Bond enthalpy calculations estimate how much heat is absorbed or released during a chemical reaction by comparing the bonds broken in reactants with the bonds formed in products. This matters because energy changes help predict whether a reaction is endothermic or exothermic. The method is especially useful for gases and for reactions where detailed experimental enthalpy data are not available.
It gives a practical first estimate of ΔHrxn using values from a table of average bond enthalpies.
Key Facts
- ΔHrxn ≈ Σ bond enthalpies of bonds broken − Σ bond enthalpies of bonds formed
- Breaking bonds absorbs energy, so bonds broken are counted as positive energy input.
- Forming bonds releases energy, so bonds formed are subtracted from the total.
- If ΔHrxn < 0, the reaction is exothermic and releases heat.
- If ΔHrxn > 0, the reaction is endothermic and absorbs heat.
- Average bond enthalpies are approximate because the same bond can have different strengths in different molecules.
Vocabulary
- Bond enthalpy
- The energy required to break one mole of a specific type of bond in the gas phase.
- Reaction enthalpy
- The overall heat energy change for a chemical reaction at constant pressure.
- Endothermic
- A process that absorbs more energy than it releases, giving a positive ΔH.
- Exothermic
- A process that releases more energy than it absorbs, giving a negative ΔH.
- Average bond enthalpy
- A mean bond energy value measured from many compounds, used to estimate reaction energy changes.
Common Mistakes to Avoid
- Adding bonds formed instead of subtracting them is wrong because bond formation releases energy and lowers the reaction enthalpy.
- Forgetting to multiply by the number of each bond is wrong because the total energy depends on how many identical bonds are broken or formed.
- Using product bonds as bonds broken is wrong because bonds broken come from reactants and bonds formed come from products.
- Treating the answer as exact is wrong because average bond enthalpies vary with molecular environment and usually apply best to gas phase reactions.
Practice Questions
- 1 Estimate ΔHrxn for H2 + Cl2 → 2HCl using H-H = 436 kJ/mol, Cl-Cl = 243 kJ/mol, and H-Cl = 431 kJ/mol.
- 2 Estimate ΔHrxn for CH4 + 2O2 → CO2 + 2H2O using C-H = 413 kJ/mol, O=O = 498 kJ/mol, C=O in CO2 = 799 kJ/mol, and O-H = 463 kJ/mol.
- 3 A reaction has stronger bonds in the products than in the reactants. Explain whether the reaction is likely to be endothermic or exothermic using the bond enthalpy formula.