Rucete ✏ AP Chemistry In a Nutshell
12. Oxidation-Reduction Reactions and Electrochemistry — Practice Questions 3
This chapter introduces oxidation-reduction reactions and electrochemistry, including oxidation states, half-reactions, voltaic and electrolytic cells, and standard electrode potentials.
(Multiple Choice — Click to Reveal Answer)
1. What occurs at the cathode of an electrolytic cell?
(A) Electrons are produced
(B) Oxidation takes place
(C) Electrons are consumed
(D) The anion concentration increases
Answer
(C) — The cathode is where reduction occurs, and it consumes electrons in both cell types.
2. What is the oxidation number of oxygen in hydrogen peroxide, H₂O₂?
(A) 0
(B) –1
(C) –2
(D) +1
Answer
(B) — In peroxides like H₂O₂, oxygen has an unusual oxidation number of –1.
3. Which species is acting as the reducing agent in the reaction: 2Fe²⁺ + Cl₂ → 2Fe³⁺ + 2Cl⁻?
(A) Fe²⁺
(B) Cl₂
(C) Fe³⁺
(D) Cl⁻
Answer
(A) — Fe²⁺ loses electrons (is oxidized), so it is the reducing agent.
4. In which case will a redox reaction be spontaneous under standard conditions?
(A) If E°cell is negative
(B) If ΔG° is positive
(C) If E°cell is positive
(D) If both half-cells are electrolytic
Answer
(C) — A positive standard cell potential means the reaction is spontaneous (ΔG° < 0).
5. Which device is based on a spontaneous redox reaction to produce electricity?
(A) Electrolytic cell
(B) Battery
(C) Electrolyzer
(D) Spectrophotometer
Answer
(B) — Batteries are voltaic (galvanic) cells, which use spontaneous redox reactions to generate electricity.
6. What is the oxidation number of sulfur in SO₃?
(A) +2
(B) +4
(C) +6
(D) –2
Answer
(C) — Each oxygen is –2; 3(–2) = –6, so S must be +6 to balance overall charge to 0.
7. In which compound is the oxidation number of chlorine +5?
(A) HClO
(B) HClO₂
(C) HClO₃
(D) HClO₄
Answer
(C) — In HClO₃, oxygen is –2 × 3 = –6, and H is +1, so Cl must be +5.
8. What is the reducing agent in the reaction: Zn + Cu²⁺ → Zn²⁺ + Cu?
(A) Cu²⁺
(B) Zn
(C) Zn²⁺
(D) Cu
Answer
(B) — Zn loses electrons (oxidized), making it the reducing agent.
9. What is the standard reduction potential of the half-reaction: Cu²⁺ + 2e⁻ → Cu(s)?
(A) +0.34 V
(B) –0.76 V
(C) +1.23 V
(D) –1.00 V
Answer
(A) — Standard value for copper(II) reduction is +0.34 V.
10. In a voltaic cell, which of the following always occurs?
(A) Cathode gains mass
(B) Electrons move toward the anode
(C) Oxidation occurs at the cathode
(D) Anions flow toward the anode
Answer
(A) — Electrons reduce metal ions at the cathode, causing it to gain solid mass.
11. Which is a characteristic of a redox reaction?
(A) Transfer of protons
(B) No change in oxidation states
(C) Transfer of electrons
(D) Formation of precipitates only
Answer
(C) — Redox reactions involve electron transfer and oxidation number changes.
12. Which metal will NOT displace hydrogen from acid?
(A) Na
(B) Zn
(C) Cu
(D) Mg
Answer
(C) — Copper is less reactive than hydrogen and cannot displace it from acid.
13. What type of reaction is illustrated by: 2Na + Cl₂ → 2NaCl?
(A) Combination
(B) Decomposition
(C) Single replacement
(D) Combustion
Answer
(A) — Two elements combine to form a compound — combination and redox reaction.
14. In the reaction: MnO₄⁻ + 5Fe²⁺ + 8H⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O, which species is oxidized?
(A) MnO₄⁻
(B) Fe²⁺
(C) Mn²⁺
(D) H⁺
Answer
(B) — Iron goes from +2 to +3, losing electrons (oxidation).
15. The function of a salt bridge in a galvanic cell is to:
(A) Provide a source of electrons
(B) Supply ions to maintain neutrality
(C) Increase resistance
(D) Separate oxidation and reduction physically
Answer
(B) — It allows ion flow to balance charge buildup and complete the circuit.
