Electricity and Chemical Change

Predict → justify → write → check

Work on paper first. For every question: identify molten or aqueous, check electrode material, list the ions, choose products, then balance atoms and charge. Compare reasoning with your regular partner after your own attempt.

Explore the electrolytic cell ↗ Compare the galvanic cell ↗

Flexible 40-minute practice lesson: 4 min diagnostic (Q1), 8 min teacher model (Q2), 18 min practice (Q3–6), 7 min correction and comparison (Q7), 3 min independent exit check (Q8). If product selection is insecure, focus on Q3–5 and use Q6–7 for homework.

1. Charge carriers. A student says: “Electrons flow through molten lead(II) bromide.” Correct this and explain why the solid does not conduct.
   Check after your attempt

   Electrons carry charge in the metal wires. Mobile Pb²⁺ and Br⁻ ions carry charge in the molten electrolyte. In the solid the ions are fixed in position.

2. Molten electrolyte. Molten PbBr₂ is electrolysed with inert electrodes. Give each electrode sign, product and half-equation.
   Check after your attempt

   Cathode (−): lead, Pb²⁺ + 2e⁻ → Pb. Anode (+): bromine, 2Br⁻ → Br₂ + 2e⁻. Check both atom count and total charge.

3. Transfer the method. Molten aluminium oxide contains Al³⁺ and O²⁻. Write both electrode half-equations. Explain where reduction occurs.
   Check after your attempt

   Cathode: Al³⁺ + 3e⁻ → Al. Anode: 2O²⁻ → O₂ + 4e⁻. Reduction occurs at the cathode because aluminium ions gain electrons. In industrial extraction the carbon anode also reacts with oxygen.

4. Aqueous competition. Concentrated aqueous sodium chloride is electrolysed using inert electrodes. Predict both gases and explain why sodium metal is not formed.
   Check after your attempt

   Hydrogen forms at the cathode because hydrogen is discharged in preference to the more reactive sodium. Chlorine forms at the anode in concentrated chloride solution. Cathode: 2H₂O + 2e⁻ → H₂ + 2OH⁻. Anode: 2Cl⁻ → Cl₂ + 2e⁻.

5. Change one condition. Compare aqueous CuSO₄ using inert electrodes with the same solution using copper electrodes. Give products and the change in blue colour.
   Check after your attempt

   Both cathodes deposit copper: Cu²⁺ + 2e⁻ → Cu. Inert anode: oxygen, 4OH⁻ → O₂ + 2H₂O + 4e⁻; blue colour fades. Copper anode: Cu → Cu²⁺ + 2e⁻; the colour stays approximately constant as Cu²⁺ is replaced.

6. Repair an answer. A student writes Cu²⁺ → Cu + 2e⁻ at the cathode. Identify the error and write the correct equation.
   Check after your attempt

   Cu²⁺ must gain electrons: Cu²⁺ + 2e⁻ → Cu. The student equation has charge +2 on the left and −2 on the right. The correct equation has total charge 0 on both sides.

7. Compare the cells. In a zinc–copper galvanic cell and an electrolytic cell, are the cathodes always positive? State what is always true at the cathode.
   Check after your attempt

   No. The galvanic cathode is positive; the electrolytic cathode is negative. Reduction (electron gain) always occurs at the cathode. Anode and cathode are defined by reaction, not by a universal sign.

8. Independent exit check. A copper spoon is to be silver-plated. Which terminal connects to the spoon? What ion must the electrolyte contain? Write the half-equation at the spoon.
   Check after your attempt

   The spoon is the negative cathode. The electrolyte contains Ag⁺. Ag⁺ + e⁻ → Ag deposits silver on the spoon.

Use your errors: Wrong products → practise Q3–5. Wrong electron side or charge → practise Q2, Q3 and Q6. Confused signs → compare the two models, then retry Q7 without notes.