Use bond energy and bond length to compare molecules
Define bond energy and length, then use supplied evidence to compare covalent molecule reactivity without overgeneralising.
Section 3 · Chemical bonding
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the supplied Pauling values when given, show particle and bond reasoning, and distinguish intramolecular bonds from intermolecular forces. Written practice is self-checked
Before you begin
- Read element symbols and simple molecular formulae; use the Periodic Table.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does bond energy measure?
What you will learn
- Define gaseous bond energy and covalent bond length
- Use values and bond length to compare molecular reactivity
Bond energy and bond length describe different properties
Bond energy is the energy required to break one mole of a particular covalent bond in the gaseous state. Bond length is the internuclear distance between two covalently bonded atoms.
For a given pair of atoms, a shorter bond is generally associated with a stronger bond and a higher bond energy. Use the data provided and identify the exact bond; values for polyatomic molecules may be average bond energies.
Relate a comparison to a reaction context
When comparing reactivity, use bond energy and bond length together with the reaction described. A bond that is harder to break may be consistent with lower reactivity in a relevant process, but one bond value alone does not determine the overall rate or every possible reaction.
Check whether the supplied energy is an exact diatomic value or an average for a bond in polyatomic molecules. Do not compare unlike bonds as though their chemical environments were identical.
Pause and explain
Which comparison supports the stronger bond, all else in the stated comparison?
Worked example
A supplied table gives X–X bond energy 160 kJ mol⁻¹ and length 145 pm, and Y–Y energy 240 kJ mol⁻¹ and length 120 pm. Which bond is stronger?
Show the worked solution
- Compare energy needed to break one mole of each bond.
- Y–Y has the larger bond energy.
- It also has the shorter internuclear distance, consistent with a stronger bond.
Answer Y–Y is stronger in the supplied comparison: it has higher bond energy and shorter bond length.
Try a topic test
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define bond energy and bond length, keeping the units or physical meaning distinct.
Give me a hint
One is an energy per amount of bonds; the other is a separation.
Compare my reasoning
- Bond energy concerns breaking one mole of a named covalent bond.
- State that the bond is in the gaseous state.
- Bond length is the internuclear distance between bonded atoms.
Bond energy is the energy required to break one mole of a particular covalent bond in the gaseous state. Bond length is the internuclear distance between the bonded atoms.
Look for these in your work
- One mole specified
- Gaseous state included
- Distance definition separate
Bond A has energy 350 kJ mol⁻¹ and length 154 pm; bond B has energy 610 kJ mol⁻¹ and length 134 pm. Compare their relative strengths.
Give me a hint
Use both supplied measures and state the conclusion only for this comparison.
Compare my reasoning
- Bond B needs more energy to break.
- Bond B is shorter.
- The two observations support B being stronger in the stated comparison.
Bond B is stronger in the supplied comparison: its bond energy is higher and its length is shorter.
Look for these in your work
- Energy evidence compared
- Length evidence compared
- Bounded conclusion stated
A student uses a larger bond energy to claim that a molecule must react more slowly in every reaction. Explain the limitation.
Give me a hint
A reaction involves more than one bond and may have a different rate-determining step.
Compare my reasoning
- Bond energy describes breaking one specified gaseous bond.
- A reaction may involve several bonds, intermediates and activation barriers.
- Use the full reaction context and evidence before comparing rates.
Bond energy is one bond property, not a complete reaction-rate model. The overall reaction and activation barrier matter, so that universal conclusion is not justified.
Look for these in your work
- Definition scope retained
- Reaction context recognised
- Overclaim rejected
Common mistakes
- Using bond energy to mean the energy of a whole molecule
- Omitting the gaseous state from the bond-energy definition
- Assuming bond energy alone predicts every reaction rate
- Ignoring that listed values may be averages
What can you explain now?
What does bond energy measure?
Compare with the explanation
Energy required to break one mole of a specified covalent bond in the gas state
The definition includes the mole and gaseous state.
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Make it stick
What does a bond-energy value describe, and why is it not a complete prediction of reaction rate?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.