Galvanic cells and electrode potential

The essentials

A galvanic (voltaic) cell converts the chemical energy of a spontaneous redox reaction into electrical energy. In the Daniell cell, zinc is oxidised at the anode (Zn → Zn²⁺ + 2e⁻) and copper ions are reduced at the cathode (Cu²⁺ + 2e⁻ → Cu). The two half-cells are joined by a salt bridge, which completes the circuit and keeps the solutions electrically neutral.

Key points

• In a galvanic cell the anode is negative and the cathode is positive; electrons flow from anode to cathode in the external wire.
• Cell notation: anode on the left, cathode on the right, e.g. Zn(s) | Zn²⁺(aq) || Cu²⁺(aq) | Cu(s).
• The standard hydrogen electrode (SHE) is assigned a potential of zero at all temperatures. It uses H₂ gas at 1 bar and H⁺ at unit concentration on platinum.
• Standard electrode potentials are written as reduction potentials. A more positive E° means a greater tendency to be reduced (a stronger oxidising agent); a more negative E° means a stronger reducing agent.
• E°cell = E°cathode − E°anode (both as reduction potentials). For the Daniell cell: 0.34 − (−0.76) = 1.10 V.
• A positive E°cell means the cell reaction is spontaneous as written.
• Electrode potential is an intensive property, so it is not multiplied when the half-equation is multiplied.

Exam tips

• Lithium has the most negative standard reduction potential among common elements and is a powerful reducing agent; fluorine has the most positive and is the strongest oxidising agent.
• A metal with a more negative E° can displace a metal with a more positive E° from its salt solution.

Common mistakes

• Multiplying E° values by stoichiometric coefficients.
• Calling the anode positive in a galvanic cell.