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Electro Chemical Series

A strip of metallic zinc is placed in an aqueous solution of copper nitrate as shown in the figure for about half an hour.

The strip becomes coated with metallic copper and the blue colour of the solution disappears. The reaction between metallic zinc and the aqueous solution of copper nitrate is
Zn (s) + Cu2+ (aq) Zn2+(aq) + Cu (s)
In this reaction zinc has lost electrons to form Zn2+ and copper ion is reduced to Copper by gaining electrons from the zinc.

A copper rod is dipped in silver nitrate solution as shown in the figure.

The solution develops a blue colour due to the formation of Cu2+ ions on account of the reaction.
Cu(s) + 2Ag +(aq) Cu2+(aq) + 2Ag(s)
Here Cu(s) is oxidized to Cu2+ and Ag + (aq) is reduced to Ag(s).
When metallic cobalt is placed in nickel sulphate solution the following reaction occurs:
Co(s) + Ni2+(aq) CO2+(aq) + Ni(s)
At equilibrium chemical tests reveal that both Ni2+ (aq) and CO2+(aq) are present at moderate concentrations. From the the above mentioned examples, by comparison we know that zinc releases electrons to copper and copper releases electrons to silver .The electron releasing tendency of the metals is the order: Zn>Cu>Ag. From this we understand that we can develop a table in which metals and their ions are listed on the basis of their tendency to release electrons. This series of metals are known as metal activity series or electrochemical series.

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