Rucete ✏ AP Chemistry In a Nutshell
12. Oxidation-Reduction Reactions and Electrochemistry — Practice Questions
This chapter introduces oxidation-reduction reactions, half-reactions, galvanic and electrolytic cells, cell potentials, and the relationship between free energy, equilibrium, and electrochemical measurements.
(Multiple Choice — Click to Reveal Answer)
1. Which of the following is always true in a redox reaction?
(A) The oxidizing agent gains electrons
(B) The reducing agent gains electrons
(C) Both elements are reduced
(D) Electrons flow from cathode to anode
Answer
(A) — The oxidizing agent gains electrons and is reduced.
2. Which device uses a spontaneous redox reaction to generate electrical energy?
(A) Electrolytic cell
(B) Voltaic cell
(C) Capacitor
(D) Electroscope
Answer
(B) — A voltaic (galvanic) cell converts chemical energy into electrical energy spontaneously.
3. Which statement correctly compares oxidation and reduction?
(A) Both involve loss of electrons
(B) Oxidation increases oxidation number; reduction decreases it
(C) Oxidation happens at the cathode
(D) Reduction always releases gas
Answer
(B) — Oxidation increases and reduction decreases the oxidation state.
4. In a galvanic cell, electrons move:
(A) From the salt bridge to the solution
(B) From cathode to anode
(C) From anode to cathode
(D) From electrolyte to electrode
Answer
(C) — Electrons always flow from the anode (oxidation) to the cathode (reduction).
5. What is the charge on the cathode in a voltaic cell?
(A) Positive
(B) Negative
(C) Zero
(D) Depends on ion concentration
Answer
(A) — The cathode is positive in voltaic cells because it gains electrons.
6. Which species is reduced in the following reaction?
Cl₂ + 2e⁻ → 2Cl⁻
(A) Cl⁻
(B) Cl₂
(C) e⁻
(D) None
Answer
(B) — Cl₂ gains electrons and is reduced to Cl⁻.
7. A salt bridge is necessary in an electrochemical cell to:
(A) Conduct electrons
(B) Separate oxidation from reduction
(C) Complete the circuit and maintain charge balance
(D) Generate potential energy
Answer
(C) — Salt bridges allow ion flow to balance charges while keeping half-cells separate.
8. In the line notation Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s), which is oxidized?
(A) Cu²⁺
(B) Cu
(C) Zn
(D) Zn²⁺
Answer
(C) — Zn(s) loses electrons and is oxidized to Zn²⁺.
9. The species that undergoes reduction:
(A) Loses electrons
(B) Gains electrons
(C) Increases oxidation state
(D) Always forms a gas
Answer
(B) — Reduction is defined as the gain of electrons.
10. What does a positive E°cell indicate?
(A) The reaction is at equilibrium
(B) The reaction is nonspontaneous
(C) The reaction is spontaneous under standard conditions
(D) The reaction is exothermic
Answer
(C) — A positive standard cell potential means a spontaneous reaction.
11. In which half-cell does oxidation occur?
(A) The cathode
(B) The salt bridge
(C) The anode
(D) The electrolyte
Answer
(C) — Oxidation always occurs at the anode.
12. Which of the following is the best description of an oxidizing agent?
(A) A substance that loses electrons
(B) A substance that gains protons
(C) A substance that gains electrons
(D) A substance that loses oxygen
Answer
(C) — The oxidizing agent gains electrons and causes oxidation of another substance.
13. What is the role of the salt bridge in a galvanic cell?
(A) To supply energy
(B) To neutralize excess charge
(C) To oxidize the anode
(D) To prevent any ion flow
Answer
(B) — It maintains electrical neutrality by allowing ions to flow between half-cells.
14. What happens at the cathode of an electrolytic cell?
(A) Oxidation occurs
(B) Ions lose electrons
(C) Electrons are released
(D) Reduction occurs
Answer
(D) — Reduction occurs at the cathode in both galvanic and electrolytic cells.
15. If Zn²⁺ is reduced to Zn, what is the number of electrons transferred per atom?
(A) 1
(B) 2
(C) 3
(D) 0
Answer
(B) — Two electrons are needed to reduce Zn²⁺ to Zn(s).
