Exam Tips for Bond Energy Calculations
Part of Bond Energies (HT) — GCSE Chemistry
This exam tips covers Exam Tips for Bond Energy Calculations 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 12 of 13 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.
Topic position
Section 12 of 13
Practice
20 questions
Recall
15 flashcards
💡 Exam Tips for Bond Energy Calculations
🎯 Common Question Types:
- "Calculate ΔH for this reaction using bond energies" (4-5 marks)
- "Explain in terms of bond breaking and making why this reaction is exothermic" (3 marks)
- "Identify the mistake in this student's calculation" (2 marks)
- "Predict whether this reaction is exothermic or endothermic given bond energy data" (2 marks)
📝 Key Command Words:
- Calculate — show all steps: bonds broken, bonds formed, ΔH = in − out
- Draw out the structures — count EVERY bond in reactants and products
- Check coefficients — 2H₂ means 2 × H-H bonds, not 1
⚠️ Common Mistakes to Avoid:
- Saying "breaking bonds releases energy" — it is MAKING bonds that releases energy!
- Forgetting that 2H₂O has 4 O-H bonds (2 per molecule × 2 molecules)
- Subtracting breaking from making — it must be ΔH = breaking MINUS making
- Not showing working — you can still get method marks even with an arithmetic error
Quick Check: A student says "in exothermic reactions, energy is needed to break bonds, and this is where the heat comes from." What is wrong with this statement?
The student has the direction wrong. Breaking bonds REQUIRES energy — it is endothermic and does not release heat. The heat in an exothermic reaction comes from the energy RELEASED when new bonds FORM in the products. The energy released by bond formation is greater than the energy needed for bond breaking, and the surplus is released as heat to the surroundings.