Questions: Galvanic Cells and Spontaneous Redox Reactions

Question 1

In a galvanic cell, at which electrode does oxidation occur, and what does that electrode do in the external circuit?

• A. The cathode; it supplies electrons to the external circuit
• B. The anode; it supplies electrons to the external circuit
• C. The cathode; it draws electrons from the external circuit
• D. The anode; it draws electrons from the external circuit

Answer: (B)

Oxidation (loss of electrons) occurs at the anode. The metal electrode dissolves as ions, releasing electrons into the electrode itself, which then flow through the external wire toward the cathode. The mnemonic 'anode = oxidation' works because both start with vowels. At the cathode, reduction occurs: ions from solution gain the arriving electrons and deposit as solid metal. The anode is the electron source; the cathode is the electron sink.

Question 2

A student builds a zinc-copper galvanic cell but omits the salt bridge. What most likely happens?

• A. The cell works normally; the salt bridge only stabilizes long-term operation
• B. The cell produces double the voltage because all current is forced through the external circuit
• C. The cell quickly stops producing current because charge imbalances in the half-cells halt the reaction
• D. The cell produces lower voltage indefinitely without the salt bridge

Answer: (C)

As oxidation proceeds in the anode compartment, Zn²⁺ ions accumulate, making that solution increasingly positive. In the cathode compartment, Cu²⁺ ions are consumed, leaving an excess of negative ions. These charge imbalances create an electrostatic force opposing further electron flow. Without a salt bridge to allow ion migration between compartments to restore neutrality, the cell stops almost immediately. The salt bridge is not optional — it is essential for sustained current.

Question 3

In a galvanic cell, the electrode with the higher standard reduction potential serves as the anode where oxidation occurs.

• T. True
• F. False

Answer: False

It is the opposite: the electrode with the higher standard reduction potential serves as the cathode, where reduction occurs. The driving force of a galvanic cell comes from the spontaneous tendency of the higher-reduction-potential half-reaction to proceed as reduction and the lower one to proceed in reverse (as oxidation). E°cell = E°cathode − E°anode is always positive for a spontaneous galvanic cell, which requires E°cathode > E°anode.

Question 4

A positive standard cell potential (E°cell > 0) indicates that the overall redox reaction is spontaneous as written.

• T. True
• F. False

Answer: True

E°cell = E°cathode − E°anode. A positive value means the system releases free energy when current flows — this is the hallmark of a spontaneous process. Through thermodynamics, ΔG° = −nFE°cell: when E°cell > 0, ΔG° < 0, confirming spontaneity. Galvanic cells harness this spontaneous electron flow to do electrical work. An electrolytic cell, by contrast, requires external energy input to force a non-spontaneous reaction (E°cell < 0).

Question 5

Explain the role of the salt bridge in a galvanic cell. What specific problem does it solve, and what would happen if it were removed?

Model answer: As oxidation proceeds at the anode, positive ions (e.g., Zn²⁺) accumulate in the anode half-cell, building up a positive charge. Simultaneously, positive ions (e.g., Cu²⁺) are consumed at the cathode half-cell, leaving a net negative charge. Without a way to neutralize these growing charge imbalances, the electrostatic repulsion would quickly prevent further electron flow through the external wire — the cell would stop producing current almost immediately. The salt bridge solves this by allowing ions to migrate between compartments: anions flow toward the anode to neutralize the accumulating positive charge, and cations flow toward the cathode. This ion flow maintains electrical neutrality in both half-cells, allowing the spontaneous redox reaction to continue.

The salt bridge completes the electrical circuit on the ion side. Electrons flow through the external wire (electronic current), and ions flow through the salt bridge (ionic current). Both pathways are required simultaneously for sustained operation. A porous membrane can serve the same function. Without either, a galvanic cell is fundamentally broken regardless of how well-matched the half-reactions are.