16. Which is the correct shorthand notation for a standard galvanic cell?
(A) Cu²⁺ | Cu || Zn | Zn²⁺
(B) Zn | Zn²⁺ || Cu²⁺ | Cu
(C) Cu | Cu²⁺ || Zn²⁺ | Zn
(D) Zn²⁺ | Zn || Cu | Cu²⁺
Answer
(B) — The anode (Zn) is on the left, and the cathode (Cu) is on the right.
17. What is the standard condition for measuring cell potential?
(A) 1 atm pressure, 25 °C, 1 M solutions
(B) 0 °C and 1 atm
(C) 25 °C and 0.1 M
(D) 273 K and 1 atm
Answer
(A) — Standard conditions include 1 atm, 25 °C, and 1 M concentrations.
18. Which species undergoes oxidation in the reaction: Fe + Cu²⁺ → Fe²⁺ + Cu
(A) Fe
(B) Cu
(C) Fe²⁺
(D) Cu²⁺
Answer
(A) — Fe loses electrons and is oxidized to Fe²⁺.
19. In an electrochemical cell, what does the voltmeter measure?
(A) Amount of electrons in solution
(B) Number of moles reacting
(C) Potential difference between electrodes
(D) Mass of precipitate formed
Answer
(C) — The voltmeter measures the potential difference (voltage) between the two half-cells.
20. If a metal is higher than hydrogen on the activity series, it will:
(A) Be reduced by hydrogen
(B) Not react with acids
(C) Be oxidized by acids
(D) React with oxygen only
Answer
(C) — Metals above hydrogen in the activity series will displace H⁺ from acids and be oxidized.
21. What is the cell potential if both half-cell potentials are zero?
(A) 1.00 V
(B) 0.00 V
(C) –1.00 V
(D) Cannot be determined
Answer
(B) — If both standard reduction potentials are zero, the overall E°cell = 0 V.
22. What happens to the mass of the anode in a galvanic cell over time?
(A) It increases
(B) It decreases
(C) It stays the same
(D) It alternates
Answer
(B) — The anode is oxidized, releasing metal ions into solution and losing mass.
23. Which of the following is NOT a redox reaction?
(A) Combustion
(B) Single-replacement
(C) Acid-base neutralization
(D) Corrosion
Answer
(C) — Acid-base reactions are not redox; they involve proton transfer, not electron transfer.
24. If E°cell is negative, the reaction:
(A) Is spontaneous
(B) Requires an external power source
(C) Will release energy
(D) Will occur rapidly
Answer
(B) — A negative E°cell indicates the need for external energy input; it's nonspontaneous.
25. In electroplating, where does the metal being plated go?
(A) Into the electrolyte
(B) Onto the cathode
(C) Onto the anode
(D) It remains in solution
Answer
(B) — The cathode gains mass as metal cations are reduced and deposited.
26. Which half-reaction has the highest tendency to occur as a reduction?
(A) Zn²⁺ + 2e⁻ → Zn E° = –0.76 V
(B) Fe³⁺ + e⁻ → Fe²⁺ E° = +0.77 V
(C) Cu²⁺ + 2e⁻ → Cu E° = +0.34 V
(D) F₂ + 2e⁻ → 2F⁻ E° = +2.87 V
Answer
(D) — The more positive the standard reduction potential, the greater the tendency for reduction to occur.
27. If the E°cell for a reaction is +1.10 V, what is the value of ΔG° at 298 K for a 2-electron process?
(A) –106.1 kJ
(B) –212.2 kJ
(C) –55.0 kJ
(D) +106.1 kJ
Answer
(A) — ΔG° = –nFE° = –(2)(96,485)(1.10) ≈ –106,133 J = –106.1 kJ.
28. Which change increases the voltage of a Zn–Cu galvanic cell?
(A) Adding HCl to the Cu²⁺ solution
(B) Diluting the Zn²⁺ solution
(C) Increasing [Cu²⁺]
(D) Decreasing temperature
Answer
(C) — Increasing the concentration of the cathode ion drives the reduction forward, increasing cell voltage.
29. Which statement best explains why electrolysis of molten NaCl requires high temperature?
(A) To reduce NaCl to Na₂
(B) To liquefy water
(C) To allow ion mobility
(D) To increase electron affinity
Answer
(C) — Solid NaCl doesn’t conduct; melting is required to allow ions to move for electrolysis.
