Energetics & electrochemistry · SL/HL
Bond enthalpy
Estimate enthalpy from bonds broken and formed.
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Number formats & units
Use decimal dots in datasets. Single values also accept decimal commas and scientific notation such as 1.2e-3 or 1.2 × 10⁻³.
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Formula & example
The relationship
What the variables mean
| Symbol | Meaning |
|---|---|
| Average bond enthalpy in kJ mol⁻¹. |
Worked example
- Bonds broken
- 1 436 1 243
- Bonds formed
- 2 431
-183 kJ mol⁻¹
Follow the example steps
- Broken: 1 × 436 = 436 kJ mol⁻¹
- Broken: 1 × 243 = 243 kJ mol⁻¹
- Formed: 2 × 431 = 862 kJ mol⁻¹
- Σ broken = 679; Σ formed = 862
- ΔH ≈ 679 − 862 = -183 kJ mol⁻¹
Common mistakes
Subtract energy released by bonds formed from energy needed to break bonds.
Incorrect approach
ΔH = 600 − 500 = +100 kJ mol⁻¹
Correct approach
ΔH = 500 − 600 = −100 kJ mol⁻¹
More checks before you submit
- Breaking bonds requires energy; forming bonds releases energy in this bookkeeping convention.
Method notes
Average bond enthalpies are gas-phase averages across compounds, so the result is an estimate rather than an exact compound-specific enthalpy.