Q. 33

On the basis of standard electrode potential values, suggest which of the following reactions would take place? (Consult the book for E\degree value).

(i) Cu + Zn2+ Cu2+ + Zn

(ii) Mg + Fe2+ Mg2+ + Fe

(iii) Br2 + 2Cl- Cl2 + 2Br-

(iv) Fe + Cd2+ Cd + Fe2+

Answer :

The electrochemical series gives a relative strength arrangement of standard electrode reduction potentials of common ions and elements. It is arranged in increasing order of Reduction Potential from bottom to top with F being at the top and Li being at the bottom.

Now whether a reaction should takes place or not would depend on the net cell EMF of the cell, given by equation



Option (i)Cu + Zn2+ Cu2+ + Zn


Here Cu undergoes oxidation so it acts as anode and Zn acts as cathode. So from the table


For cathode E°cathode = -0.76 V


For anode E°anode = 0.52 V




As the EMF of cell is negative the given reaction will not occur spontaneously if they were to form a cell placed as electrodes.


Option (ii) Mg + Fe2+ Mg2+ + Fe


Similarly, we can say that Mg undergoes oxidation and Fe undergoes reduction.


cathode = -0.44 V


anode = -2.36 V




Positive EMF implies that the reaction will give out energy and attain stability, thus it will occur spontaneously. So the given redox reaction will occur.


Option (iii) Br2 + 2Cl- Cl2 + 2Br-


Here Br undergoes reduction thus acting as cathode and Cl acting as anode.


For cathode E°cathode = 1.09 V


For anode E°anode = 1.36 V




The negative potential prevents easy reaction, so the redox reaction will not occur.


Option (iv) Fe + Cd2+ Cd + Fe2+


Fe is the cathode and Cd is the anode


For cathode E°cathode = -0.44 V


For anode E°anode = -0.40 V




The negative potential prevents easy reaction, so the redox reaction will not occur.


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