Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- In a reaction, bonds in the reactants break and new bonds in the products form. Both change the potential energy of the system.
- Breaking a bond is endothermic (energy in). Forming the same bond is exothermic and releases the same amount.
- A bond enthalpy is the energy to break one mole of a bond in gaseous molecules. It is always positive.
- Tables give average bond enthalpies, taken over many molecules. Answers from them are estimates.
Key relationships
| Idea | Rule |
|---|---|
| Estimate of ΔH | ΔH ≈ Σ(bonds broken) − Σ(bonds formed) |
| Exothermic | energy released forming bonds > energy needed to break bonds; ΔH < 0 |
| Endothermic | energy needed > energy released; ΔH > 0 |
| Counting | bonds per molecule (from Lewis structure) × coefficient |
| Shortcut | bonds unchanged on both sides cancel; the answer does not change |
| Unknown bond | rearrange the same equation and solve for the missing value |
Assumptions behind the method
- All reactants and products are gases. Energy from intermolecular forces in liquids or solids is not counted.
- Each bond has its average strength, whatever molecule it is in.
- The imaginary path “break everything, then form everything” gives the same ΔH as the real path, because ΔH depends only on the start and the end.
Mistakes to avoid
- Formed − broken. Wrong sign. It is broken − formed.
- “Breaking bonds releases energy.” Never.
- Double bond = two single bonds. Use the C=C or O=O value itself.
- Missing coefficients. 2NH₃ has 6 N–H bonds.
- Wrong bond order. O₂ is O=O, N₂ is N≡N, CO₂ is O=C=O (two C=O).
- Expecting the exact measured value. It is an estimate.
Quick self-check
- Estimate ΔH for H₂(g) + F₂(g) → 2HF(g) using H–H 432, F–F 154, H–F 565 kJ mol⁻¹. (586 − 1130 = −544 kJ mol⁻¹, exothermic.)
- How many C–H bonds are broken when 2 mol of CH₄ react completely? (8 mol.)
- Give one reason a bond-enthalpy estimate differs from the measured ΔH. (Table values are averages over many molecules; or a substance is not a gas.)
Next: practice questions.