Organic Chemistry/Introduction to reactions/Bond dissociation energy
Bond dissociation energyis the energy needed to break a chemical bond. Also known as the Bond Enthalpy. |
The bond dissociation energy, or bond enthalpy, for a diatomic molecule X-Y is defined as the energy required to break one mole of X-Y bonds, as illustrated in the following process...
- X-Y(g) → X(g) + Y(g)
Bond enthalpies always refer to breaking bonds under gaseous conditions.
The mean molar bond enthalpy is an average value that is quoted for a bond that can occur in different molecular environments. An example is methane, CH4
- CH4(g) → C(g) + 4H(g)
Bond enthalpy values are used in Hess's Law Calculations.
The standard enthalpy of a reaction can be found by considering the bond enthalpies of the products and reactants of the reaction -
- Standard Enthalpy = Σ Enthalpy of formation of the products - Σ Enthalpy of formation of reactants
For stronger bonds, bond dissociation energy is higher as more energy is needed to break the bond.
A carbon-carbon double bond is stronger than a single bond and requires more energy to be broken. However, a carbon-carbon double bond is not twice as strong as a single one, it is only 1.5 times stronger.
All chemical bonds need an input of energy to be broken, as bonds allow a lower energy state for the component atoms. If a bond did not offer a lower energy state for the atoms that form it, a bond would not form.