AQA GCSE Chemistry Chemistry Paper 1 (Foundation), June 2024: Question 7

📐 Calculation

Four identical cells are connected in series:

Total Voltage = number of cells × voltage per cell

Total Voltage = 4 × 1.5 V = 6 V

✅ Correct Answer

6 (V)

💡 Key Knowledge

A battery consists of two or more cells joined together in series. When joined in series, their potential differences (voltages) are additive:

V_total = V₁ + V₂ + V₃ + V₄

❌ Common Errors

• Writing 1.5 V: Confusing cells connected in parallel with cells connected in series.
• Dividing instead of multiplying (e.g. 1.5 / 4 = 0.375 V).

✅ Correct Answer

Select box: Sodium chloride solution

💡 Key Knowledge

• An electrolyte is an ionic compound that conducts electricity because its ions are free to move.
• In solid lead bromide, the ions (Pb²⁺ and Br⁻) are held tightly in a rigid giant ionic lattice and cannot move.
• Pure water consists of neutral molecules with virtually no free ions, making it a very poor conductor.
• Sodium chloride solution is an aqueous ionic compound where Na⁺ and Cl⁻ ions are free to move through the liquid to carry electrical current.

❌ Common Misconceptions

Students often remember lead bromide from electrolysis practicals and tick solid lead bromide. Remember: ionic substances only conduct when molten or dissolved in water (aqueous)!

✅ Model 6-Mark Plan (Level 3)

1. Keep one electrode constant: Set Metal A as copper throughout all trials.
2. Vary the second electrode: Use at least 4 different metals of known differing reactivities for Metal B (e.g. magnesium, zinc, iron, tin).
3. Set up the apparatus: Measure a fixed volume (e.g. 50 cm³) of the same concentration of electrolyte (e.g. 0.5 mol dm⁻³ sodium chloride solution) into a beaker.
4. Complete the circuit: Place Metal A and Metal B into the electrolyte, ensuring they do not touch. Connect them across a voltmeter using connecting leads.
5. Record: Read and record the potential difference (voltage) shown on the voltmeter.
6. Repeat: Replace Metal B with the next metal, keeping Metal A unchanged. Use fresh electrolyte for each test.
7. Control variables:
   • Keep the volume, concentration, and type of electrolyte identical.
   • Keep the surface area / depth of immersion of electrodes the same.
   • Keep temperature constant.

🧠 Exam Technique: Securing Level 3 (5–6 Marks)

To reach Level 3, your plan must lead to a valid outcome with logically sequenced steps:

• Validity key: You must keep one metal the same (control) and vary only the second metal. If you change both metals at random, you cannot determine which metal caused the voltage difference!
• Name specific equipment: State voltmeter (to measure potential difference) and measuring cylinder (to measure electrolyte volume).
• State controls clearly: Explicitly mention controlling the volume, concentration, and type of electrolyte.

❌ Common Errors & Lost Marks

• Changing both metals: Comparing "Mg + Cu" then "Zn + Fe" makes the data invalid because two variables change at once.
• Omitting a voltmeter: Saying "measure the electricity" or "see if a bulb lights" rather than explicitly stating measure the voltage using a voltmeter.
• Forgetting control variables: Failing to specify that the electrolyte must remain constant in type, volume, and concentration.
• Touching electrodes: Forgetting that electrodes touching inside the beaker creates a short circuit, giving 0 V.

💡 Expected Outcome & Scientific Principle

A chemical cell produces electricity because of the difference in reactivity between the two metals:

• The greater the difference in reactivity between Metal A and Metal B, the higher the voltage produced.
• For instance, a pair with a large difference (e.g. Magnesium and Copper) produces a higher voltage than a pair close together in the reactivity series (e.g. Iron and Copper).
• If two identical metals are used (e.g. Copper and Copper), the voltage produced is 0 V.