30. A nonspontaneous redox reaction is used in:
(A) Galvanic cell
(B) Dry cell battery
(C) Fuel cell
(D) Electrolytic cell
Answer
(D) — Electrolytic cells use electrical energy to drive nonspontaneous redox reactions.
31. Which metal will displace Ag⁺ from solution?
(A) Au
(B) Cu
(C) Zn
(D) Hg
Answer
(C) — Zn has a more negative reduction potential than Ag⁺ and will reduce it to Ag(s).
32. What is the oxidation number of Mn in KMnO₄?
(A) +2
(B) +4
(C) +7
(D) +6
Answer
(C) — K = +1, O = –2 × 4 = –8 → Mn = +7 to balance the overall charge.
33. Which factor affects the standard electrode potential of a half-reaction?
(A) Volume of electrolyte
(B) Type of voltmeter
(C) Nature of the element and ions
(D) Salt bridge length
Answer
(C) — Standard potential is intrinsic to the identity and chemistry of the species involved.
34. In the Nernst equation, what does an increase in Q (reaction quotient) do to Ecell?
(A) Increases Ecell
(B) Decreases Ecell
(C) Has no effect
(D) Makes Ecell negative
Answer
(B) — As Q increases, the term –(RT/nF)lnQ becomes more negative, lowering Ecell.
35. Which condition would cause plating to occur faster during electrolysis?
(A) Decreasing current
(B) Using a lower molar solution
(C) Increasing voltage
(D) Shortening electrode distance
Answer
(C) — Higher voltage increases current flow, speeding up the rate of electroplating.
36. Define oxidation in terms of both electron transfer and oxidation number.
Answer
Oxidation is the loss of electrons and an increase in oxidation number. For example, Fe → Fe²⁺ + 2e⁻ is oxidation.
37. What is the purpose of using a salt bridge in an electrochemical cell?
Answer
To maintain electrical neutrality. It allows the flow of ions to prevent charge buildup in half-cells.
38. Write the half-reaction for the reduction of Cu²⁺ to Cu.
Answer
Cu²⁺ + 2e⁻ → Cu(s) — This is the standard reduction half-reaction for copper ions.
39. Explain why E°cell must be positive for a spontaneous reaction.
Answer
Because ΔG° = –nFE°. If E° is positive, ΔG° is negative, indicating spontaneity.
40. What two half-reactions make up the cell Zn | Zn²⁺ || Cu²⁺ | Cu?
Answer
Zn → Zn²⁺ + 2e⁻ (oxidation at anode), Cu²⁺ + 2e⁻ → Cu (reduction at cathode)
41. How can you determine if a redox reaction will occur using standard reduction potentials?
Answer
By calculating E°cell = E°cathode – E°anode. If E°cell > 0, the reaction is spontaneous.
42. What is the oxidation number of sulfur in H₂SO₄?
Answer
+6 — H = +1 (×2), O = –2 (×4), so S must be +6 to balance the +2 and –8 charges.
43. In electroplating, why must the object being plated be the cathode?
Answer
Because reduction occurs at the cathode. Metal cations gain electrons and deposit on the cathode surface.
44. What is the function of the external wire in a galvanic cell?
Answer
To allow electron flow from anode to cathode. This flow powers any external circuit connected.
45. What is meant by standard conditions in electrochemistry?
Answer
1 M concentration, 1 atm pressure, and 25 °C (298 K). These are used to report standard electrode potentials.
46. How does increasing the concentration of the oxidizing agent affect Ecell?
Answer
It increases Ecell. More oxidizing agent drives the reduction half-reaction forward.
47. Why is fluorine the strongest oxidizing agent among elements?
Answer
Because it has the highest standard reduction potential. It strongly attracts electrons.
48. What type of cell uses electrical energy to drive a chemical change?
Answer
Electrolytic cell. It performs nonspontaneous reactions using external electricity.
49. How is ΔG related to Ecell?
Answer
ΔG = –nFEcell, where n = moles of electrons, F = Faraday constant, and E = cell potential.
50. Explain why the anode of a galvanic cell loses mass over time.
Answer
Because metal atoms oxidize to ions. They leave the anode and enter solution, reducing the anode’s mass.