Q. 16

Which of the following electrodes will act as anodes, when connected to Standard Hydrogen Electrode?
A. Al/Al2+ -E\degree = –1.66

B. Fe/Fe2+ -E\degree = – 0.44

C. Cu/Cu2+ -E\degree= + 0.34

D. F2 (g)/2F- (aq) -E\degree= + 2.87

Answer :

According to thermodynamics, an element undergoes changes spontaneously only if it attains a stable state as a product, or after that change it attains lower energy state, or reaches equilibrium with its surrounding. This process of reaching lower energy in the case of ion formation occurs along with its release in the form of electrode potential energy. This is considered as an primary thermodynamic stability criteria.

As far by the convention the release in energy from the system is explained by positive sign. Thus the option with positive EMF will undergo spontaneous changes and the reduction reaction is favoured. The Standard Hydrogen Electrode (SHE) has a potential of 0.0 V as considered as a reference also called a Reference Electrode. There are many reference electrodes whose initial half-cell electrode potential is known. When another element is connected as the second electrode it may undergo reduction or oxidation, depending on whether it can do that in accordance with thermodynamics stability criteria mentioned earlier.

So for the electrodes containing negative potential, the reduction reaction are not favoured, as it shows the half-cell reactions are non-spontaneous and requires input of energy from external source.

Elements having negative potential will readily undergo oxidation and those with positive electrode potential wil undergo reduction.

With hydrogen as one electrode, from the given options Al and Fe will act as anode and undergo oxidation. This configuration will give a positive E°cell.

Al and H2 cell representation

Similar can be built for Fe and H2.

The positive EMF value shows the feasibility of the configuration chosen.

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