Rucete ✏ AP Chemistry In a Nutshell
12. Oxidation-Reduction Reactions and Electrochemistry — Practice Questions 2
This chapter introduces redox reactions and electrochemistry, including oxidation numbers, half-reactions, voltaic/electrolytic cells, electrode potentials, and the Nernst equation.
(Multiple Choice — Click to Reveal Answer)
1. In an electrochemical cell, what is always true about the direction of electron flow?
(A) From cathode to anode
(B) From the electrolyte to the anode
(C) From anode to cathode
(D) From salt bridge to electrodes
Answer
(C) — Electrons always flow from the anode (oxidation site) to the cathode (reduction site).
2. What is the oxidation number of chromium in Cr2O72–?
(A) +6
(B) +3
(C) +2
(D) +7
Answer
(A) — Let x be the oxidation number of Cr. 2x + (–14) = –2 → x = +6.
3. Which species is oxidized in the following reaction? Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
(A) Zn(s)
(B) Cu2+
(C) Zn2+
(D) Cu(s)
Answer
(A) — Zinc loses electrons, which means it is oxidized.
4. What is the role of the salt bridge in a voltaic cell?
(A) To provide a path for electron flow
(B) To separate the electrodes physically
(C) To balance charge by allowing ion flow
(D) To supply energy for the reaction
Answer
(C) — The salt bridge maintains electrical neutrality by allowing ions to migrate.
5. Which electrode in a voltaic cell is where reduction occurs?
(A) Anode
(B) Cathode
(C) Salt bridge
(D) Electrolyte
Answer
(B) — The cathode is the site of reduction in both voltaic and electrolytic cells.
6. In the reaction 2Fe3+ + Sn2+ → 2Fe2+ + Sn4+, which element is reduced?
(A) Fe
(B) Sn
(C) O
(D) H
Answer
(A) — Iron goes from +3 to +2, meaning it gains electrons (reduction).
7. What is the standard electrode potential of a hydrogen electrode defined as?
(A) +1.00 V
(B) –1.00 V
(C) 0.00 V
(D) +0.76 V
Answer
(C) — The standard hydrogen electrode (SHE) is the reference with 0.00 V.
8. Which metal will displace copper from a copper(II) sulfate solution?
(A) Silver
(B) Gold
(C) Zinc
(D) Mercury
Answer
(C) — Zinc is more reactive and will displace Cu2+ ions from solution.
9. In a galvanic cell, which reaction occurs at the anode?
(A) Oxidation
(B) Reduction
(C) Neutralization
(D) No reaction
Answer
(A) — Oxidation always occurs at the anode.
10. Which of the following is true for a spontaneous redox reaction under standard conditions?
(A) E°cell is negative
(B) ΔG is positive
(C) Electrons flow from cathode to anode
(D) E°cell is positive
Answer
(D) — Positive E°cell indicates the reaction is spontaneous.
11. Which of the following would best complete the half-reaction: Ag⁺ + ___ → Ag(s)?
(A) 1 electron
(B) 2 electrons
(C) 2 protons
(D) 1 proton
Answer
(A) — Ag⁺ + e⁻ → Ag is the correct half-reaction for reduction of silver.
12. Which of the following is always true in any redox reaction?
(A) Only one substance changes oxidation state
(B) Electrons are transferred
(C) All atoms are reduced
(D) Oxidation number of oxygen increases
Answer
(B) — Redox reactions involve transfer of electrons between species.
13. Which of the following changes indicates oxidation?
(A) Loss of oxygen
(B) Gain of electrons
(C) Gain of hydrogen
(D) Loss of electrons
Answer
(D) — Oxidation is defined as loss of electrons.
14. Which element is being reduced in this redox reaction: Cl₂ + 2Br⁻ → 2Cl⁻ + Br₂?
(A) Br
(B) Cl
(C) Cl₂
(D) Br₂
Answer
(B) — Chlorine is reduced from 0 in Cl₂ to –1 in Cl⁻.
15. Which measurement determines the spontaneity of a redox reaction?
(A) ΔH
(B) Keq
(C) E°cell
(D) Molarity
Answer
(C) — The standard cell potential E°cell is used to assess spontaneity.
