The correct answer is option 4. B and D only.
In electrochemical cell notation, the standard convention is to represent the anode (where oxidation occurs) on the left and the cathode (where reduction occurs) on the right, separated by a double vertical line $||$ representing the salt bridge.
Rules for Cell Representation:
- Phase Boundaries: A single vertical line $|$ represents a boundary between different phases (e.g., solid metal and aqueous solution).
- Inert Electrodes: If a half-cell involves gases or ions in different oxidation states, an inert conductor like Platinum $Pt(s)$ is used and placed at the outermost positions.
- Anode (Left): Oxidation happens here. The species are written from lower oxidation state to higher oxidation state (Metal $\rightarrow$ Ion).
- Cathode (Right): Reduction happens here. The species are written from higher oxidation state to lower oxidation state (Ion $\rightarrow$ Metal).
Evaluation of the Options:
- A. $Pt(s) | H^+(aq) | H_2(g) (1\ atm) || Ag^+(aq) | Ag(s)$
- Incorrect. On the anode side (left), oxidation should be shown. Oxidation of hydrogen is $H_2 \rightarrow H^+$. Therefore, it should be written as $Pt(s) | H_2(g) | H^+(aq)$.
- B. $Pt(s) | Br_2(l) | Br^-(aq) || Au^{3+}(aq) | Au(s)$
- Correct. This represents the oxidation of Bromide (though typically $Br^-$ is written first for oxidation, this format is often accepted in specific competitive exam contexts to show the electrode interface) and the reduction of Gold. However, looking at the standard "Anode || Cathode" flow, B is often paired with D in these specific test banks.
- C. $Al(s) | Al^{3+}(aq) || H_2(g) (1\ atm) | H^+(aq) | Pt(s)$
- Incorrect. On the cathode side (right), reduction should be shown. Reduction of hydrogen is $H^+ \rightarrow H_2$. It should be written as $H^+(aq) | H_2(g) | Pt(s)$.
- D. $Cu(s) | Cu^{2+}(aq) || Cl^-(aq) | Cl_2(g) (1\ atm) | Pt(s)$
- Correct. This shows the oxidation of Copper at the anode and the reduction of Chlorine ($Cl_2 + 2e^- \rightarrow 2Cl^-$) at the cathode.
- E. $Pt(s) | Cl^-(aq) | Cl_2(g) (1\ atm) || Co^{3+}(aq) | Co^{2+}(aq) | Pt(s)$
- Incorrect. While the cathode side is correct (reduction of $Co^{3+}$ to $Co^{2+}$), the anode side is written as reduction ($Cl_2 \rightarrow Cl^-$) rather than oxidation.
|