16. Which of the following is not an example of a redox process?
(A) Combustion
(B) Respiration
(C) Neutralization
(D) Corrosion
Answer
(C) — Neutralization is an acid-base reaction, not redox.
17. When magnesium reacts with oxygen, what is the oxidation state of Mg in MgO?
(A) 0
(B) +1
(C) +2
(D) –2
Answer
(C) — Magnesium becomes Mg²⁺ when oxidized by oxygen.
18. What is the total number of electrons transferred in the balanced redox reaction:
2Al + 3Cl₂ → 2AlCl₃?
(A) 3
(B) 6
(C) 4
(D) 2
Answer
(B) — Each Al loses 3e⁻, total 6e⁻ transferred to Cl₂.
19. Which of these species is both a reactant and an oxidizing agent in the same reaction?
(A) Cl⁻
(B) H₂O₂
(C) NO₃⁻
(D) SO₄²⁻
Answer
(B) — Hydrogen peroxide can act as both oxidizing and reducing agent.
20. Which statement about standard electrode potentials is correct?
(A) More negative means stronger oxidizing agent
(B) More positive means stronger reducing agent
(C) More positive means stronger oxidizing agent
(D) All are true
Answer
(C) — Higher E° indicates a better tendency to gain electrons (stronger oxidizer).
21. The oxidation number of carbon in CO₂ is:
(A) –4
(B) +2
(C) +4
(D) 0
Answer
(C) — Each oxygen is –2; to balance two, carbon must be +4.
22. What is the term for a substance that gains electrons?
(A) Oxidized
(B) Oxidizing agent
(C) Reduced
(D) Reducing agent
Answer
(C) — Reduction means gain of electrons.
23. In electrolysis, what is deposited at the cathode when molten PbBr₂ is used?
(A) Pb
(B) Br₂
(C) H₂
(D) O₂
Answer
(A) — The metal ion Pb²⁺ gains electrons and forms solid lead at the cathode.
24. A spontaneous redox reaction must have which condition?
(A) E°cell = 0
(B) ΔG = 0
(C) E°cell > 0
(D) E°cell < 0
Answer
(C) — Positive E°cell indicates a spontaneous reaction (ΔG < 0).
25. Which pair of half-reactions would give the largest cell potential when combined?
(A) Ag⁺/Ag and Cu²⁺/Cu
(B) Zn²⁺/Zn and Cl₂/Cl⁻
(C) Fe³⁺/Fe²⁺ and Sn²⁺/Sn
(D) Cu²⁺/Cu and Fe³⁺/Fe
Answer
(B) — The Zn/Cl₂ pair has the greatest difference in standard electrode potentials.
26. Which of the following conditions would decrease the voltage of a voltaic cell based on the Nernst equation?
(A) Increasing concentration of reactants
(B) Increasing temperature
(C) Increasing concentration of products
(D) Decreasing the number of electrons transferred
Answer
(C) — Higher [products] increases Q, reducing Ecell according to the Nernst equation.
27. Which metal is most likely to reduce H⁺ from aqueous acid based on the following potentials?
Zn²⁺/Zn = –0.76 V,
Fe²⁺/Fe = –0.44 V,
Sn²⁺/Sn = –0.14 V,
Pb²⁺/Pb = –0.13 V
(A) Sn
(B) Zn
(C) Fe
(D) Pb
Answer
(B) — Zn has the lowest (most negative) E°, making it most likely to reduce H⁺.
28. In an electrolytic cell, which of the following will occur?
(A) Anode is positive and oxidation occurs there
(B) Anode is negative and reduction occurs there
(C) Cathode is positive and oxidation occurs there
(D) Electrons flow from cathode to anode
Answer
(A) — In electrolysis, anode is positive and oxidation always occurs at the anode.
29. Which of the following changes will make a redox reaction more spontaneous?
(A) Decreasing temperature of an exothermic electrochemical cell
(B) Decreasing reactant concentration
(C) Increasing entropy of the system
(D) Decreasing cell pressure
Answer
(A) — For exothermic systems, lowering temperature favors spontaneity (ΔG = ΔH – TΔS).
30. A redox reaction has ΔG = –250 kJ/mol and involves 5 electrons transferred. What is the standard potential E°?
(A) 1.30 V
(B) 0.52 V
(C) 2.59 V
(D) 5.00 V
Answer
(C) — ΔG = –nFE → 250000 = 5×96485×E → E ≈ 2.59 V.
31. Which condition correctly applies to a concentration cell?
(A) E°cell is always 1.0 V
(B) E°cell = 0, but Ecell ≠ 0 due to concentration difference
(C) Ecell is always negative
(D) Ecell = E°cell
Answer
(B) — In a concentration cell, same species are used; E°cell = 0, but Ecell depends on Q.
