other than H and O
B) Balance electrons
C) Balance energy
D) Add H₂O, H⁺, or OH⁻ as needed
E) Combine both half-reactions
⚙ Oxidation Number Change Method
6. What is the first step in the oxidation number change method?
A) Add electrons
B) Identify oxidized and reduced elements
C) Add H₂O
D) Balance charges
E) Multiply half-reactions
7. In the oxidation number method, the key is:
A) Balancing OH⁻ first
B) Using molar ratios
C) Ensuring total change in oxidation numbers is equal
D) Calculating pH
E) Mixing all species at once
8. Which is a correct pair of oxidation number changes in this reaction?
Fe²⁺ + Cr₂O₇²⁻ → Fe³⁺ + Cr³⁺ (acidic)
A) Fe: +2 → +3; Cr: +6 → +3
B) Fe: +3 → +2; Cr: +3 → +6
C) Fe: 0 → +2; Cr: +2 → +3
D) Fe: +2 → 0; Cr: 0 → +3
E) Fe: +3 → 0; Cr: +6 → 0
9. In acidic solutions, which species is commonly added to balance oxygen?
A) H₂O₂
B) O₂
C) OH⁻
D) H₂O
E) HCl
10. In acidic solutions, which species is added to balance hydrogen?
A) H₂O
B) OH⁻
C) H₂
D) H⁺
E) Cl⁻
⚛ Half-Reaction Method
11. What is the final step in the half-reaction method?
A) Add electrons
B) Multiply half-reactions
C) Cancel electrons and combine
D) Add water
E) Check oxidation numbers
12. Which half-reaction represents oxidation?
A) Cu²⁺ + 2e⁻ → Cu
B) Zn → Zn²⁺ + 2e⁻
C) O₂ + 4e⁻ → 2O²⁻
D) Fe³⁺ + e⁻ → Fe²⁺
E) Cl₂ + 2e⁻ → 2Cl⁻
13. In basic solutions, what is used to neutralize H⁺ ions?
A) H₂
B) H₂O
C) OH⁻
D) Cl⁻
E) NO₃⁻
14. Which step is exclusive to balancing redox reactions in basic solution?
A) Adding H₂O
B) Balancing O atoms
C) Adding OH⁻ to both sides
D) Balancing electrons
E) Identifying oxidizing agent
15. In basic solution, the combination of H⁺ and OH⁻ forms:
A) H₂
B) HO₂
C) H₃O⁺
D) H₂O
E) OH₂
🔬 Examples & Calculations
16. What is the coefficient of MnO₄⁻ in this balanced acidic reaction?
MnO₄⁻ + Fe²⁺ → Mn²⁺ + Fe³⁺
A) 1
B) 2
C) 3
D) 4
E) 5
17. In this half-reaction: Cr₂O₇²⁻ → Cr³⁺ (acidic), how many electrons are transferred per Cr₂O₇²⁻?
A) 2
B) 3
C) 4
D) 6
E) 8
18. Which of these is not required when balancing in acidic solution?
A) OH⁻
B) H⁺
C) H₂O
D) Electrons
E) Oxidation numbers
19. In the redox reaction: H₂O₂ → O₂, what is the oxidation number change of oxygen?
A) –2 → 0
B) 0 → –1
C) +1 → –1
D) –1 → 0
E) 0 → –2
20. How many electrons are lost in: Zn → Zn²⁺?
A) 1
B) 2
C) 3
D) 4
E) 0
🧪 Mixed Application
21. What is the oxidizing agent in: 2Al + 3Cu²⁺ → 2Al³⁺ + 3Cu?
A) Al
B) Cu
C) Al³⁺
D) Cu²⁺
E) O₂
22. Which method is best when working with reactions in basic solution?
A) Oxidation number method
B) Half-reaction with OH⁻ and H₂O
C) Only mass balance
D) Gas law approach
E) Titration
23. Identify the balanced number of electrons in this half-reaction:
NO₃⁻ → NO (acidic)
A) 1
B) 2
C) 3
D) 4
E) 5
24. Which of the following is true for balancing redox reactions?
A) Electrons can be unequal
B) Charge doesn’t matter
C) Use of OH⁻ in acidic solutions
D) H₂O balances oxygen atoms
E) H⁺ always balances electrons
25. In basic solution, after balancing with H⁺, what must you do?
A) Convert H⁺ to OH⁻
B) Add salt
C) Subtract electrons
D) Multiply oxidation states
E) Add acid
26. In balancing: ClO⁻ → Cl⁻ (basic), what is added to balance O?
A) OH⁻
B) H⁺
C) H₂O
D) Cl₂
E) ClO₄⁻
27. Which of these reactions requires balancing in basic solution?
A) Zn + Cu²⁺ → Zn²⁺ + Cu
B) MnO₄⁻ + SO₃²⁻ → MnO₂ + SO₄²⁻
C) Na + H₂O → NaOH + H₂
D) Cl₂ + H₂ → 2HCl
E) Fe + CuSO₄ → FeSO₄ + Cu
28. What is the oxidation number of Mn in MnO₄⁻?
A) +2
B) +4
C) +5
D) +7
E) –1
29. In basic solution, the half-reaction OH⁻ + Cr(OH)₃ → CrO₄²⁻ involves what key step?
A) Adding electrons to both sides
B) Adding H⁺
C) Adding OH⁻ and H₂O
D) Ignoring oxygen atoms
E) Multiplying both sides by 3
30. In redox balancing, charges on both sides of the equation must be:
A) Maximum
B) Ignored
C) Equal
D) Opposite
E) Negative
Answers and Explanations
1. C – Both atoms and charges must be balanced.
2. B – Electrons lost = electrons gained.
3. C – Oxidation numbers help track e⁻ transfer.
4. C – Half-reaction method separates ox/red.
5. C – Energy balance is not part of the method.
6. B – Identify oxidation and reduction first.
7. C – Equal total oxidation number change is key.
8. A – Fe: +2 → +3; Cr: +6 → +3.
9. D – H₂O is added to balance O.
10. D – H⁺ is added to balance H in acidic media.
11. C – After balancing, combine and cancel electrons.
12. B – Zn is oxidized, loses electrons.
13. C – OH⁻ neutralizes H⁺ in basic media.
14. C – Adding OH⁻ to neutralize H⁺ is unique to basic.
15. D – H⁺ + OH⁻ → H₂O.
16. A – 1 MnO₄⁻ reacts per balanced equation.
17. D – Each Cr goes from +6 to +3 → 6 e⁻ total.
18. A – OH⁻ is not used in acidic solutions.
19. D – O in H₂O₂ (–1) goes to 0 in O₂.
20. B – Zn loses 2 electrons.
21. D – Cu²⁺ is reduced → oxidizing agent.
22. B – Use OH⁻ and H₂O for basic balancing.
23. E – 5 e⁻ needed to balance NO₃⁻ → NO.
24. D – Use H₂O to balance O atoms.
25. A – Convert H⁺ to H₂O and add OH⁻.
26. C – Add H₂O to balance oxygen.
27. B – MnO₄⁻ → MnO₂ in basic solution.
28. D – Mn = +7 in MnO₄⁻.
29. C – Use OH⁻ and H₂O to balance in basic.
30. C – Charges must be equal on both sides.
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