Breaking bonds costs energy.
Breaking bonds requires energy; forming bonds releases it. A reaction is exothermic if forming product bonds releases more energy than is required to break reactant bonds. The reverse statement describes an endothermic change.
Estimate ΔH by adding bond enthalpies for bonds broken and subtracting those for bonds formed. Count every bond and use balanced equation coefficients. Average bond enthalpies refer to gas-phase bonds across multiple compounds, so values differ from a precise measurement for a specific reaction.
Breaking versus making bonds
Explore the relationship
ΔH ≈ bond energies broken − bond energies formed. Average gas-phase bond energies give estimates.
Tap or focus a plotted point to read its values.
View plotted data as a table
| Series | Energy to break bonds / kJ mol⁻¹ | Estimated ΔH / kJ mol⁻¹ |
|---|---|---|
| Estimated ΔH / kJ mol⁻¹ | 200 | -400 |
| Estimated ΔH / kJ mol⁻¹ | 213.333 | -386.667 |
| Estimated ΔH / kJ mol⁻¹ | 226.667 | -373.333 |
| Estimated ΔH / kJ mol⁻¹ | 240 | -360 |
| Estimated ΔH / kJ mol⁻¹ | 253.333 | -346.667 |
| Estimated ΔH / kJ mol⁻¹ | 266.667 | -333.333 |
| Estimated ΔH / kJ mol⁻¹ | 280 | -320 |
| Estimated ΔH / kJ mol⁻¹ | 293.333 | -306.667 |
| Estimated ΔH / kJ mol⁻¹ | 306.667 | -293.333 |
| Estimated ΔH / kJ mol⁻¹ | 320 | -280 |
| Estimated ΔH / kJ mol⁻¹ | 333.333 | -266.667 |
| Estimated ΔH / kJ mol⁻¹ | 346.667 | -253.333 |
| Estimated ΔH / kJ mol⁻¹ | 360 | -240 |
| Estimated ΔH / kJ mol⁻¹ | 373.333 | -226.667 |
| Estimated ΔH / kJ mol⁻¹ | 386.667 | -213.333 |
| Estimated ΔH / kJ mol⁻¹ | 400 | -200 |
| Estimated ΔH / kJ mol⁻¹ | 413.333 | -186.667 |
| Estimated ΔH / kJ mol⁻¹ | 426.667 | -173.333 |
| Estimated ΔH / kJ mol⁻¹ | 440 | -160 |
| Estimated ΔH / kJ mol⁻¹ | 453.333 | -146.667 |
| Estimated ΔH / kJ mol⁻¹ | 466.667 | -133.333 |
| Estimated ΔH / kJ mol⁻¹ | 480 | -120 |
| Estimated ΔH / kJ mol⁻¹ | 493.333 | -106.667 |
| Estimated ΔH / kJ mol⁻¹ | 506.667 | -93.3333 |
| Estimated ΔH / kJ mol⁻¹ | 520 | -80 |
| Estimated ΔH / kJ mol⁻¹ | 533.333 | -66.6667 |
| Estimated ΔH / kJ mol⁻¹ | 546.667 | -53.3333 |
| Estimated ΔH / kJ mol⁻¹ | 560 | -40 |
| Estimated ΔH / kJ mol⁻¹ | 573.333 | -26.6667 |
| Estimated ΔH / kJ mol⁻¹ | 586.667 | -13.3333 |
| Estimated ΔH / kJ mol⁻¹ | 600 | 0 |
| Estimated ΔH / kJ mol⁻¹ | 613.333 | 13.3333 |
| Estimated ΔH / kJ mol⁻¹ | 626.667 | 26.6667 |
| Estimated ΔH / kJ mol⁻¹ | 640 | 40 |
| Estimated ΔH / kJ mol⁻¹ | 653.333 | 53.3333 |
| Estimated ΔH / kJ mol⁻¹ | 666.667 | 66.6667 |
| Estimated ΔH / kJ mol⁻¹ | 680 | 80 |
| Estimated ΔH / kJ mol⁻¹ | 693.333 | 93.3333 |
| Estimated ΔH / kJ mol⁻¹ | 706.667 | 106.667 |
| Estimated ΔH / kJ mol⁻¹ | 720 | 120 |
| Estimated ΔH / kJ mol⁻¹ | 733.333 | 133.333 |
| Estimated ΔH / kJ mol⁻¹ | 746.667 | 146.667 |
| Estimated ΔH / kJ mol⁻¹ | 760 | 160 |
| Estimated ΔH / kJ mol⁻¹ | 773.333 | 173.333 |
| Estimated ΔH / kJ mol⁻¹ | 786.667 | 186.667 |
| Estimated ΔH / kJ mol⁻¹ | 800 | 200 |
| Estimated ΔH / kJ mol⁻¹ | 813.333 | 213.333 |
| Estimated ΔH / kJ mol⁻¹ | 826.667 | 226.667 |
| Estimated ΔH / kJ mol⁻¹ | 840 | 240 |
| Estimated ΔH / kJ mol⁻¹ | 853.333 | 253.333 |
| Estimated ΔH / kJ mol⁻¹ | 866.667 | 266.667 |
| Estimated ΔH / kJ mol⁻¹ | 880 | 280 |
| Estimated ΔH / kJ mol⁻¹ | 893.333 | 293.333 |
| Estimated ΔH / kJ mol⁻¹ | 906.667 | 306.667 |
| Estimated ΔH / kJ mol⁻¹ | 920 | 320 |
| Estimated ΔH / kJ mol⁻¹ | 933.333 | 333.333 |
| Estimated ΔH / kJ mol⁻¹ | 946.667 | 346.667 |
| Estimated ΔH / kJ mol⁻¹ | 960 | 360 |
| Estimated ΔH / kJ mol⁻¹ | 973.333 | 373.333 |
| Estimated ΔH / kJ mol⁻¹ | 986.667 | 386.667 |
| Estimated ΔH / kJ mol⁻¹ | 1000 | 400 |
Explore: Raise the energy required to break bonds and find where ΔH changes sign.
For H₂ + Cl₂ → 2HCl, use H–H = 436, Cl–Cl = 243 and H–Cl = 431 kJ mol⁻¹.
- Breaking requires 436 + 243 = 679 kJ mol⁻¹.
- Forming releases 2 × 431 = 862 kJ mol⁻¹; estimated ΔH = −183 kJ mol⁻¹.
Assumed knowledge
Bond counting and signed energy changes.
Learning checkpoints & source
YOUR LEARNING CHECKPOINT- Estimate reaction enthalpy from average bond enthalpies.
- Read energy-level diagrams and explain limitations.