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17.2 Galvanic Cells
It is often convenient to separate oxidation-reduction reactions into half-reactions in order to balance the equation and emphasize the actual chemical transformations.
When a clean piece of copper metal is placed in a solution of silver nitrate, silver metal begins to form and copper ion pass into the solution.
The blue color of the solution shows the presence of copper.
The reaction may be split into two parts.
Each reaction can be considered individually, because half-reactions separate oxidation from the reduction.
An oxidation reduction reaction occurs when a piece of copper is placed into a solution of silver nitrate.
As the reaction proceeds, the solution becomes blue because of the copper ion present, and silver metal is deposited on the copper strip as the silver ion are removed from solution.
There is a metal in this picture.
The anode is connected to a wire and the other terminal is connected to a silver electrode.
There is no current flow at this point because the circuit is open.
The circuit is closed using a bridge.
The salt bridge consists of a concentrated, nonreactive, electrolyte solution.
When electrons flow from left to right through the wire, they pass through a porous plug on the left into the copper nitrate solution.
The beaker on the left is neutral by the charge on the copper(II) ion that are produced in the solution.
The added cations keep the beaker on the neutral side.
Without the bridge, the compartments wouldn't stay neutral and the current wouldn't flow.
There is no need for a salt bridge if the two compartments are in direct contact.
The energy per unit charge available from the oxidation-reduction reaction is a measure of the cell potential.
A is the current in amperes and C is the charge in coulombs.
The energy in joules can be obtained if the charge in coulombs is multiplied by the volts.
In a standard galvanic cell, the half-cells are separated and the electrons can flow through an external wire.
There is a lot going on in this system so it is useful to summarize things.
oxidation occurs in the left half-cell and the anode is in the left half-cell.
The right half-cell has the cathode because reduction occurs here.
The left half-cell isOxidation in the figure.
The right half-cell is where the reduction occurs.
The inherent differences in the nature of the materials used to make the two half-cells lead to the cell potential being +0.46 V.
The salt bridge must be present to close the circuit and both an oxidation and reduction must occur for the current to flow.
A shorthand notation is used to describe galvanic cells.
It works for other types of cells as well.
A double line and a phase boundary are marked by a vertical line.
Information about the anode is written to the left, followed by the solution, then the bridge, and finally the solution to the right.
The initial concentrations of the various ion are usually included.
The Daniell cell is one of the least complicated cells.
It is possible to make a battery by placing a copper electrode at the bottom of a jar and covering the metal with a copper sulfate solution.
A zinc sulfate solution is floated on top of a copper sulfate solution and a zinc electrode is placed in the zinc sulfate solution.
An electric current can flow if the copper and zinc are connected.
An example of a cell without a salt bridge is this one.
Some oxidation-reduction reactions involve species that are poor conductors of electricity, and so an electrode is used that does not participate in the reactions.
There are many chemical reactions in which the gold, Platinum, and Graphite are not present.
At the anode on the left, magnesium undergoes oxidation, while hydrogen ion reduction takes place on the right.
Platinum and gold are generally unreactive.
The oxidation and reduction half-reactions can be written using cell notation.
When three electrons are lost to form Cu, Cu is reduced as it gains two electrons to form Cu.
Oxidation occurs at the anode.
When one electron is lost to form Fe3+, and five electrons are gained to form Mn2+, it undergoes oxidation.
Oxidation occurs at the anode.
The galvanic cell is where copper and zinc are reduced to copper and zinc, respectively.
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