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Galvanic cells, also called voltaic cells are basically batteries.
If a shiny piece of copper is placed into a solution of silver nitrate, a spontaneous reaction occurs.
A grayish white silver deposit if formed on the copper and the solution turns blue because of copper (II) nitrate.
2Ag+ + Cu ==> Cu2+ + 2Ag
No usable energy could be harnessed from this reaction because it is dissipated as heat.
The same chemical reaction can occur and produce electricity if it was placed in a galvanic cell.
A galvanic cell consists of two containers with a salt bridge between them.
The two containers each store the half-reactions of the equation above.
reduction: Ag+ + e- ==> Ag
oxidation: Cu ==> Cu2+ + 2e-
This works because the solutions in both compartments, or half-cells, remain electrically neutral.
The salt bridge permits ions to enter or leave the solutions.
The electron flow from the anode to the cathode is what creates electricity.
In a galvanic cell, the cathode is positive while the anode is negative, while in a electrolytic cell, the cathode is negative while the anode is positive.
How much electricity is made by the reaction above?
To find that out we need to find the reduction potential of the substances reduced and oxidized.
The electricity produced by the cell (E°cell) in volts is equal to the difference of the standard reduction potential of the substance reduced and the substance oxidized.
A table of standard reduction potentials is needed.