Electrochemistry Marking Scheme

This is the marking scheme for quiz: C4 - Electrochemistry Quiz
Topic Guide: C4 - Electrochemistry

Section A: Multiple Choice (5 Marks)

  1. D [1] (Anode is positive; oxidation is loss of electrons)
  2. C [1] (Bromine gas $\text{Br}_2$ is reddish-brown)
  3. B [1] (In aqueous $\text{NaCl}$, $\text{H}^+$ is reduced at cathode instead of $\text{Na}^+$)
  4. B [1] (Object to be plated is always the cathode)
  5. B [1] (Water is the only product)

Section B: Short Answer (15 Marks)

  1. Process:

    • Cathode: Steel object [1]
    • Anode: Pure copper strip [1]
    • Electrolyte: Soluble copper salt solution (e.g., $\text{CuSO}_4$) [1]
    • Mechanism: $\text{Cu}^{2+}$ ions move to the cathode and are reduced to $\text{Cu}$ metal, forming a layer on the steel. [2]

  2. Advantages:

    • Zero pollutants/emissions (only water produced) [2]
    • Higher energy efficiency compared to internal combustion engines [2]

  3. a) Molten KF:

    • Cathode: Potassium ($\text{K}$) [1]
    • Anode: Fluorine ($\text{F}_2$) [1] b) Dilute aqueous NaCl:
    • Cathode: Hydrogen ($\text{H}_2$) [2] (Since it is dilute/aqueous, $\text{H}^+$ is discharged)
    • Anode: Oxygen ($\text{O}_2$) [2] (Since it is dilute, $\text{OH}^-$ is discharged instead of $\text{Cl}^-$)

Section C: Extended Response (10 Marks)

  1. a) $\text{Cu}^{2+} + 2\text{e}^- \rightarrow \text{Cu}$ [2] b) $\text{Cu} \rightarrow \text{Cu}^{2+} + 2\text{e}^-$ [2] c) Cathode mass increases [1] because copper ions are deposited as metal. [1] Anode mass decreases [1] because copper atoms are oxidized to ions and dissolve into the solution. [1] d) If graphite is used at the anode, oxygen gas ($\text{O}_2$) would be produced instead of copper dissolving, and the anode mass would remain constant. [2]