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Write the half-reaction for the deposition of copper.

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2H+(aq)+2e−→H2(g)2\text{H}^+(aq) + 2\text{e}^- \to \text{H}_2(g)

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Here's the solution to questions 2 and 3:

2. During purification of copper by electrolysis, 1.48 g1.48 \text{ g} of copper were deposited when a current was passed through aqueous copper (II) sulphate for 2122 \frac{1}{2} hours. Calculate the amount of current passed. (Cu=63.5\text{Cu} = 63.5, 1 Faraday=96500 C1 \text{ Faraday} = 96500 \text{ C})

  1. Write the half-reaction for the deposition of copper.

    Cu2+(aq)+2e−→Cu(s)\text{Cu}^{2+}(aq) + 2\text{e}^- \to \text{Cu}(s)

    This shows that 2 moles of electrons are required to deposit 1 mole of copper.

  2. Calculate the moles of copper deposited.

    Moles of Cu=Mass of CuMolar mass of Cu\text{Moles of Cu} = \frac{\text{Mass of Cu}}{\text{Molar mass of Cu}} Moles of Cu=1.48 g63.5 g/mol≈0.023307 mol\text{Moles of Cu} = \frac{1.48 \text{ g}}{63.5 \text{ g/mol}} \approx 0.023307 \text{ mol}
  3. Calculate the moles of electrons required.

    Moles of electrons=Moles of Cu×2\text{Moles of electrons} = \text{Moles of Cu} \times 2 Moles of electrons=0.023307 mol×2=0.046614 mol\text{Moles of electrons} = 0.023307 \text{ mol} \times 2 = 0.046614 \text{ mol}
  4. Calculate the total charge (Q) passed.

    Q=Moles of electrons×Faraday constant\text{Q} = \text{Moles of electrons} \times \text{Faraday constant} Q=0.046614 mol×96500 C/mol≈4500.85 C\text{Q} = 0.046614 \text{ mol} \times 96500 \text{ C/mol} \approx 4500.85 \text{ C}
  5. Convert the time to seconds.

    Time (t)=212 hours=2.5 hours\text{Time (t)} = 2 \frac{1}{2} \text{ hours} = 2.5 \text{ hours} t=2.5 hours×3600 s/hour=9000 s\text{t} = 2.5 \text{ hours} \times 3600 \text{ s/hour} = 9000 \text{ s}
  6. Calculate the current (I).

    Q=I×t\text{Q} = \text{I} \times \text{t} I=Qt\text{I} = \frac{\text{Q}}{\text{t}} I=4500.85 C9000 s≈0.500 A\text{I} = \frac{4500.85 \text{ C}}{9000 \text{ s}} \approx 0.500 \text{ A}

    The amount of current passed is 0.500 A\boxed{0.500 \text{ A}}

    3. The diagram below represents a set-up that can be used for the electrolysis of dilute sulphuric acid

    a) Name the electrodes A and B
    Electrode A is connected to the positive terminal of the battery.
    Electrode B is connected to the negative terminal of the battery.
    Anode: Electrode A
    Cathode: Electrode B

    b) Write an equation for the reaction taking place at electrode B
    Electrode B is the cathode (negative electrode). In dilute sulphuric acid, the species present are H+\text{H}^+, SO42−\text{SO}_4^{2-}, and H2O\text{H}_2\text{O}. At the cathode, reduction occurs. Hydrogen ions are preferentially reduced over water.

    2H+(aq)+2e−→H2(g)\boxed{2\text{H}^+(aq) + 2\text{e}^- \to \text{H}_2(g)}

    c) What happens to the concentration dilute sulphuric acid as the reaction continues?
    At the cathode (B), H+\text{H}^+ ions are consumed to produce hydrogen gas.
    At the anode (A), water is oxidized to produce oxygen gas and H+\text{H}^+ ions: 2H2O(l)→O2(g)+4H+(aq)+4e−2\text{H}_2\text{O}(l) \to \text{O}_2(g) + 4\text{H}^+(aq) + 4\text{e}^-.
    The overall reaction is the decomposition of water: 2H2O(l)→2H2(g)+O2(g)2\text{H}_2\text{O}(l) \to 2\text{H}_2(g) + \text{O}_2(g).
    Since water is consumed and sulphuric acid acts as a catalyst (its ions are regenerated or not consumed), the amount of solvent (water) decreases. Therefore, the concentration of the dilute sulphuric acid increases.

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