Final answer:
The cell potential of a mercury cell remains constant throughout its life because the reaction in the cell is nonspontaneous and the standard reduction potentials of the half-reactions involved are fixed. The concentrations of the reacting species also remain constant.
Step-by-step explanation:
The cell potential of a mercury cell remains constant throughout its life because the reaction in the cell is nonspontaneous, meaning it requires an external source of electrical energy to occur. The cell potential is determined by the difference in potential between the anode and the cathode, which remains constant as long as the concentrations and other conditions in the cell remain the same.
One reason for the constant cell potential is that the reaction in a mercury cell involves the reduction of mercury(II) ions at the cathode and the oxidation of zinc at the anode. Both of these reactions have fixed standard reduction potentials, which determine the voltages of the respective half-reactions. The overall cell potential is the difference between these two voltages, and it remains unchanged because the standard reduction potentials are constant.
Additionally, the concentrations of the reacting species can affect the cell potential, but in a mercury cell, the concentrations remain constant because the reactants are not consumed during the reaction. Instead, the reaction relies on an external electrical circuit for the reduction and oxidation to occur, rather than the reactants being directly consumed in the cell.