Test yourself on Bond Energies (HT) with exam style questions, marked instantly.
Flashcards that come back just before you would forget them.
A revision plan that remembers what you know and shows what to do next.
See which topics you have nailed and which need another look.
Quiz yourself on Bond Energies (HT)Get your flashcardsBuild my revision planTrack my progress📝 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.
Make this stick
Signing in gives access to all PrepWise features.