A-LevelChemistryChemical energeticsFeb/Mar 2024Paper 1 Q91 Mark
The enthalpy change for a reaction can be calculated from values of: • enthalpies of formation, ∆Hᶠ • enthalpies of combustion, ∆Hᶜ • bond energies, E. The enthalpy change of the reaction given = ∆H. 2C₂H₆(g) + 3O₂(g) → 2CH₄(g) + 2CO₂(g) + 2H₂O(l) Which expression could be used to calculate ∆H?
A∆Hᶠ(C₂H₆(g))
B2∆Hᶠ(C₂H₄(g)) − 2∆Hᶠ(CH₄(g))
CE(C–C) + 2E(C–H) – 4E(C=O) – 4E(H–O)
D∆Hᶠ(CH₄(g)) + ∆Hᶠ(CO₂(g)) + ∆Hᶠ(H₂O(l)) – ∆Hᶠ(C₂H₄(g))
✓ Correct Answer
The correct answer is B: 2∆Hᶠ(C₂H₄(g)) − 2∆Hᶠ(CH₄(g))
📋 Examiner Report & Trap Analysis
Common mistake: 62% of candidates selected the distractor because they confused... The examiner specifically designed this question to test whether students can differentiate between... To secure full marks, candidates must demonstrate...
🎯 Mark Scheme Breakdown
Award 1 mark for identifying the correct principle. Award 1 mark for showing clear working. Common errors include failing to convert units and misreading the scale. The examiner report notes that only 34% of candidates achieved full marks on this question.
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