Lesson focus: Overall reaction energy can be estimated from the energy needed to break bonds and the energy released when new bonds form.
Use mean bond energyAn average value for the energy needed to break one type of bond. to estimateA rough value worked out using simple numbers or averages. reaction energy changes and explain the limits of the calculation.
Build on earlier learning
Energy-level diagramsEnergy-level diagrams show the overall change. Bond energies provide a molecular explanation and a numerical estimate.
You will be successful when you can…
- State energy directions for bond changes.
- Count bonds from displayed structures.
- Calculate an estimated energy change.
- Explain why mean values give estimates.
Words that carry the science
- bond energy
- The energy required to break one mole of a specified bond in gaseous molecules.
- mean bond energy
- An average bond-breaking value across several molecular environments.
- bond breaking
- A process that requires energy transfer into the reacting system.
- bond making
- A process that transfers energy from the reacting system.
- enthalpy change
- A measure of energy transferred as heat at constant pressure; introduced here through an estimate.
- estimate
- A value obtained using simplifying assumptions or average data.
Account for bond breaking and bond making separately
01Breaking bonds always requires energy
Energy must be transferred in to separate bonded atoms. bond breakingPulling atoms apart, which always takes in energy. is therefore always an energy-requiring step, even in an exothermic reaction.
The activation barrier partly reflects the need to weaken or break bonds before new arrangements can form. Stronger bonds generally require more energy to break.
02Making bonds releases energy
When atoms form new bonds and move to a lower-energy arrangement, energy is transferred to the surroundings. The overall estimateA rough value worked out using simple numbers or averages. is energy to break reactant bonds − energy released making product bonds.
A negative result means more energy is released in bond makingAtoms joining together, which always gives out energy. than taken in for bond breaking, so the reaction is exothermic. A positive result indicates endothermic overall.
03Mean bond energies are approximate
Tabulated mean values average the same bond type across different gaseous molecules. Actual bond energyThe amount of energy needed to break a particular bond between atoms. depends on molecular environment, and state changes or intermolecular forces may also matter.
The calculation is therefore an estimate and may differ from a measured reaction enthalpy. It remains useful for explaining trends and checking whether a proposed value is plausible.
Bond energies
Breaking bonds takes in energy. Making bonds gives out energy. If more energy is given out making the new bonds than is taken in breaking the old ones, the reaction is exothermic overall. Energy change = bonds broken − bonds made.
Controls
Bond energies in kJ per mole are average values; data books differ slightly.
Estimate hydrogen chloride formation
H₂ + Cl₂ → 2HCl. Use H–H 436, Cl–Cl 243 and H–Cl 431 kJ mol⁻¹.
- Break one H–H and one Cl–Cl: 436 + 243 = 679 kJ mol⁻¹.
- Make two H–Cl bonds: 2 × 431 = 862 kJ mol⁻¹ released.
- Calculate 679 − 862 = −183 kJ mol⁻¹.
- The negative estimate indicates an exothermic reaction.
How to make the reasoning visible: Multiply each bond energy by the number of that bond broken or formed.
Separate bond-energy contributions
Use the simplified values shown; all are mean gaseous bond energies.
| Step | Bond count | Energy per bond / kJ mol⁻¹ | Contribution / kJ mol⁻¹ | Direction |
|---|---|---|---|---|
| Break H–H | 1 | 436 | +436 | Into system |
| Break Cl–Cl | 1 | 243 | +243 | Into system |
| Make H–Cl | 2 | 431 | −862 | Out of system |
| Overall | — | — | −183 | Exothermic |
Why is the bond-making contribution doubled?
Why might a measured value differ from −183 kJ mol⁻¹?
Check your understanding
Answer from memory, then check against the suggested answer.
Does bond breaking require or release energy?
What happens energetically when bonds form?
What does a negative bond-energy estimate indicate?
A calculation gives +75 kJ mol⁻¹. What does this mean?
Explain why a molecule with strong bonds can still take part in an exothermic reaction.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
A reaction needs 500 kJ mol⁻¹ for bond breaking and releases 640 kJ mol⁻¹ in bond making.
Calculate the overall energy change. Show your working and include the sign.
(3)Explain what the sign tells you about the reaction.
(2)Total for this question: 5 marks
Command word: For calculate and explainA command word pair: work out the number answer, then say what it means or why your method works., show the signed calculation and translate the sign into a physical transfer.