In which dish, the solution is acidic? — Electrochemistry Chemistry Question

💡 Solution & Explanation

Step 1 - Write and Balance the Half-Cell Reactions To determine which compartment contains the acidic solution, we analyze the unbalanced overall redox reaction: $$\ce{Hg2^{2+}(aq) + NO3^-(aq) + H3O^+(aq) -> 2Hg^{2+}(aq) + HNO2(aq) + 4H2O(l)}$$ We can separate this overall chemical equation into its individual oxidation and reduction half-reactions: * **Oxidation half-reaction:** The dimerized mercury(I) ion (mercurous ion, $\ce{Hg2^{2+}}$) undergoes oxidation to form mercury(II) ions (mercuric ions, $\ce{Hg^{2+}}$): $$\ce{Hg2^{2+}(aq) -> 2Hg^{2+}(aq) + 2e^-}$$ This oxidation process occurs at the **anode** (located in **Dish A**). This half-reaction does not involve hydronium ions ($\ce{H3O^+}$). * **Reduction half-reaction:** The nitrate ion ($\ce{NO3^-}$) is reduced to nitrous acid ($\ce{HNO2}$) in the presence of acid: $$\ce{NO3^-(aq) + 3H3O^+(aq) + 2e^- -> HNO2(aq) + 4H2O(l)}$$ This reduction process occurs at the **cathode** (located in **Dish B**). Step 2 - Identify the Role of Hydronium Ions ($\ce{H3O^+}$) From the balanced reduction half-reaction taking place at the cathode, we observe that: * Hydronium ions ($\ce{H3O^+}$) are a direct reactant in the reduction of nitrate to nitrous acid. * For the reaction to proceed spontaneously under standard conditions, a standard concentration of $1.0\text{ M}$ hydronium ions ($\ce{H3O^+}$) must be maintained in the cathode compartment. * Therefore, the solution in the cathode chamber, which corresponds to **Dish B**, is acidic. Step 3 - Evaluate each Option * **Option (A) is incorrect:** Dish A is the anode compartment containing the $\ce{Hg2^{2+}}/\ce{Hg^{2+}}$ redox couple. The oxidation process does not involve or require hydronium ions as a reactant. * **Option (B) is correct:** Dish B is the cathode compartment containing the $\ce{NO3^-}/\ce{HNO2}$ couple. This reduction reaction consumes hydronium ions ($\ce{H3O^+}$), meaning the solution in Dish B must be acidic. * **Option (C) is incorrect:** Since only Dish B requires an acidic medium to facilitate the reduction of nitrate ions, both solutions are not acidic. * **Option (D) is incorrect:** Since the solution in Dish B is acidic, this option is incorrect. $$\text{Correct Option: } \boxed{B}$$