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+

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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