Batteries: The Galvanic Cell
Separate the two halves of a redox reaction and force the electrons through a wire — that detour is every battery ever made.
Builds on: 6.1 Redox: Electron Bookkeeping
The trick: separate the halves
In the beaker, zinc handed Cu²⁺ its electrons directly — energy released as useless heat. A galvanic cell splits the reaction into two rooms: zinc in one beaker, copper ions in another, the metals joined by a wire and the solutions by a salt bridge (which lets ions drift across to keep both sides electrically neutral). Zinc still insists on shedding electrons — but now the only route to the waiting Cu²⁺ is through the wire. An electron current you can harvest: chemistry become electricity.
Every half-reaction has a measured standard potential E° — the activity series with numbers attached (Zn: −0.76 V, Cu: +0.34 V, Ag: +0.80 V). The voltage of a cell is simply the difference: Zn/Cu gives 1.10 V. Want more? Pick metals farther apart, or stack cells in series — a 9 V block battery is literally six 1.5 V cells in a trench coat.
Anode, cathode, and the naming fog
The electrode being oxidized (zinc, dissolving away) is the anode — the battery’s − terminal. The electrode where reduction happens (copper, growing plating) is the cathode, the + terminal. Electrons flow anode → cathode through the outside wire. Every battery you own — from the AA in a remote to the lithium cell in your phone — is this same architecture with fancier chemistry: two half-reactions of different eagerness, separated, and taxed at the wire.
This is where the two courses shake hands: the voltage source you treated as a given in every circuit — the “charge pump” — is a redox reaction held apart. The pump’s pressure is E°cell; the pump runs down when a reactant runs out. And internal resistance? Mostly the sluggishness of ions crossing the electrolyte — as your lemon will demonstrate, pointedly, in the capstone.
⚗️ Lab — Build-a-Battery
Two half-cells, a salt bridge, a lamp — and your choice of electrodes.
- Classic Zn/Cu: confirm 1.10 V. Then chase the maximum — which pair wins?
- Pick the same metal twice. Why exactly does the lamp stay dark?
- Swap anode and cathode into a negative voltage — which way would electrons actually flow?