Bond Energies

Q: What is bond energy?

A: The energy required to break one mole of a particular bond in gaseous molecules.

Q: Why is bond breaking endothermic?

A: Energy must be absorbed to overcome the attraction between bonded atoms.

Q: Why is bond formation exothermic?

A: Energy is released when atoms form a more stable bonded arrangement.

Q: What is the formula for enthalpy change using bond energies?

A: ΔH = Σ bond energies of bonds broken − Σ bond energies of bonds formed.

Q: Why can a reaction be exothermic even though bonds are broken?

A: More energy is released when new bonds form than is absorbed breaking the original bonds.

Q: Why are bond energies average values?

A: Bond energy varies depending on the molecule, so the values are averages for that type of bond.

Q: Why is the bond energy method approximate?

A: It uses average bond energies, so it does not account for the exact bonding environment in each molecule.

Calculation method

Identify all bonds in the reactants.

Count the bonds broken.

Identify all bonds in the products.

Count the bonds formed.

Use bonds broken − bonds formed.

Memory trick: Breaking costs energy. Forming gives energy back.