Galvanic (Voltaic) Cells and Half-Cells

A galvanic (voltaic) cell uses a spontaneous redox reaction to produce electricity: oxidation takes place at the anode (negative electrode), reduction at the cathode (positive electrode), electrons flow through the external wire from the anode to the cathode and a salt bridge completes the circuit. In brief: in the Daniell cell, zinc oxidizes (Zn → Zn²⁺ + 2e⁻) at the anode, copper(II) is reduced (Cu²⁺ + 2e⁻ → Cu) at the cathode, and the standard cell potential is 1.10 V.

Every battery is made of one or more galvanic cells. To understand it, you need to know each part of the cell and the direction in which electrons and ions move. This article describes the components of a galvanic cell, explains each half-cell, gives a table of the signs and charge flows, and works through the calculation of the mass changes at the electrodes.

What are the parts of a galvanic cell?

• Two half-cells: each consists of an electrode (a conductor) in contact with an electrolyte solution that contains the ions of the redox couple.
• The anode: the electrode where oxidation takes place. In a galvanic cell it is the negative electrode, since the electrons it releases accumulate there.
• The cathode: the electrode where reduction takes place. It is the positive electrode.
• An external circuit: a wire (and a load such as a lamp or voltmeter) through which electrons flow from the anode to the cathode.
• A salt bridge or porous separator: connects the two solutions and allows ions to move.

A useful mnemonic is “An Ox and a Red Cat”: oxidation at the Anode, reduction at the Cathode.

How does the Daniell cell work?

The Daniell cell has a zinc electrode in a solution of zinc sulfate and a copper electrode in a solution of copper(II) sulfate. When they are connected:

1. At the zinc anode, zinc atoms are oxidized: Zn(s) → Zn²⁺(aq) + 2e⁻. The zinc rod gets thinner, and the Zn²⁺ concentration in the left compartment increases.
2. The electrons travel through the wire to the copper electrode.
3. At the copper cathode, Cu²⁺ ions take up the electrons: Cu²⁺(aq) + 2e⁻ → Cu(s). Copper deposits on the electrode, and the blue Cu²⁺ concentration falls.
4. To balance the charges, anions (SO₄²⁻) in the salt bridge move toward the anode compartment and cations (K⁺) toward the cathode compartment.

The overall reaction is Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s), with E°_cell = +1.10 V. The cell works until the reactants are used up or the cell reaches equilibrium, when the voltage falls to zero.

Summary of the electrodes

What does the salt bridge do?

Without a salt bridge, the cell would stop at once. As Zn²⁺ is formed, the left solution would become positively charged, and as Cu²⁺ is removed, the right solution would become negatively charged, which would oppose any further electron flow. The salt bridge, a U-tube filled with an inert electrolyte such as KNO₃ or KCl in a gel, supplies ions to neutralize the charge build-up, and it completes the electrical circuit by ion transport. The electrolyte is chosen so that its ions do not react with the half-cell solutions, and have similar mobilities. A porous ceramic barrier can be used instead of a bridge.

What if there is no metal electrode?

When a half-reaction involves only dissolved species or a gas, an inert electrode such as platinum or graphite is used to carry the electrons. For example, in the half-cell for Fe³⁺ + e⁻ → Fe²⁺ the platinum wire only supplies or collects electrons. The same is done in the standard hydrogen electrode, where H₂ gas is bubbled over a platinum surface in acid.

Worked examples

Example 1: electrode reactions in a zinc–silver cell

Zn is the more active metal. Anode: Zn → Zn²⁺ + 2e⁻. Cathode: 2Ag⁺ + 2e⁻ → 2Ag (the silver half-reaction is doubled to balance the electrons). Overall: Zn + 2Ag⁺ → Zn²⁺ + 2Ag, with E° = 0.80 − (−0.76) = 1.56 V.

Example 2: change of mass at the electrodes

A Daniell cell supplies a current for a time in which a total charge of 965 C flows through the circuit. How much does each electrode change in mass?

1. Moles of electrons: n = 965 / 96,485 = 0.0100 mol.
2. Zinc: 2 mol of electrons per mole of Zn, so 0.00500 mol of Zn dissolves, and the mass lost is 0.00500 × 65.38 = 0.327 g.
3. Copper: 0.00500 mol of Cu deposits, and the mass gained is 0.00500 × 63.55 = 0.318 g.

Example 3: direction of ion movement

In the cell of Example 1, the anode compartment accumulates Zn²⁺, so nitrate (anions) must move into it from the salt bridge. In the cathode compartment, Ag⁺ ions are removed, and cations from the bridge move in to replace the lost positive charge.

Who invented the cell?

Alessandro Volta’s pile of 1800 was the first source of a steady electric current, but it polarized quickly as hydrogen bubbles formed on the copper. John Frederic Daniell, an English chemist, developed in 1836 a cell with two electrolytes separated by a porous barrier, which gave a much steadier voltage of about 1.1 V, and it was widely used to power telegraph systems. The name galvanic cell honors Luigi Galvani and the name voltaic cell honors Volta.

Which mistakes are common?

• Mixing up the signs of the electrodes: the anode is negative in a galvanic cell, but positive in an electrolytic cell.
• Saying that electrons flow through the salt bridge: only ions do.
• Placing oxidation at the cathode: oxidation is always at the anode.
• Forgetting that the cell stops when there is no salt bridge: the circuit must be complete.
• Confusing the direction of current with that of the electrons: by convention, current flows opposite to electrons.