Bond Enthalpies

Bond enthalpies provide another method for calculating enthalpy changes, particularly useful for reactions involving covalent molecules in the gas phase. This approach considers the energy required to break bonds in the reactants and the energy released when new bonds form in the products.

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What is a Bond Enthalpy?

A bond enthalpy (also called bond energy or bond dissociation enthalpy) is the energy required to break one mole of a specific covalent bond in the gaseous state, with all species in the gas phase.

For example, the O–H bond enthalpy in water refers to:

H–O–H(g) → H(g) + OH(g) ΔH = +463 kJ mol⁻¹

Notice that bond breaking is always endothermic (positive ΔH) — you must put energy in to overcome the attractive forces holding atoms together. Conversely, bond making is always exothermic (negative ΔH) — energy is released when atoms come together and form bonds.

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Mean Bond Enthalpies

The exact energy needed to break a particular bond depends on the molecular environment. For example, the energy to break the first O–H bond in water is different from the energy to break the second O–H bond:

H₂O(g) → H(g) + OH(g) ΔH = +498 kJ mol⁻¹