The Currency of Chemical Reactions
📖 The Currency of Chemical Reactions
Bond energy is like a bank balance for the reaction! Breaking bonds = the reaction withdraws money (energy must be put IN). Making bonds = the reaction deposits money (energy is given OUT). If the reaction earns more than it spends — more energy released making bonds than was needed to break them — the reaction is exothermic overall. If it spends more than it earns, it is endothermic. Note: always count from the reaction's perspective. Breaking bonds costs the reaction energy (energy flows in from the surroundings); making bonds pays the reaction energy (energy flows out to the surroundings).
Chemistry glossary
- What is bond energy?
- The energy needed to break 1 mole of a particular bond
Why does breaking a bond always require energy? Think about pulling a magnet off a fridge: you have to put energy in to separate them. Bonded atoms are similar — they are held together by electrical attraction between positively charged nuclei and shared electrons, and you must supply energy to pull them apart. This is
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🧠 Memory Aids
Breaking = Bad (needs energy), Making = Magic (releases energy)
This is the most important thing to remember for bond energy questions. Break = ENDO (energy in). Make = EXO (energy out).
The formula: "IN minus OUT" = ΔH
ΔH = energy IN (breaking) − energy OUT (making)
If ΔH is negative → more OUT than IN → exothermic (more energy released than absorbed)
If ΔH is positive → more IN than OUT → endothermic (more energy absorbed than released)
Double check rule: "Negative ΔH = nice and warm (exothermic). Positive ΔH = pulls energy from surroundings (endothermic)."
Now try it yourself
Quiz · Question 1 of 25
Which statement correctly describes the energy change when chemical bonds are broken?
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This topic in real past papers
Every real exam question we've found on bond energies (ht), with a full worked answer.
AQA Paper 1
Complete or interpret a reaction profile diagram showing activation energy and overall energy change, or calculate an unknown bond energy using given bond energies and the overall energy change.
Edexcel Paper 2
Use bond energy data to calculate the overall energy change for a reaction