bond energy
Summary
Bond energy is the energy needed to break one mole of a particular bond in the gas phase, usually in kJ/mol. A higher value means a stronger bond, which is usually shorter. Most testable: ΔH ≈ energy of bonds broken − energy of bonds formed.
Detail
Meaning: Bond energy is the energy absorbed to break one mole of a particular bond in the gas phase. Breaking a bond is endothermic; forming it releases the same energy. A larger value means a stronger bond. Between the same two atoms, energy rises and length falls from single to double to triple. Values for bonds like C–H are averages over many compounds.
Equation: ΔH°rxn ≈ Σ(bond energies broken) − Σ(bond energies formed), where ΔH°rxn is the reaction enthalpy change and each bond energy is a positive value, both in J/mol (usually quoted in kJ/mol).
On the MCAT: A passage gives a bond energy table and a reaction such as combustion. You total bonds broken and formed, then decide whether the reaction is exothermic (ΔH negative) or endothermic. Other questions ask you to rank bonds by strength or link shorter bonds to higher bond energy.
Common confusion: Students subtract in the wrong order and get the wrong sign. Energy goes in to break bonds and comes out when bonds form, so use broken minus formed (reactants minus products). This is the reverse of ΔHf° problems, which use products minus reactants. The result is an estimate.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.