Lesson 1 of 12 · 9 min
Galvanic and electrolytic cells
NCERT §2.1
Two rods, two beakers and a strip of salt-soaked gel, and the voltmeter reads 1.13 V. Where does that push come from, and what happens when a stronger supply pushes back?
The lesson in notes
In short
An electrochemical cell couples a redox reaction to an electric circuit. In a galvanic (voltaic) cell a spontaneous reaction produces electrical energy; in an electrolytic cell electrical energy from outside forces a non-spontaneous reaction.
The Daniell cell runs Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s). With both ions at 1 mol dm⁻³ its potential is 1.1 V.
Connect an external source that opposes the cell. While E_ext < 1.1 V, electrons still flow from the Zn rod to the Cu rod (current from Cu to Zn), zinc dissolves and copper deposits.
At E_ext = 1.1 V exactly, no current flows and the reaction stops.
When E_ext > 1.1 V the flow reverses: electrons go from Cu to Zn, zinc is deposited on the zinc rod and copper dissolves. The same hardware is now an electrolytic cell.
Strictly, activities replace concentrations in these equations; in dilute solutions the two are equal.