8.1Voltaic cells

A voltaic cell, also called a galvanic cell, turns the energy of a spontaneous redox reaction into electrical energy.

The two halves of the reaction happen in separate beakers. Electrons leave one half and are taken up by the other, and the only path between them is a wire. That flow of electrons through the wire is the current.

• Oxidation is the loss of electrons. It happens at the anode: metal atoms give up electrons and leave as ions. Zn → Zn²⁺ + 2e⁻. Over time the anode wears away.
• Reduction is the gain of electrons. It happens at the cathode: ions from the solution take electrons and settle on it as metal. Cu²⁺ + 2e⁻ → Cu. Over time the cathode builds up, and a blue copper solution fades.
• Electrons always travel through the wire from the anode to the cathode.

As the cell runs, the anode’s beaker gains positive ions and the cathode’s beaker loses them. Left alone, that charge would build up and stop the reaction within moments.

The salt bridge joins the two beakers with a solution of ions that do not react. Its negative ions drift toward the anode and its positive ions toward the cathode, so both solutions stay neutral and the current keeps flowing.

Every half-reaction has a standard reduction potential, E°: how strongly it pulls electrons in, measured against the hydrogen electrode at 0.00 V. In a cell, the half with the higher E° is reduced and becomes the cathode.

E°_cell = E°_cathode − E°_anode

A copper and zinc cell: 0.34 − (−0.76) = 1.10 V. Copper is reduced, zinc is oxidized.

The reference half-cell is platinum in hydrochloric acid with hydrogen gas bubbled over it. Platinum carries the electrons and does not react. Paired with a metal that pulls electrons harder, such as copper, hydrogen gas is oxidized to H⁺. Paired with one that pulls less, such as zinc, H⁺ is reduced and hydrogen bubbles form on the platinum.