{
  "problem_id": "Exam-02-T3",
  "header": "Problem 3: Bi-Ion Removal Battery. A bi-ion removal battery (BDI) can intercalate cations and anions from seawater into crystalline phases analogous to those in lithium-ion batteries and store them through conversion reactions in the cathode and anode, enabling efficient seawater desalination with high ion-removal capacity and high charge efficiency. Studies show that a $\\mathrm{NaBi_3O_4Cl_2}$ electrode can simultaneously remove $\\mathrm{Na^+}$ and $\\mathrm{Cl^-}$ from seawater.",
  "header_images": [
    {
      "file": "Exam-02-T3-header-1.png",
      "description": "On the left is a stacked structure of red blocks. Following the process indicated by the blue arrow “$\\mathrm{-Na^+, -Cl^-}$,” it becomes the central stacked structure of red and orange blocks. The process indicated by the red arrow “$\\mathrm{+Na^+, +Cl^-}$” then converts it to the structure on the right, composed of red and blue blocks. A further process indicated by the blue arrow “$\\mathrm{-Na^+, -Cl^-}$” returns it to the central structure. The legend identifies red blocks as $\\mathrm{NaBi_3O_4Cl_2}$, blue blocks as substance X, and orange blocks as $\\mathrm{Bi}$."
    }
  ],
  "subquestions": [
    {
      "sub_id": "Exam-02-T3-1",
      "tags": [
        "2.1",
        "3.5"
      ],
      "text_open": "Substance X is $BiOCl$.\n3-2-3 Write the electrode-reaction equations occurring at the $\\mathrm{NaBi_3O_4Cl_2}$ electrode during charging.",
      "text_images": [],
      "full_mark": 4,
      "rubric": [
        "$\\mathrm{Na^+ + 3Bi + 2Cl^- + 4H_2O - 9e^- = NaBi_3O_4Cl_2 + 8H^+}$ (2 points; $\\mathrm{Na^+ + 3Bi + 2Cl^- + 8OH^- - 9e^- = NaBi_3O_4Cl_2 + 4H_2O}$ receives 1 point).",
        "$\\mathrm{Bi + Cl^- + H_2O - 3e^- = BiOCl + 2H^+}$ (2 points)",
        "$\\mathrm{Bi + Cl^- + 2OH^- - 3e^- = BiOCl + H_2O}$ (1 point).",
        "Combining the two equations above gives:",
        "$\\mathrm{3xBi + yNa^+ + (3x-y)Cl^- + (3x+y)H_2O - 9e^- = yNaBi_3O_4Cl_2 + (3x - 3y)BiOCl + (6x + 2y)H^+}$ (4 points)"
      ],
      "rubric_images": []
    },
    {
      "sub_id": "Exam-02-T3-2",
      "tags": [
        "3.5"
      ],
      "text_open": "3-3 To construct a complete battery, $\\mathrm{NaTi_2(PO_4)_3@C}$ is used as the electrode on the other side. 3-3-1 State the type of the $\\mathrm{NaTi_2(PO_4)_3@C}$ electrode during seawater desalination.",
      "text_images": [],
      "full_mark": 2,
      "rubric": [
        "Cathode (2 points)"
      ],
      "rubric_images": []
    },
    {
      "sub_id": "Exam-02-T3-3",
      "tags": [
        "2.1",
        "3.5"
      ],
      "text_open": "3-3-2 Write the electrode-reaction equation occurring at the $\\mathrm{NaTi_2(PO_4)_3@C}$ electrode during seawater desalination.",
      "text_images": [],
      "full_mark": 2,
      "rubric": [
        "$\\mathrm{NaTi_2(PO_4)_3@C + 2Na^+ + 2e^- = Na_3Ti_2(PO_4)_3@C}$ (2 points)",
        "Alternatively, $\\mathrm{NaTi_2(PO_4)_3@C + xNa^+ + xe^- = Na(1+x)Ti_2(PO_4)_3@C}$ (2 points)"
      ],
      "rubric_images": []
    }
  ]
}