Question

A cell representation is given: Ag/AgCl $\parallel$ FeCl$_2$, FeCl$_3$ / Pt
Which of the following can increase the EMF of the cell?
(i) By increasing concentration of Fe$^{2+}$
(ii) By increasing concentration of Fe$^{3+}$
(iii) By decreasing concentration of Fe$^{2+}$
(iv) By decreasing concentration of Fe$^{3+}$
(v) By increasing concentration of Cl$^-$

• A. ii, iv, v
• B. iii, iv, v
• C. iii, ii, v
• D. i, iii, v

Hint:

In electrochemical cells, changes in ion concentration can influence the EMF according to the Nernst equation. A lower concentration of Fe$^{2+}$ or higher concentration of Fe$^{3+}$ results in a higher EMF.

Answer 1

The Correct Option is B

Step 1: Use of Nernst equation.
The EMF of the cell can be calculated using the Nernst equation: \[ E_{\text{cell}} = E^\circ_{\text{cell}} - \frac{0.0591}{n} \log \left( \frac{[ \text{Fe}^{3+} ]}{[ \text{Fe}^{2+} ]} \right) \] Where \(E^\circ_{\text{cell}}\) is the standard electrode potential and \(n\) is the number of electrons involved.
Step 2: Effect of concentration.
- Increasing the concentration of Fe$^{3+}$ would decrease the EMF because the ratio \( \frac{[\text{Fe}^{3+}]}{[\text{Fe}^{2+}]} \) increases, leading to a more negative value of the logarithmic term. Hence, this will decrease the EMF. - Increasing the concentration of Fe$^{2+}$ would increase the EMF since the ratio decreases, increasing the value of \(E_{\text{cell}}\). - Decreasing the concentration of Fe$^{3+}$ would increase the EMF. - Decreasing the concentration of Fe$^{2+}$ would decrease the EMF. - Increasing the concentration of Cl$^-$ would increase the EMF by decreasing the potential difference between the electrodes.
Step 3: Conclusion.
The correct answer is (2) iii, iv, v, as these changes increase the EMF of the cell.