Chapter 1:- Concept of Electrode Potential
B.sc 2nd year Book
Advance Inorganic Chemistry
A Daniell cell is an electrochemical cell(or voltaic cell or galvanic cell) in which oxidation and reduction reactions take place in two separate containers and energy is produced during these reactions. This Daniell cell consists of two beakers, one of which contains 1M solution of ZnSO4 and the other contains 1M solution of CuSO4 . A Zn -rod is dipped into 1M ZnSO4 solution and Cu -rod is dipped into 1MCuO4 solution. These metallic rods are called ‘electrodes’. The solutions in the two beakers are connected together by an inverted U-tube containing saturated solution of an inert electrolyte like KCI. KNO3, K2SO4, NH4NO3, etc. or solidified solution of such an electrolyte in agar-agar and gelatin, the U- tube is called ‘salt bridge’ as shown in figure1.0. Two electrodes (metallic rods) are connected with an ammeter by means of an insulated wire and a key is also inserted to complete the circuit. As the circuit is completed the needle of the ammeter is deflected which shows the electric current flows in the external circuit.
The cell cannot operate without a transfer of ions. As the zinc electrode goes into solution, the concentration of Zn2+ becomes greater, whereas the deposition of Cu at the other electrode decreases the concentration of Cu2+ in the second vessel. Consequently, an excess of SO42- ions accumulates around the Cu electrode and an excess of Zn2+ ions around the Zn electrode. In any ionic solution the sum of the negative charges must be equal to the total positive charges. Hence, if no provision is made for the migration of ions from one vessel to the other, the current will cease to flow.
In the above cell, the following observations are made-
(i) Zn– rod dipped in ZnSO4 solution acts as anode at which oxidation reaction takes place while Cu -rod dipped in CuSO4 solution acts as cathode at which reduction reaction takes place.
(ii) A beaker containing Zn – rod dipped in ZnSO4 solution is called ‘oxidation half-cell’ and other beaker containing Cu- rod is called ‘reduction half cell’.
(iii) Oxidation reaction taking place at Zn- rod is called ‘oxidation half-cell reaction’ and reduction reaction taking place at Cu – rod is called ‘reduction half-cell reaction’.
(iv) At anode (Zn- electrode), Zn (s) oxidized to Zn2+(aq) ions which results Zn – rod losses its weight and concentration of Zn2 ions increase in ZnSO4 solution.
Zn(s)—————-> Zn2+ (aq) + 2e–
(v) At cathode(Cu-electrode), Cu2+(aq) ions reduced to Cu(s) by accepting the electrons lost by Zn(s) which results deposition of reduced copper on Cu – rod and also the concentration of Cu2+ ions decreases in CuSO4 solution.
(vi) Electrons released at Zn – electrode move towards the Cu – electrode through the wire in the external circuit.
(vii) Electric current flows from Cu – electrode to Zn – electrode through salt bridge.
(viii) Since electrons move from Zn- electrode to Cu – electrode, Zn – rod is also called as ‘negative terminal’ and Cu – rod as ‘positive terminal’. Sometimes negative( − ) and positive(+) signs are also put on the electrodes to show the release and loss of electrons taking place on them.
(ix) An inert electrolyte used in salt bridge is one whose ions do not participate in the redox reaction and also do not react with the electrolytes used.
(x) The function of the salt bridge is to allow the movement of ions from one half-cell to the other without mixing of the two electrolytes. moreover, it helps to maintain the electrical neutrality of the solution in the two half-cells.
(xi) Electrical double layers are developed on the electrodes due to oxidation or reduction half-reactions (see figure 1.04 and 1.05 ).
(xii) By convention this electrochemical cell is represented as-
Zn(s); Zn2+(1M) || Cu2(1M) ; Cu(s)