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Bond energies

The enthalpy change ($$\Delta H$$) of a reaction is related to the bond energies.

Every bond in a molecule has a set bond energy. A bond with a higher energy is a stronger bond.

The total bond energy of the reactants is found by adding the bond energies of all bonds in the reactants.

Likewise, the total bond energy of the products is found by adding the bond energies of all the bonds in the products.

$$\Delta H$$ is equal to the difference between the total bond energy of the reactants and the total bond energy of the products. Essentially:

$$\Delta H$$ = Energy of reactant bonds broken $$-$$ Energy of product bonds formed

A reaction is therefore endothermic ($$\Delta H$$ is positive) if the bonds broken are stronger than the bonds formed.

A reaction is exothermic ($$\Delta H$$ is negative) if the bonds formed are stronger than the bonds broken.