Electrochemical Cell

Galvanic cell or Voltaic cell or electrochemical cell converts the chemical energy of a spontaneous redox reaction into electrical energy.

In the Galvanic cell, the Gibbs energy of the spontaneous redox reaction is converted into electrical work which may be used for running motor or other electrical gadgets like heater, fan, geyser, etc.

Difference between an electrolytic cell and a galvanic cell

Electrochemical cell (Galvanic Cell)	Electrolytic cell
A Galvanic cell converts chemical energy into electrical energy.	An electrolytic cell converts electrical energy into chemical energy.
The redox reaction is spontaneous and is responsible for the production of electrical energy.	The redox reaction is not spontaneous and electrical energy has to be supplied to initiate the reaction.
The two half-cells are set up in different containers, being connected through the salt bridge or porous partition.	Both the electrodes are placed in a same container in the solution or molten electrolyte.
The anode is negative and cathode is positive electrode. The reaction at the anode is oxidation and that at the cathode is reduction.	The anode is positive and cathode is the negative electrode. The reaction at the anode is oxidation and that at the cathode is reduction.
The electrons are supplied by the species getting oxidized. They move from anode to the cathode in the external circuit.	The external battery supplies the electrons. They enter through the cathode and come out through the anode.

Daniel cell

An electrochemical cell using zinc and copper metals as electrodes, is known as Daniell cell.

Zn(s) + Cu²⁺⁽aq) → Zn²⁺(aq) + Cu(s)

It has an electrical potential equal to 1.1V when concentration of Zn²⁺ and Cu²⁺ ions is unity (1 mol dm^–3).

If an external opposite potential is applied in the galvanic cell and increased slowly, the reaction continues till the opposing voltage reaches the value 1.1V. The reaction stops altogether when the external opposing voltage reaches 1.1 Volt and no current flows through the cell.

Any further increase in the external potential again starts the reaction but in the opposite direction. It now functions as an electrolytic cell, a device for using electrical energy to carry non-spontaneous chemical reactions.

When E_ext < 1.1 V

i. Electrons flow from Zn rod to Cu rod hence current flows from Cu to Zn.

ii. Zn dissolves at anode and copper deposits at cathode.

When E_ext = 1.1 V

i. No flow of electrons or current.

ii. No chemical reaction.

When E_ext > 1.1 V

i. Electrons flow from Cu to Zn and current flows from Zn to Cu.

ii. Zinc is deposited at the zinc electrode and copper dissolves at copper electrode

Cell half reactions

The reaction is a combination of two half reactions whose addition gives the overall cell reaction:

i. Cu²⁺ + 2e^– → Cu(s) (reduction half reaction)

ii. Zn(s) → Zn²⁺ + 2e^– (oxidation half reaction)

The reduction half reaction occurs on the copper electrode while the oxidation half reaction occurs on the zinc electrode.

These two portions of the cell are called half-cells or redox couples.

The copper electrode is called the reduction half-cell and the zinc electrode, the oxidation half-cell.