(a) Following reactions occur at cathode during the electrolysis of aqueous sodium chloride solution:
Na+(aq)+e−→Na(s) E∘=−2.71V
H+(aq)+e−→12H2(g)E∘=0.00V
On the basis of their standard reduction electrode potential (E∘) values, which reaction is feasible at the cathode and why?
(b) Why does the cell potential of mercury cell remain constant throughout its life?
(a) Following reactions occur at cathode during the electrolysis of aqueous sodium chloride solution:
Na+(aq)+e−→Na(s) E∘=−2.71V
H+(aq)+e−→12H2(g)E∘=0.00V
On the basis of their standard reduction electrode potential (E∘) values, which reaction is feasible at the cathode and why?
(b) Why does the cell potential of mercury cell remain constant throughout its life?
A redox reaction is feasible only if the species with high reduction potential is reduced (accepts an electron) and the species with lower standard reduction potential is oxidised (loses an electron). For a reaction to occur, it is necessary that the species with a greater tendency to lose an electron, loses its electron, which is gained by the species with a greater tendency to gain the electron. The reactions taking place during the electrolysis of aqueous sodium chloride solution are
Na+(aq)+e−→Na(s) E∘=−2.71V (I)
H+(aq)+e−→12H2(g) E∘=0.00V (II)
One more reaction which takes place during the electrolysis of aqueous sodium chloride solution is
Cl+2(aq)+2e−→2Cl−(g)E∘=1.36V (III)
From the above three reactions, we observe that the standard reduction potential sodium (reaction I) is negative, while the standard reduction potential for chlorine (reaction III) is positive.
Hence, reaction III is feasible at the cathode.
b) A mercury cell consists of a zinc container as the anode, a carbon rod as the cathode and a paste of mercuric oxide mixed with KOH as the electrolyte.
· In this cell, the overall cell reaction does not involve any ion whose concentration may change.
· Therefore, this cell gives a constant potential of 1.5 V throughout its life.
· However, these cells should be reprocessed for mercury recovery or treated to prevent mercury or mercury compounds from entering the atmosphere and causing pollution.
A redox reaction is feasible only if the species with high reduction potential is reduced (accepts an electron) and the species with lower standard reduction potential is oxidised (loses an electron). For a reaction to occur, it is necessary that the species with a greater tendency to lose an electron, loses its electron, which is gained by the species with a greater tendency to gain the electron. The reactions taking place during the electrolysis of aqueous sodium chloride solution are
Na+(aq)+e−→Na(s) E∘=−2.71V (I)
H+(aq)+e−→12H2(g) E∘=0.00V (II)
One more reaction which takes place during the electrolysis of aqueous sodium chloride solution is
Cl+2(aq)+2e−→2Cl−(g)E∘=1.36V (III)
From the above three reactions, we observe that the standard reduction potential sodium (reaction I) is negative, while the standard reduction potential for chlorine (reaction III) is positive.
Hence, reaction III is feasible at the cathode.
b) A mercury cell consists of a zinc container as the anode, a carbon rod as the cathode and a paste of mercuric oxide mixed with KOH as the electrolyte.
· In this cell, the overall cell reaction does not involve any ion whose concentration may change.
· Therefore, this cell gives a constant potential of 1.5 V throughout its life.
· However, these cells should be reprocessed for mercury recovery or treated to prevent mercury or mercury compounds from entering the atmosphere and causing pollution.
