Read the passage carefully and answer.
When a metal is placed in a solution of its ions, the metal can lose electron and can go in the solution as ion or the metal ion in the solution can take electron from the electrode and get deposited as metal. Thus, the electrode acquires either positive or negative charge with respect to the solution leading to the development of a potential difference between the metal electrode and the solution. This potential difference is called the electrode potential of the metal. The electrode potential of a metal is a measure of relative tendency to undergo oxidation (loss of electron) or reduction (gain of electron). When the concentration of the ions in the solution is unity, the electrode potential is called standard electrode potential. The magnitude of potential depends upon the nature of electrode, concentration of ions in solution and the temperature. In a galvanic cell, the electrode at which oxidation occurs is known as anodic half-cell and the electrode at which reduction occurs is known as cathodic half cell. The emf of the cell in terms of standard reduction potential is given by \( E°_{cell} = E°_{cathode} - E°_{anode} \). The standard free energy change of the redox reaction taking place in the cell is related to \( E°_{cell} \) by \( ΔG°= -nFE° \). A redox reaction would occur spontaneously if the free energy change is negative. The equilibrium constant of the reaction is related to the standard free energy change by \( ΔG°= -RT \ln K \).
The measured potential for \( Ag^+ + e^- → Ag(s) \) does not change with
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Changing the pressure of the solution
The potential of an electrode depends on the nature of the electrode, the concentration of its ions and the temperature, as the passage states. For the silver electrode the Nernst equation shows all three:
\( \displaystyle \ce{Ag+(aq) + e- -> Ag(s)} \)
\( \displaystyle E_{(\ce{Ag+}/\ce{Ag})} = E^\circ_{(\ce{Ag+}/\ce{Ag})} - \frac{RT}{F}\ln\frac{1}{[\ce{Ag+}]} \)
No gas takes part in this half-reaction, so no pressure term appears in the equation. Pressure has a negligible effect on a solid and on ions in a liquid solution, so changing it does not change the measured potential.
Pressure matters only where a gas is involved, as in the hydrogen electrode, whose potential depends on the pressure of \( \ce{H2} \) gas.
Option (1) Changing the temperature — Changes the measured potential. T appears in the Nernst equation, and \( E^\circ \) itself changes with temperature.
Option (2) Varying the concentration of Ag+ ion — Changes the measured potential. The potential depends on \( [\ce{Ag+}] \) through the log term of the Nernst equation.
Option (3) Purity of Ag plate — Changes the measured potential. The equation treats the solid as pure silver; impurities change the nature of the electrode surface, which is one of the factors that decide the potential.
Option (4) Changing the pressure of the solution — Does not change the measured potential. No gas takes part in the half-reaction, so the Nernst equation has no pressure term.