16. What is the oxidation number of sulfur in H₂SO₄?
(A) +4
(B) +6
(C) –2
(D) +2
Answer
(B) — 2(+1) + x + 4(–2) = 0 → x = +6.
17. What component is reduced in the following half-reaction: MnO₄⁻ → Mn²⁺?
(A) Mn
(B) O
(C) H
(D) The entire molecule
Answer
(A) — Manganese changes from +7 to +2, indicating reduction.
18. What is the purpose of using a standard reduction potential table?
(A) To measure pH
(B) To determine spontaneity of redox reactions
(C) To calculate molarity
(D) To assign oxidation numbers
Answer
(B) — The table helps compare tendencies of reduction and cell voltage.
19. What happens to the mass of the anode in a voltaic cell over time?
(A) It increases
(B) It decreases
(C) It remains the same
(D) It becomes radioactive
Answer
(B) — The anode undergoes oxidation and loses mass as it dissolves into the solution.
20. Which is the strongest oxidizing agent?
(A) H₂
(B) Na
(C) F₂
(D) Cl⁻
Answer
(C) — Fluorine has the highest standard reduction potential and is the strongest oxidizer.
21. What factor does not affect the cell potential?
(A) Concentration of ions
(B) Temperature
(C) Surface area of electrodes
(D) Identity of electrodes
Answer
(C) — Surface area affects rate, not cell potential directly.
22. Which of the following processes is nonspontaneous and requires electrical energy?
(A) Voltaic cell
(B) Electrolysis
(C) Precipitation
(D) Combustion
Answer
(B) — Electrolysis drives nonspontaneous reactions using electricity.
23. What type of reaction occurs in a rechargeable battery when it is charging?
(A) Spontaneous redox
(B) Oxidation at cathode
(C) Electrolytic reaction
(D) Combustion reaction
Answer
(C) — Charging requires electrical input and thus is electrolytic.
24. What is the standard condition for electrode potentials?
(A) 100°C, 2 atm
(B) 25°C, 1 atm, 1 M
(C) 0°C, 1 atm, 1 M
(D) 37°C, 2 M
Answer
(B) — Standard conditions are 25°C, 1 atm, and 1 M concentration.
25. In a redox reaction, what is the agent that causes reduction called?
(A) Oxidizing agent
(B) Reducing agent
(C) Catalyst
(D) Spectator ion
Answer
(B) — The reducing agent donates electrons and is oxidized itself.
26. A galvanic cell is constructed using a Zn electrode in ZnSO₄ and a Ni electrode in NiSO₄. The standard reduction potentials are:
Zn²⁺ + 2e⁻ → Zn(s), E° = –0.76 V
Ni²⁺ + 2e⁻ → Ni(s), E° = –0.23 V
What is the standard cell potential?
(A) +1.00 V
(B) +0.53 V
(C) +0.23 V
(D) –0.53 V
Answer
(B) — E°cell = E°cathode – E°anode = (–0.23) – (–0.76) = +0.53 V
27. A nonspontaneous redox reaction can be driven using:
(A) A salt bridge
(B) A galvanic cell
(C) External electricity
(D) A spontaneous oxidizing agent
Answer
(C) — Electrolytic cells apply electrical energy to drive nonspontaneous reactions.
28. Which change increases the voltage of a voltaic cell based on the Nernst equation?
(A) Decreasing the temperature
(B) Increasing ion concentration in the cathode compartment
(C) Diluting the anode compartment
(D) Adding inert gas to the cell
Answer
(B) — Increasing [products] in cathode favors the forward reaction, increasing voltage.
29. Which of the following best describes the role of a platinum electrode in a redox cell?
(A) Acts as the reducing agent
(B) Serves as a nonreactive surface for electron transfer
(C) Precipitates ions
(D) Provides additional protons
Answer
(B) — Platinum is inert and used when no solid metal is involved in the half-reaction.
30. Which statement correctly applies the Nernst equation?
(A) E = E° – (0.059/n)logQ
(B) E = E° + RTQ
(C) E = E° – nFlogK
(D) E = Q – RT/F
Answer
(A) — This is the simplified form of the Nernst equation at 25°C.