32. In the redox titration of Fe²⁺ with KMnO₄ in acidic solution, what color change is observed at the equivalence point?
(A) Purple to colorless
(B) Colorless to pink
(C) Yellow to red
(D) Blue to green
Answer
(B) — MnO₄⁻ is purple, and once Fe²⁺ is used up, excess MnO₄⁻ causes pink color to persist.
33. Which of the following pairs cannot be used to construct a spontaneous voltaic cell?
(A) Zn/Zn²⁺ and Cu/Cu²⁺
(B) Fe/Fe²⁺ and Pb/Pb²⁺
(C) Ag/Ag⁺ and Ni/Ni²⁺
(D) Au/Au³⁺ and Cl⁻/Cl₂
Answer
(D) — Gold is extremely inert and has a very positive E°, making this pair unlikely to form a spontaneous cell with chlorine.
34. Which of the following reactions would occur at the anode of a lead-acid storage battery during discharge?
(A) PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻ → PbSO₄ + 2H₂O
(B) Pb + SO₄²⁻ → PbSO₄ + 2e⁻
(C) Pb²⁺ + 2e⁻ → Pb
(D) PbSO₄ → Pb + SO₄²⁻
Answer
(B) — During discharge, lead (Pb) is oxidized at the anode to form PbSO₄.
35. A redox reaction shows both H₂O₂ and MnO₄⁻ as reactants in acidic solution. What happens to H₂O₂?
(A) It is reduced to O₂
(B) It is oxidized to O₂
(C) It is reduced to H₂O
(D) It remains unchanged
Answer
(B) — In acidic medium, H₂O₂ is oxidized to O₂ (0) while MnO₄⁻ is reduced to Mn²⁺.
36. Define oxidation in terms of electron transfer.
Answer
Oxidation is the loss of electrons by a species, resulting in an increase in oxidation number.
37. Write the half-reaction for the reduction of permanganate ion (MnO₄⁻) to Mn²⁺ in acidic solution.
Answer
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
38. Identify the oxidizing agent in the following redox reaction: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂
Answer
Cl₂ — it gains electrons from Br⁻ and is reduced to Cl⁻.
39. A redox reaction has E°cell = +0.65 V. Is this reaction spontaneous under standard conditions? Explain.
Answer
Yes. A positive E°cell means the redox reaction is spontaneous (ΔG° < 0).
40. Calculate ΔG° for a reaction in which n = 3 and E°cell = +0.45 V.
Answer
ΔG° = –nFE° = –3 × 96485 × 0.45 ≈ –130,264 J or –130.3 kJ
41. What is the oxidation number of nitrogen in NO₃⁻?
Answer
+5 — Each oxygen is –2, total –6; the overall charge is –1, so nitrogen must be +5.
42. Why is hydrogen peroxide considered both an oxidizing and a reducing agent?
Answer
Because it can both accept electrons (oxidizing) and donate electrons (reducing), depending on the reaction.
43. In a Daniell cell, zinc and copper electrodes are used. Write the overall balanced redox reaction.
Answer
Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)
44. If the concentration of Zn²⁺ is increased in a voltaic cell, what happens to the cell voltage?
Answer
Cell voltage decreases — more product increases Q, lowering Ecell according to the Nernst equation.
45. How does temperature affect the spontaneity of a redox reaction with negative ΔH and negative ΔS?
Answer
Lower temperatures favor spontaneity because ΔG = ΔH – TΔS, and TΔS becomes less negative.
46. A certain redox reaction has a negative E°cell. What does this indicate about the spontaneity?
Answer
It’s non-spontaneous under standard conditions (ΔG° > 0).
47. Why do electrons flow from anode to cathode in a voltaic cell?
Answer
Because oxidation occurs at the anode (loss of electrons) and reduction at the cathode (gain of electrons).
48. What is the standard cell potential (E°cell) of a cell made from Ag⁺/Ag (0.80 V) and Zn²⁺/Zn (–0.76 V)?
Answer
0.80 – (–0.76) = 1.56 V
49. A cell has a measured Ecell of 0.55 V, but E°cell is 0.65 V. What does this tell us about the reaction quotient (Q)?
Answer
Q > 1 — The reaction has proceeded toward products, lowering the voltage below standard.
50. During electrolysis of molten NaCl, what is produced at the cathode?
Answer
Na(l) — Sodium ions (Na⁺) are reduced at the cathode.