AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/9/9-8)
Unit 9 · Topic 9.8
9.8 Galvanic (Voltaic) and Electrolytic Cells
A galvanic (voltaic) cell uses a thermodynamically favored redox reaction to produce electricity, while an electrolytic cell uses outside electrical energy to drive an unfavored one. In both, oxidation happens at the anode and reduction at the cathode, electrons travel through the wire, and ions move through the solutions and salt bridge to keep charges balanced.
Key terms
- galvanic (voltaic) cell
- electrolytic cell
- anode
- cathode
- salt bridge
- oxidation and reduction
Parts of a galvanic cell
A classic galvanic cell has two half-cells. In one, a zinc strip sits in a Zn²⁺ solution; in the other, a copper strip sits in a Cu²⁺ solution. A wire (often through a voltmeter) connects the two metal strips, which are the electrodes. A salt bridge, a tube of an unreactive salt solution like KNO₃, connects the two solutions.
Keeping the half-reactions apart forces the electrons to travel through the wire, where they can do useful work, instead of transferring directly from Zn atoms to Cu²⁺ ions.
What happens at each electrode
Anode: oxidation. Zn(s) → Zn²⁺(aq) + 2e⁻. The zinc electrode loses mass as zinc atoms become ions in solution.
Cathode: reduction. Cu²⁺(aq) + 2e⁻ → Cu(s). Copper metal deposits, so the copper electrode gains mass, and the blue color of Cu²⁺ fades.
Overall: Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). A memory aid: 'An Ox' (anode, oxidation) and 'Red Cat' (reduction, cathode). This is true for every electrochemical cell, galvanic or electrolytic.
Electron and ion flow
- Electrons flow through the wire from the anode to the cathode. They never travel through the solutions or the salt bridge.
- In the anode compartment, Zn²⁺ ions build up, adding positive charge. Anions from the salt bridge (like NO₃⁻) flow toward the anode to balance it.
- In the cathode compartment, Cu²⁺ ions are used up. Cations from the salt bridge (like K⁺) flow toward the cathode to replace the lost positive charge.
- Without a salt bridge, charge builds up in each half-cell almost at once and the current stops.
Electrolytic cells
An electrolytic cell uses a power source, such as a battery, to push electrons in the unfavored direction. The power source pulls electrons from the anode (where oxidation is forced to happen) and pushes them into the cathode (where reduction is forced).
Example: electrolysis of molten NaCl. At the cathode, Na⁺ + e⁻ → Na(l). At the anode, 2Cl⁻ → Cl₂(g) + 2e⁻. Bubbles of chlorine gas form at the anode. Electroplating, refining copper and recharging a battery are all electrolytic processes.
Labeling electrodes as positive or negative isn't tested; focus on anode/oxidation and cathode/reduction.
| Feature | Galvanic cell | Electrolytic cell |
|---|---|---|
| Reaction | Thermodynamically favored (ΔG° < 0) | Unfavored (ΔG° > 0), driven by outside energy |
| Energy | Chemical → electrical | Electrical → chemical |
| Cell potential | Positive E°cell | Negative E°cell; the applied voltage must be larger than its size |
| Anode | Oxidation | Oxidation |
| Cathode | Reduction | Reduction |
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Describing a galvanic cell
A galvanic cell is built from a Ni electrode in 1.0 M Ni²⁺ and a Ag electrode in 1.0 M Ag⁺, with a KNO₃ salt bridge. The overall reaction is Ni(s) + 2Ag⁺(aq) → Ni²⁺(aq) + 2Ag(s). Identify the anode and cathode, the direction of electron flow, how the electrode masses change, and which way K⁺ moves in the salt bridge.
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- Step 1: Ni is oxidized (Ni → Ni²⁺ + 2e⁻), so the Ni electrode is the anode.
- Step 2: Ag⁺ is reduced (Ag⁺ + e⁻ → Ag), so the Ag electrode is the cathode.
- Step 3: Electrons flow through the wire from the Ni anode to the Ag cathode.
- Step 4: The Ni electrode loses mass (atoms become ions); the Ag electrode gains mass (silver deposits).
- Step 5: Ag⁺ is used up in the cathode compartment, so K⁺ moves from the salt bridge toward the Ag (cathode) compartment, and NO₃⁻ moves toward the Ni (anode) compartment.
Answer: Anode: Ni; cathode: Ag; electrons flow Ni → Ag through the wire; Ni loses mass and Ag gains mass; K⁺ moves toward the Ag half-cell.
- Example 2
Trap: electrons in the salt bridge
A student's diagram of the Zn/Cu cell shows electrons flowing from the Zn electrode, through the wire to the Cu electrode, and then back through the salt bridge to complete the loop. What's wrong, and what actually completes the circuit?
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- Step 1: Electrons can't travel through an aqueous solution or salt bridge. They move only through the metal wire and electrodes.
- Step 2: Inside the cell, the circuit is completed by moving ions. Anions in the salt bridge move toward the anode (the Zn half-cell, where Zn²⁺ is building up), and cations move toward the cathode (the Cu half-cell, where Cu²⁺ is being used up).
- Step 3: So charge flows around the whole loop, carried by electrons in the wire and by ions in the solutions.
Answer: Electrons never go through the salt bridge; ion migration through the salt bridge and solutions completes the circuit.
Common mistakes
- Swapping anode and cathode. Oxidation is always at the anode, in both galvanic and electrolytic cells.
- Drawing electrons moving through solutions or the salt bridge.
- Sending salt-bridge ions the wrong way. Anions move toward the anode; cations move toward the cathode.
- Saying the cathode loses mass in a galvanic cell. Metal plates out at the cathode, so it gains mass.
On the exam
- Cell-diagram questions ask you to label the anode and cathode, show electron flow, show ion movement in the salt bridge, and predict mass changes or gas formation. Use the reaction to decide what is oxidized and reduced.
- Experimental questions may ask what happens if the salt bridge is removed (the current stops) or a different electrode is used.
Connected topics
Videos
Check yourself
4 questions on 9.8 Galvanic (Voltaic) and Electrolytic Cells. Pick an answer to see if you got it, and why.
Molten NaCl is electrolyzed to produce sodium metal and chlorine gas, a process with a large positive ΔG°. Which of the following correctly describes the cell?
A spoon is silver-plated in an electrolytic cell containing AgNO₃(aq), with a silver bar as the other electrode. Which of the following correctly describes the spoon?
A galvanic cell is built with a zinc electrode in 1.0 M Zn(NO₃)₂ and a copper electrode in 1.0 M Cu(NO₃)₂, at 25 °C. The electrodes are joined by a wire, and the solutions are joined by a salt bridge containing KNO₃(aq).
Cu²⁺(aq) + 2e⁻ → Cu(s) E° = +0.34 V
Zn²⁺(aq) + 2e⁻ → Zn(s) E° = −0.76 V
Described cell and standard reduction potentials
Which of the following correctly describes the cell as it operates?
As the cell operates, which of the following happens in the salt bridge?
0 of 4 answered