Explain bond-energy changes in reactions
Relate energy transfer to breaking and making bonds, and distinguish exact from average bond-energy values.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Report signs, states, units and significant figures consistently. Standard conditions use 298 K and 101 kPa in this syllabus. Written practice is self-checked
Before you begin
- Rearrange a formula, balance a chemical equation and use temperature differences.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens energetically when a covalent bond is broken?
What you will learn
- Explain energy changes using bonds broken and formed
- State that bond breaking is endothermic and bond making is exothermic
- Distinguish exact and average bond energies
Breaking bonds takes energy; making bonds releases energy
Chemical reactions transfer energy because bonds are broken in reactants and new bonds are made in products. Breaking a bond requires energy and is endothermic. Making a bond releases energy and is exothermic.
If forming product bonds releases more energy than is needed to break reactant bonds, the net reaction is exothermic. If bond breaking requires more energy than bond making releases, the net reaction is endothermic.
Some bond-energy values are exact and others are averages
Bond energy is defined for breaking one mole of a specified covalent bond in the gaseous state. Some tabulated values are exact, such as values for a specified diatomic molecule; values for bonds in polyatomic molecules are often average values across different molecular environments.
Use the supplied data and label the bond type. An average bond energy is an estimate for a particular bond in its molecular contexts, not an exact value for every individual bond in every substance.
Pause and explain
What kind of bond-energy data are values for bonds in many polyatomic molecules?
Worked example
Explain why bond breaking and bond making have opposite energy signs.
Show the worked solution
- Separating bonded atoms requires energy input.
- Forming an attractive bond lowers the system's energy.
- Therefore bond breaking is endothermic and bond making is exothermic.
Answer Breaking bonds takes in energy; forming bonds releases energy.
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Describe the energy transfers when reactant bonds break and product bonds form.
Give me a hint
Track the energy change for each direction of bond change.
Compare my reasoning
- Breaking reactant bonds requires energy.
- Making product bonds releases energy.
- The net reaction enthalpy depends on the balance of these transfers.
Bond breaking is endothermic and bond making is exothermic; their energy balance determines the reaction enthalpy.
Look for these in your work
- Breaking energy sign correct
- Making energy sign correct
- Net balance linked
A data table labels a C–H energy in a polyatomic molecule as average and H–H energy as exact. Explain the difference in how to use them.
Give me a hint
Consider whether the bond occurs in one defined diatomic molecule or varied molecular settings.
Compare my reasoning
- The H–H value applies to the specified diatomic molecule.
- A C–H average represents bonds across differing molecular environments.
- Use each value with its stated meaning and appropriate calculation.
The H–H value is exact for that specified diatomic bond; the C–H value is an average because C–H bonds occur in different polyatomic environments.
Look for these in your work
- Exact value bounded to specified molecule
- Average context explained
- Data labels respected
A reaction forms 300 kJ mol⁻¹ of product-bond energy but requires 220 kJ mol⁻¹ to break reactant bonds. Predict the sign and type of reaction enthalpy.
Give me a hint
Compare released energy from making bonds with energy absorbed in breaking them.
Compare my reasoning
- Bond making releases 300 kJ mol⁻¹.
- Bond breaking absorbs 220 kJ mol⁻¹.
- Net energy released is 80 kJ mol⁻¹.
ΔH is negative (−80 kJ mol⁻¹); bond making releases more energy than bond breaking absorbs, so the reaction is exothermic.
Look for these in your work
- Energy difference correct
- Negative sign correct
- Exothermic classification linked
Common mistakes
- Saying bond breaking releases energy
- Ignoring new bonds when explaining reaction enthalpy
- Treating an average value as exact for every molecule
- Confusing one mole of bonds with one mole of molecules
What can you explain now?
What happens energetically when a covalent bond is broken?
Compare with the explanation
Energy is absorbed
Separating bonded atoms requires energy.
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Make it stick
Which part absorbs energy, which releases energy, and how do exact and average bond values differ?
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