Chemistry: Animation, Video, and Laboratory Activity

Oxidation-reduction involve the transfer of electrons. The transfer of electrons between metals and metal ions is a common type of redox reaction. It's illustrates what happened when a strip of zinc is placed in a solution of copper(II) sulfate.

In this reaction there is a transfer of electrons from the zinc metal to the copper(II) ions. The transfers of electrons from Zn to Cu2+ takes place on the surface of the zinc strip. As the reaction proceeds the zinc metal dissolves and copper metal is formed. As the Cu2+ ions are reduced to Cu metal the blue coloured solution fades, eventually becoming colourless. The Cu2+ ions have a greater tendency to gain electrons than Zn2+ ions. As a result, Cu2+ ions will accept electrons from atoms Zn metal, resulting in the formation of Cu atoms and Zn2+ ions.
This reaction illustrates how all redox reactions are the result of competition for electrons.
If a strip of copper is placed in a solution of zinc sulfate no reaction occurs. This is because Zn2+ ions have a lesser tendency to gain electrons than Cu2+ ions. As a result, Cu atoms will not give up electrons to Zn2+ ions.
Redox reactions can be used to generate electricity if the two half-reactions are
physically separated. The electrons which are transferred can then pass through an external wire or circuit rather than being transferred through direct contact. A redox reaction in which the reactants are physically separated so that the transferred electrons can be directed through an external circuit is called an electrochemical cell.
One of the earliest electrochemical cell was the Daniel cell, developed in 1835. This
cell was based on the reactions between metallic zinc and copper(II) sulfate reviously described. The only difference is that half-reactions are physically separated so that the transferred electrons can be directed through an external circuit. In this way usable electrical energy can be degenerated.
The Daniel cell was extensively used for many years in telegraph and telephone work
as a reliable source of electricity. This electrochemical cell can be demonstrated by partly immersing zinc and copper strips in 1 mol L-1 solutions of zinc sulfate and copper(II) sulfatemrespectively in separate containers. The solutions are joined by a salt bridge which consists of U-tube filled with an electrolyte solution such as potassium nitrate. Each of two parts, consisting of a metal strip in an electrolyte solution, is called a half-cell.
When the metal strips are connected with a piece of wire an electric current flows
through the circuit. If a voltmeter is connected across the metal strips the voltmeter should read 1.1 volts. As the cell operates, the zinc strip dissolves slowly, while the copper strip becomes coated with more copper. The blue colour of the copper(II) sulfate solution fades and the voltage gradually falls.
These observations can be explained in terms of oxidation and reduction processes. The zinc loses electrons to form zinc ions according to the following equation.
Zn(s) --> Zn2+(aq) + 2e-
The zinc ions go into solution and hence the zinc strip gradually dissolves. The electrons given up by the zinc pass through the external circuit to the copper strip. There they are accepted by copper(II) ions in the solution surrounding the copper strip. The copper(II) ions are reduced to copper atoms which deposit on the strip forming a copper layer. The equation is as follows.
Cu2+(aq) + 2e- --> Cu(s)
The removal of Cu2+ ions from the solution causes the blue colour of the solution to fade.
Each time two electrons are released by a zinc atom in the left-hand half-cell, two
electrons are accepted by copper(II) ion in the other half-cell. Thus, for each zinc ion produced, a copper(II) ion is removed from solution. Electrical neutrality in the half-cell solutions is maintained by the migration of positive ions towards the copper half-cell and negative ions towards the zinc half cell. Thus Zn2+ ions move into the salt bridge and K+ ions move into the copper half-cell. At the same time, SO4 2h ions from the copper half-cell move into the salt bridge while NO3 ions move into the zinc half-cell. Thus, the flow of current through the external circuit is by movement of electrons through a metallic wire conductor and the flow of current in the internal circuit is by movement of ions.
The cell will continue to generate an electric current until chemical equilibrium is
reached. In this reaction the reactants are almost completely converted to products before equilibrium is achieved. At equilibrium most of the copper(II) ions will have been lost from the solution and the concentration of the zinc ions will be nearly double the original 1 mol L-1. The voltage, ore.m.f., drops to zero at equilibrium.
Any electrochemical cell consists of two half-cells. Each half-cell consists of an
electrode, which is a conductive metal or graphite strip in contact with an electrolyte solution.
The electrode at which oxidation takes place is called the anode. As electrons are generated by oxidation, the terminal of the anode is marked as the negative (-). The electrode at which reduction takes place is called the cathode. Because electrons are accepted in reduction, the terminal of the cathode is marked as the positive (+).
When the anode and cathode are connected, an electric current flows through the
external circuit. This electron flow can be used as source of energy to heat a lamp filament, run a motor, operate a transistor radio and so on. In other words, chemical energy is being used to generate electrical energy. Electrochemical cells, in which chemical change is used to generate electrical energy, are also called galvanic or voltaic cells.
An electrochemical cell does not necessarily have reactive metal electrodes. Consider
the cell in Figure 19.5 in which the platinum electrodes are chemically inert. hydrogen and chlorine gases are bubbled over the surfaces of the platinum electrodes as shown. In this cell the half-cells share a common electrolyte which is 1 mol L-1 hydrochloric acid. When the external circuit is connected by a voltmeter an e.m.f. of 1.36 V register on the voltmeter.