AQA A-Level Chemistry Paper 1, 2023: Question 2

✅ Mark Scheme Answers

Purpose (1 mark):

• To complete the circuit
  OR
• To allow ions to move (between the half-cells) / maintain electrical neutrality.

Identity (1 mark):

• Potassium nitrate ( KNO₃ ) or Sodium nitrate ( NaNO₃ )
• Accept: Any soluble ionic compound that does not react with H⁺, Mg²⁺, or Cl⁻ ions.

💡 Key Knowledge

• A salt bridge usually consists of filter paper soaked in an aqueous solution of an unreactive electrolyte.
• As oxidation and reduction occur, charge builds up in the beakers. The salt bridge supplies ions (cations migrate to the cathode, anions migrate to the anode) to prevent charge accumulation.
• Inertness requirement: The salt must not form a precipitate (e.g. avoid Ag⁺ which would precipitate with Cl⁻) and must not react with the acid.

❌ Common Errors

• Writing "allows electrons to flow": This scores 0 marks. Electrons only ever travel through the external wires, never through the solution or salt bridge.
• Suggesting a salt that reacts, such as AgNO₃ (Ag⁺ forms insoluble AgCl with chloride ions) or carbonates/bases (react with H⁺).

🧠 Exam Technique

Always learn the standard phrase: "Allows the movement of ions to complete the circuit and maintain electrical neutrality." This guarantees the purpose mark on any AQA paper.

✅ Correct Answer

No change

💡 Key Knowledge

• Electromotive force (EMF) is a thermodynamic property determined solely by the position of equilibrium of the half-reactions under the given temperature and concentration conditions.
• Increasing the surface area increases the rate of electron transfer (kinetics), but has zero effect on electrode potentials or the overall voltage (EMF).

❌ Common Misconceptions

• Confusing rate of reaction with cell potential. Students often guess "increases" thinking more surface area produces more current or voltage.

✅ Correct Answer

Tick: [✔] EMF increases

📐 Step-by-Step Explanation

1. Consider the equilibrium:
   Mg²⁺(aq) + 2e⁻ ⇌ Mg(s)    (E° = -2.38 V)
2. Effect of adding water:
   Adding water dilutes the solution, so [Mg²⁺] decreases.
3. Le Chatelier’s shift:
   The equilibrium shifts to the left to oppose the decrease in [Mg²⁺], releasing more electrons onto the electrode.
4. Change in electrode potential:
   The electrode becomes more negative (e.g. from -2.38 V to -2.45 V).
5. Overall cell EMF:
   EMF = E°(cathode) - E°(anode) = 0.00 V - (-2.45 V) = +2.45 V.
   Therefore, the magnitude of EMF increases.

❌ Common Errors

Students often forget that the magnesium electrode is negative. If a potential becomes "more negative", its numerical value decreases, but the difference between it and the standard hydrogen electrode (0.00 V) becomes larger. Pay careful attention to the word "magnitude".

✅ Correct Equation (either form)

• Full molecular:
  Mg + 2HCl → MgCl₂ + H₂
• Ionic:
  Mg + 2H⁺ → Mg²⁺ + H₂

Notes: State symbols are not required. Multiples are accepted.

💡 Deducing Spontaneous Direction

• Compare the standard potentials:
  • Mg²⁺ + 2e⁻ ⇌ Mg   (E° = -2.38 V, more negative: oxidises)
  • 2H⁺ + 2e⁻ ⇌ H₂   (E° = 0.00 V, more positive: reduces)
• The half-cell with the more negative E° proceeds in reverse (oxidation):
  Mg → Mg²⁺ + 2e⁻
• The half-cell with the more positive E° proceeds forwards (reduction):
  2H⁺ + 2e⁻ → H₂
• Combine and cancel electrons to yield the overall reaction.

🧠 Exam Technique

Replacing the high-resistance voltmeter with a bulb allows current to flow. The reaction will always proceed in the thermodynamically feasible direction: the more negative system releases electrons (oxidation) and transfers them to the more positive system (reduction).