Daniell Electrochemical Cell (Galvanic Cell) and the Gravity Galvanic Cell

One version of a historical Daniell cell uses gravity to separate two aqueous solutions in one beaker.  A copper electrode is placed at the bottom of a beaker and immersed in the 2.0 M copper(II) sulfate solution.  A zinc electrode is clipped near the top portion of the beaker and 0.80 M zinc sulfate solution is slowly and carefully added to the top of the copper(II) sulfate solution until the solution covers the bottom half of the zinc electrode. A voltmeter is used to measure the emf of the cell.

The zinc electrode serves as the anode - the site of the oxidation half-reaction. The zinc atoms at the surface of the metal electrode are oxidized to Zn²⁺ ions, which go into solution. The copper electrode serves as the cathode, the site of the reduction half-reaction. The Cu²⁺ ions in solution migrate toward the surface of the copper electrode and receive electrons from the copper electrode and plate copper atoms onto the copper electrode.

In the original version of the Daniell cell a porous cup containing sulfuric acid and a zinc electrode is placed in a copper container filled with 1.0 M copper(II) sulfate solution.

The two half-reactions are:

Zn(s) –> Zn²⁺(aq) + 2e^-        (oxidation)

Cu²⁺(aq) + 2e^- –> Cu(s)        (reduction)

The net ionic equation for the galvanic cell is:

Zn(s) + Cu²⁺(aq) –> Zn²⁺(aq)+ Cu(s)  E_cell = +1.0 V

Electrons produced at the zinc anode bump electrons in the copper wire in the external circuit to the copper cathode.  Cations and anions in solution migrate in opposite directions, Cu²⁺ ions towards the copper electrode, sulfate ions toward the zinc electrode. The combined simultaneous movement of electrons in the wire and ions in solution creates an electric current and completes an electrical circuit. 

This demonstration helps students learn about the historical development of batteries, shows the necessary factor of allowing ions to migrate in the solution of the cell  in order to create a complete circuit, and shows students that the salt bridge discussed in textbooks and in lecture is part of the development of electrochemical cells - but the function of the salt-bridge is to provide for efficient on migration.