This exam focus covers Worked Model Answer within Bond Energies (HT) for GCSE Chemistry. Revise Bond Energies (HT) in Energy Changes for GCSE Chemistry with 20 exam-style questions and 15 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 14 of 15 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.
Topic position
Section 14 of 15
Practice
20 questions
Recall
15 flashcards
📝 Worked Model Answer
Question: "Using the bond energies given, calculate the overall energy change for the reaction CH₄ + 2O₂ → CO₂ + 2H₂O and state whether the reaction is exothermic or endothermic." (5 marks)
Bond energy data: C–H = 413 kJ/mol; O=O = 498 kJ/mol; C=O = 805 kJ/mol; O–H = 463 kJ/mol
First, I calculate the energy needed to break all bonds in the reactants. CH₄ contains 4 C–H bonds: 4 × 413 = 1652 kJ/mol. There are also 2 O=O bonds: 2 × 498 = 996 kJ/mol. Total energy in = 1652 + 996 = 2648 kJ/mol. [2] Next, I calculate the energy released when new bonds form in the products. CO₂ contains 2 C=O bonds: 2 × 805 = 1610 kJ/mol. Two molecules of H₂O each contain 2 O–H bonds, giving 4 O–H bonds total: 4 × 463 = 1852 kJ/mol. Total energy out = 1610 + 1852 = 3462 kJ/mol. [2] Overall energy change: ΔH = 2648 − 3462 = −814 kJ/mol. [1] Because ΔH is negative, more energy is released forming bonds in the products than is needed to break bonds in the reactants, so the reaction is exothermic.
Examiner note: Method marks are awarded at each stage — you can still score 3 or 4 marks even with an arithmetic error, as long as your working is shown. The most common mistake is forgetting that 2H₂O contains 4 O–H bonds (not 2). Always draw out structural formulae and count every bond before starting the calculation.