R1.3 Exam Practice
Test your knowledge on Energy from Fuels
Section B: Data Analysis
Calculator and Data Booklet permitted.
Question 1: Bond Enthalpies Calculate
5 marksBond enthalpies: C–H = 413, C=C = 614, H–H = 436, C–C = 347 kJ mol⁻¹
(a) Define average bond enthalpy. [1]
(b) Calculate ΔH for C₂H₄ + H₂ → C₂H₆ using bond enthalpies. [2]
(c) Explain why bond enthalpy values are averages and why this limits accuracy. [2]
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(a) Energy required to break 1 mole of bonds in the gaseous state, averaged over different molecules [1]
(b) Broken: C=C(614) + H-H(436) = 1050; Formed: C-C(347) + 2×C-H(826) = 1173; ΔH = 1050 − 1173 = −123 kJ mol⁻¹ [1][1]
(c) Bond enthalpies vary depending on molecular environment [1]; using averages introduces error vs actual bond strengths [1]
Section C: Structured Questions
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Question 2: Incomplete Combustion Explain
4 marks(a) Write equations for the complete and incomplete combustion of methane. [2]
(b) Explain why incomplete combustion releases less energy. [1]
(c) State one environmental consequence of incomplete combustion. [1]
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(a) CH₄ + 2O₂ → CO₂ + 2H₂O [1]; 2CH₄ + 3O₂ → 2CO + 4H₂O [1]
(b) Fewer C=O bonds formed (CO vs CO₂), so less energy released [1]
(c) CO is toxic / carbon particulates cause respiratory problems [1]
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