http://www.ck12.org Chapter 23. Electrochemistry
TABLE23.1:(continued)
Element Oxidation Half Reaction
Least active or most difficult to oxi-
dizeGold Au(s)→Au^3 +(aq) + 3e−Notice that zinc is listed above copper on the activity series, which means that zinc is more easily oxidized than
copper. That is why copper(II) ions can act as an oxidizing agent when put into contact with zinc metal. Ions of any
metal that is below zinc, such as lead or silver, would oxidize the zinc in a similar reaction. However, no reaction
will occur if a strip of copper metal is placed into a solution of zinc ions, because the zinc ions are not able to oxidize
the copper. In other words, such a reaction is nonspontaneous.
Cu(s) + Zn^2 +(aq)→NRThe reaction of zinc metal with copper(II) ions described above is called a direct redox process or reaction. The
electrons that are transferred in the reaction go directly from the Zn atoms on the surface of the strip to the Cu^2 +
ions that are in the solution adjacent to the zinc strip. In this case, no electricity is generated. Electricity requires
the passage of electrons through a conducting medium, such as a wire, in order to do work. This work could be
used to light a light bulb, power a refrigerator, or heat a house. When the redox reaction is direct, those electrons
cannot be made to do work. Instead, we must separate the oxidation process from the reduction process and force the
electrons to travel from one place to another in order for the reaction to proceed. That is the key to the structure of
the electrochemical cell. Anelectrochemical cellisany device that converts chemical energy into electrical energy
or electrical energy into chemical energy.
Voltaic Cells
In 1800, Italian physicist, Alessandro Volta (1745-1827), constructed the first electrochemical cell that was able to
generate a direct current (DC). Avoltaic cellisan electrochemical cell that uses a spontaneous redox reaction to
produce electrical energy. There are other types of electrochemical cells that use an external source of electricity to
drive an otherwise nonspontaneous reaction. You will learn about these in a later lesson. The figure below (Figure
23.1) shows a diagram of a voltaic cell.
The voltaic cell consists of two separate compartments. Ahalf-cellisone part of a voltaic cell in which either the
oxidation or reduction half-reaction takes place. The half-cell on the left consists of a strip of zinc metal immersed
in a solution of zinc nitrate. The half-cell on the right consists of a strip of copper metal immersed in a solution of
copper(II) nitrate. The strips of metal are called electrodes. Anelectrodeisa conductor in a circuit that is used to
carry electrons to a nonmetallic part of the circuit. The nonmetallic part of the circuit is the electrolytic solutions
in which the electrodes are placed. A metal wire connects the two electrodes to one another. In the above figure,
that wire is equipped with a switch to open or close the circuit, and a voltmeter to measure the electrical potential
generated by the cell. The half-cells are also connected by a salt bridge, the u-shaped tube in the figure. Asalt
bridgeisa tube containing an inert electrolyte that allows the passage of ions between the two half-cells. Without
the salt bridge, the voltaic cell will not function because the circuit will not be complete. The inert electrolyte in the
salt bridge is often potassium chloride (KCl) or sodium nitrate (NaNO 3 ).
The various electrochemical processes that take place in a voltaic cell occur simultaneously. It is easiest to describe
them in the following steps, using the above zinc-copper cell as an example.
- Zinc atoms from the zinc electrode are oxidized to zinc ions. This happens because zinc is higher than copper
on the activity series and, therefore, is more easily oxidized.The electrode at which oxidation occursis called
theanode. The zinc anode gradually diminishes as the cell operates because zinc metal is being consumed by