Bond energy: The energy required to break one mole of a particular type of covalent bond. Units: kJ/mol. The same amount of energy is released when the same bond forms.
ÎH (enthalpy change): The overall energy change in a reaction. Calculated as: ÎH = ÎŁ(energy to break bonds) â ÎŁ(energy released making bonds). Negative = exothermic; positive = endothermic.
Endothermic bond breaking: Breaking any covalent bond always requires energy â it is always endothermic. Work must be done against the electrical attractive force holding the atoms together (like pulling a magnet off a fridge).
Exothermic bond forming: Forming any covalent bond always releases energy â it is always exothermic. When atoms come together, they move into a lower, more stable energy state and release the excess energy to the surroundings.
This definitions covers Key Definitions within Bond Energies (HT) for GCSE Chemistry. Revise Bond Energies (HT) in Energy Changes for GCSE Chemistry with 25 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 8 of 15 in this topic. Make sure you can use the exact wording confidently, because definition marks are often lost through vague language.
Practice questions for Bond Energies (HT)
Which statement correctly describes the energy change when chemical bonds are broken?
Explain how you would determine, from a bond energy calculation, whether a reaction is exothermic or endothermic.