Calculating the energy change of a reaction from the energies of the bonds broken and made.
What you will learn
- Count the bonds broken and made in a reaction
- Calculate ΔH from bond energies
- Find an unknown bond energy
Break, then make
Every bond has a bond energy: the energy needed to break one mole of it (the same energy is released when it forms).
ΔH = energy to break bonds in reactants − energy released making bonds in products
- H₂ + Cl₂ → 2HCl; H−H 436, Cl−Cl 243, H−Cl 432 kJ/mol
- break: 436 + 243 = 679 kJ/mol
- make: 2 × 432 = 864 kJ/mol
- ΔH = 679 − 864 = −185 kJ/mol: exothermic
The rest of this lesson and its 12 practice questions are locked.
In this lesson
- Break, then make
- Count every bond
12 practice questions, marked as you go; about 25 minutes in all.
In this unit: Energy changes
Exothermic and endothermic reactions, the temperature change practical, reaction profiles, bond energies, cells and fuel cells.