BRAINCAKEBond Enthalpies
Chemical reactions involve breaking bonds in the reactants and making new bonds in the products.
Breaking a bond needs energy, so it is endothermic. Making a bond releases energy, so it is exothermic. The overall enthalpy change depends on the balance between the two: if more energy is released making bonds than is taken in breaking them, the reaction is exothermic.
Bond enthalpy is the enthalpy change when one mole of a particular bond is broken, with all species in the gaseous state. For example, the H-H bond enthalpy is +436 kJ mol-1:
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