Edexcel A-Level Chemistry Paper 1, June 2025: Question 3

✅ Acceptable Answers (Any One)

• Heat energy must be continuously supplied, so any measured temperature change is not solely from the decomposition reaction itself.
• It is difficult to determine when the thermal decomposition reaction is complete.
• It is not possible to accurately measure the temperature change of a solid while actively heating it.

❌ Common Misconceptions

• Do NOT just write: "The reaction is incomplete" or "it's too slow" without reference to heating.
• Vague temperature points: Stating simply "heat is lost" does not explain why direct measurement is fundamentally impossible during thermal decomposition.

🧠 Exam Technique: Alternative Method

You can also show the maximum moles of CaCO₃ that could react:

0.090 / 2 = 0.045 mol CaCO₃

Since 0.045 mol > 0.041 mol used, HCl must be in excess.

❌ Common Error

Forgetting the 1:2 stoichiometric ratio! Simply comparing 0.090 mol to 0.041 mol scores zero for the comparison mark because it ignores the reaction stoichiometry.

❌ Major Traps

• Wrong mass in q: Do not use the mass of CaCO₃ (4.10 g) in q = mcΔT. The question states mass of solution = 30 g.
• Wrong moles: Dividing by moles of HCl (0.090 mol) instead of the limiting reactant (0.041 mol).
• Missing negative sign: Losing the final mark by writing +16.8 instead of −16.8 kJ mol⁻¹.

💡 Units Check

Always check whether your intermediate energy is in J or kJ before dividing by moles. 689.7 J ÷ 0.041 mol = 16822 J mol⁻¹ = 16.8 kJ mol⁻¹.

           [ CaCO₃(s) ] ───────── Δ_rH₁ ─────────▶ [ CaO(s) + CO₂(g) ]
                 │                                         │
  + 2HCl(aq)     │                                         │ + 2HCl(aq)
     (Δ_rH₂)      ▼                                         ▼ (Δ_rH₃)
          ┌────────────────────────────────────────────────────────┐
          │          CaCl₂(aq) + CO₂(g) + H₂O(l)                  │
          └────────────────────────────────────────────────────────┘

✅ Required Entries

• Bottom Box: CaCl₂(aq) + CO₂(g) + H₂O(l) (all formulas and state symbols must be completely correct).
• Left Arrow: Arrow pointing downwards from CaCO₃(s) to the bottom box, labelled with + 2HCl(aq) (or + 2HCl and/or Δ_rH₂).

❌ Common Errors

• Omitting state symbols in the bottom box (costing M1).
• Forgetting that CO₂(g) passes straight down on the right-hand side, so it must appear in the bottom box alongside CaCl₂(aq) and H₂O(l).
• Pointing the left arrow upwards instead of downwards.

❌ Unit Inconsistency Trap

ΔH is given in kJ mol⁻¹, while ΔS is in J K⁻¹ mol⁻¹! You must divide ΔS by 1000 to convert to kJ K⁻¹ mol⁻¹ before substituting into ΔG = ΔH − TΔS.

🧠 Alternative Method (ΔS_total)

ΔS_surroundings = −ΔH / T = −178200 / 298 = −597.99 J K⁻¹ mol⁻¹

ΔS_total = 160.4 − 597.99 = −437.59 J K⁻¹ mol⁻¹

Since ΔS_total is negative, the reaction is not feasible.

✅ Model 3-Mark Answer

1. The Mg²⁺ ion is smaller than the Ca²⁺ ion (has a smaller ionic radius / higher charge density).
2. The Mg²⁺ ion polarises the carbonate (CO₃²⁻) ion (or distorts its electron cloud) more strongly.
3. This weakens the C−O bond within the carbonate ion to a greater extent, requiring less energy to break.

❌ Costly Examiner Penalties

• Saying "atom" instead of "ion": Referring to "magnesium atom" or "Mg" instead of the Mg²⁺ ion penalises M1 immediately!
• Confusing bond breaking: Saying the "ionic bond between Mg and CO₃ is weakened" is wrong. It is the covalent C−O bond within the carbonate anion that is polarised and weakened.
• Shielding arguments alone: Simply stating "less shielding" without comparing ionic radius or charge density does not secure M1.