31. In electrolysis of molten NaCl, which of the following occurs?
(A) Chlorine is reduced at the cathode
(B) Sodium is oxidized at the anode
(C) Sodium ions are reduced at the cathode
(D) Water is split into H₂ and O₂
Answer
(C) — Na⁺ gains electrons at the cathode: Na⁺ + e⁻ → Na(s).
32. A voltaic cell produces 0.75 V under nonstandard conditions, but E°cell is 1.10 V. What does this imply about the reaction quotient Q?
(A) Q < 1
(B) Q = 1
(C) Q > 1
(D) Reaction is at equilibrium
Answer
(C) — A lower Ecell than E° means more products than reactants; Q > 1.
33. What is the limiting factor in the total amount of product formed in an electrolysis reaction?
(A) Temperature
(B) Cell voltage
(C) Amount of current and time
(D) Type of salt bridge
Answer
(C) — Faraday's Law: amount of product ∝ current × time.
34. The half-reaction: 2H⁺ + 2e⁻ → H₂(g) is used in electrochemistry because:
(A) H⁺ is always oxidized
(B) It is highly exothermic
(C) It provides a universal reference with E° = 0 V
(D) It occurs at all temperatures
Answer
(C) — The SHE is the reference point for all standard potentials.
35. In a redox titration involving KMnO₄ and Fe²⁺, which is acting as the oxidizing agent?
(A) Fe²⁺
(B) Fe³⁺
(C) Mn²⁺
(D) MnO₄⁻
Answer
(D) — Permanganate ion (MnO₄⁻) is reduced, so it acts as the oxidizing agent.
36. Define oxidation in terms of electron transfer.
Answer
Oxidation is the loss of electrons by a substance — This results in an increase in oxidation number.
37. What is the function of the salt bridge in an electrochemical cell?
Answer
It allows ions to migrate and maintain electrical neutrality — Without it, the reaction would stop due to charge buildup.
38. Explain why a metal with a lower reduction potential is more likely to act as the anode.
Answer
It more readily loses electrons (oxidized) — Anodes are where oxidation occurs, and lower reduction potential means it’s less likely to gain electrons.
39. Describe the electron flow in a voltaic cell.
Answer
Electrons flow from anode to cathode through an external wire — This flow powers any connected device.
40. How can the Nernst equation predict changes in cell potential under nonstandard conditions?
Answer
By incorporating concentrations (reaction quotient Q) — The equation shows how Ecell varies with Q and temperature.
41. Why is the standard hydrogen electrode assigned a potential of 0.00 V?
Answer
It serves as a reference point for all other electrode potentials — This allows comparison across cells.
42. A solution contains both Cu²⁺ and Zn²⁺. Which metal will be plated first when current is applied and why?
Answer
Copper (Cu²⁺) will plate first — It has a higher (more positive) reduction potential than zinc.
43. In a redox reaction, what happens to the oxidizing agent?
Answer
It is reduced by gaining electrons — That’s how it causes another substance to be oxidized.
44. What physical property is measured when determining standard reduction potential?
Answer
Voltage (in volts) — It reflects the tendency of a substance to gain electrons under standard conditions.
45. Explain how increasing the concentration of a reactant in a voltaic cell affects the cell potential.
Answer
It increases cell potential — More reactants drive the forward reaction, boosting Ecell according to the Nernst equation.
46. What is the balanced half-reaction for the reduction of MnO₄⁻ in acidic solution?
Answer
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O
47. Why do electrolytic cells require a power source?
Answer
Because they drive nonspontaneous reactions — External energy is needed to force the reaction to proceed.
48. Identify the anode and cathode in the electrolysis of molten MgCl₂.
Answer
Anode: Cl⁻ → Cl₂; Cathode: Mg²⁺ → Mg — Oxidation of chloride and reduction of magnesium occur.
49. How many moles of electrons are needed to deposit 2 mol of Cu from Cu²⁺?
Answer
4 moles of electrons — Each Cu²⁺ requires 2 electrons for reduction.
50. What is the relationship between Gibbs free energy (ΔG) and cell potential (Ecell)?
Answer
ΔG = –nFEcell — This equation links free energy change with the electrochemical cell’s voltage.