How It Works: Calculating Energy Change from Bond Energies
Part of Bond Energies (HT) — GCSE Chemistry
This how it works covers How It Works: Calculating Energy Change from Bond Energies 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 4 of 15 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 4 of 15
Practice
20 questions
Recall
15 flashcards
⚙️ How It Works: Calculating Energy Change from Bond Energies
Now that you understand the concept — breaking costs energy, making releases energy — you can calculate the overall energy change for any reaction.
The bond energy value (in kJ/mol) for each type of bond tells you exactly how much energy is needed to break one mole of that bond — or equivalently, how much energy is released when one mole of that bond forms. These values are determined experimentally and given in data books.
To calculate the overall energy change (ΔH) for a reaction: first count ALL the bonds broken in the reactants and add up their energies (this is "energy in"). Then count ALL the bonds formed in the products and add up their energies (this is "energy out"). Finally, subtract: ΔH = energy in − energy out.
If energy in is greater than energy out → ΔH is positive → endothermic (you spent more than you earned). If energy out is greater than energy in → ΔH is negative → exothermic (you earned more than you spent). The sign tells you the direction of energy transfer relative to the surroundings.
THE FORMULA:
ΔH = Energy to break bonds − Energy released making bonds
ΔH = Σ(bonds broken) − Σ(bonds formed)