The correct option for the question regarding the electrolysis of sodium hydroxide solution using platinum electrodes is **A. Na+ ions are discharged at the cathode**. ### Detailed Explanation: 1. **Understanding Electrolysis**: - Electrolysis is a process that uses electrical energy to drive a non-spontaneous chemical reaction. In this case, we are electrolyzing a sodium hydroxide (NaOH) solution. - During electrolysis, the solution is broken down into its constituent ions. In sodium hydroxide, the ions present are sodium ions (Na⁺), hydroxide ions (OH⁻), and water molecules (H₂O). 2. **Electrode Reactions**: - **At the Cathode**: The cathode is the electrode where reduction occurs (gain of electrons). In an aqueous solution, the possible species that can be reduced are: - Na⁺ ions (sodium ions) - H⁺ ions (from water, as water can dissociate into H⁺ and OH⁻) - The reduction potential of H⁺ (which forms hydrogen gas) is more favorable than that of Na⁺. Therefore, at the cathode, H⁺ ions from water are reduced to form hydrogen gas (H₂): \[ 2H^+ + 2e^- \rightarrow H_2(g) \] - However, in a concentrated NaOH solution, the discharge of Na⁺ ions is less favorable compared to the discharge of H⁺ ions from water. 3. **At the Anode**: - The anode is where oxidation occurs (loss of electrons). The hydroxide ions (OH⁻) can be oxidized to form oxygen gas (O₂) and water: \[ 4OH^- \rightarrow O_2(g) + 2H_2O + 4e^- \] 4. **Concentration Changes**: - As electrolysis proceeds, hydroxide ions are consumed at the anode, leading to a decrease in the concentration of NaOH in the solution. - At the cathode, while hydrogen gas is produced, the concentration of NaOH does not increase; rather, it remains relatively constant because the Na⁺ ions do not participate in the reduction process. ### Evaluating Other Options: - **Option B: Hydrogen ions are discharged at the cathode**: - This option is misleading. While H⁺ ions from water are indeed reduced to form hydrogen gas, the statement implies that H⁺ ions are the primary species being discharged, which is not accurate in the context of sodium hydroxide solution where Na⁺ ions are present. The reduction of H⁺ ions is a secondary reaction, and the primary discharge at the cathode is the reduction of water. - **Option C: The concentration of sodium hydroxide decreases at both electrode compartments**: - This statement is incorrect. While the concentration of NaOH decreases at the anode due to the oxidation of OH⁻ ions, it does not decrease at the cathode. The Na⁺ ions remain in solution and do not participate in the reduction process. - **Option D: The concentration of sodium hydroxide increases at the cathode only**: - This option is also incorrect. The concentration of NaOH does not increase at the cathode; it remains relatively stable. The primary reaction at the cathode is the production of hydrogen gas, not an increase in NaOH concentration. ### Summary: - The correct answer is **A. Na⁺ ions are discharged at the cathode**. - During electrolysis, H⁺ ions from water are reduced to form hydrogen gas at the cathode. - The concentration of NaOH decreases at the anode due to the oxidation of OH⁻ ions. - The overall process leads to the production of hydrogen gas at the cathode and oxygen gas at the anode, with a net decrease in NaOH concentration in the solution. ### Revision Summary: - Electrolysis of NaOH involves the reduction of H⁺ ions to form hydrogen gas at the cathode. - Na⁺ ions do not discharge at the cathode; instead, they remain in solution. - The concentration of NaOH decreases at the anode due to the oxidation of hydroxide ions. - Understanding the reactions at each electrode is crucial for predicting the outcomes of electrolysis.