When you go to the chemistry laboratory and you want to work with electrochemical cells, firstly do the voltaic cell of Zn - Cu.A voltaic cell is a spontaneous reaction, so that you have to seperate the solution into two half-cells, the first part is oxidation and the other is reduction.
Prepare all the apparatus needed and arrange them as the above picture. For a Zn-Cu cell, please dip a zinc rod in ZnSO4(aq) and a Cu rod in CuSO4(aq). Do not dip Zn rod in CuSO4(aq), because they will react. Connect the Zn rod to the copper rod by using a wire. You can use a galvanometer and a plug key.
The two aqueous solutions in the beackers, ZnSO4 and CuSO4 are interlinked one of another through a salt bridge which is an inverted 'u' tube which is filled with saturated semi solid paste of Agar-agar saturated in NaCl, KCl or KNO3 solution (Agar-agar is a carbohydrate which forms a jelly like substance when dissolved in hot water). You can use filter paper, wet with NaCl(aq). It is more simple than the first.
When the circuit is completed, a deflection is observed in the (G) towards the zinc electrode indicating that the electrons are flowing from the Zn electrode to Cu electrode. The Zn electrode will oxidize.
The electrons lost by the Zn metal and move through electrode and reach the Cu electrode at which they are accepted by Cu2+ ions of the solution to form neutral copper atoms.
In an electrochemical cell, each electrode constitutes one half of the cell and the reaction taking place at the electrode is called half-cell reaction. The overall cell reaction is obtained by adding the two half-cell reactions.
When Zn is added to CuSO4(aq), Zn directly displaces Cu2+(aq) from CuSO4(aq). However, in the electrochemical cell there is no direct contact between Zn and CuSO4(aq). Here, the elecrons can flow from Zn rod to Cu. So the chemical energy changes to the electrical energy. Hence the electrochemical cell acts as a source of electrical current although for a short interval.
The electrode at which oxidation takes place, the metal rod becomes negative charged is called negative electrode. The electrode at which reduction takes place, the metal rod becomes positive charged is called positive electrode.
Accordingly, in the above electrochemical cell, Zn electrode acts as negative electrode while Cu electrode acts as positive.
The function of salt bridge:
a) acts as a link between the two aqueous solution
b) overcomes liquid junction potential.
c) maintains the electrical neutrality of the aqueous solution of the electrodes by releasing or sending oppositely charged ions into the solution.
The cell notation of Zn - Cu is written as : Zn(s)/Zn2+(aq)//C reu2+(aq)/Cu(s) or Zn(s)/ZnSO4(aq)//CuSO4(aq)/Cu(s).
The chemical equation is :
(Anode) Oxidation : Zn(s) --> Zn2+ + 2e Eo = 0,76 volt
(Cathode) Reduction : Cu2+ + 2e --> Cu(s) Eo = 0,34 volt
Redox Reaction : Zn(s)+Cu2+(aq) --> Zn2+(aq)+Cu(s) Eo cell = 1,10 volt
Sometimes you can directly caculate the Eo cell without write down the chemical equation.
Eo cell = Eo Cu2+/Cu - Eo Zn/Zn2+ (Eo of Red at Cathode - Eo of Ox at Anode)
Eo cell = 0,34 volt - . -0,76 volt = 1,10 volt.
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