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1.4.5Physical Chemistry

Bond Enthalpies

5 min read 5 parts Lesson 26 of 63

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Section 1 of 5Bond Enthalpy

1.4.5 Bond Enthalpies
Definition
  • Bond enthalpy is the energy needed to break one mole of a particular covalent bond in gaseous molecules.
Gas phase
  • Bond breaking is considered in the gas phase so that intermolecular forces are not included.
Endothermic
  • Breaking bonds always absorbs energy, so bond enthalpies are positive, while forming bonds releases the same energy.
Mean values
  • A mean bond enthalpy is an average over several different molecules, so values from data tables are approximate.

Section 2 of 5Calculating ΔH

1
Write the balanced equation and draw out the structures so that all bonds are visible.
2
Add up the bond enthalpies of all bonds broken in the reactants.
3
Add up the bond enthalpies of all bonds formed in the products.
4
Calculate ΔH = sum of bonds broken − sum of bonds formed.
5
A negative answer means more energy is released forming bonds than is absorbed breaking them, so the reaction is exothermic.

Section 3 of 5Comparing Results

Comparing Results
Mean values
  • Bond enthalpy values are averages, so a particular bond in a specific molecule may differ slightly.
State symbols
  • Bond enthalpy calculations assume gases, so a calculation for liquid water differs from a Hess's law value.
Accuracy
  • Calculated values are therefore estimates and usually differ a little from those found by calorimetry.
Strong bonds
  • Reactions are more likely to be exothermic when the products contain stronger bonds than the reactants.

Section 4 of 5Worked Example

Question Estimate ΔH for CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) using bond enthalpies (kJ mol⁻¹): C–H 413, O=O 498, C=O 805, O–H 464.

  1. Bonds broken4 C–H and 2 O=O = 4(413) + 2(498) = 1652 + 996 = 2648 kJ.
  2. Bonds formed2 C=O and 4 O–H = 2(805) + 4(464) = 1610 + 1856 = 3466 kJ.
  3. ΔH = 2648 − 3466 = −818 kJ mol⁻¹.

Answer ΔH ≈ −818 kJ mol⁻¹ (exothermic).

Section 5 of 5Exam Checklist & Tips

1.4.5 – Key Points for the Exam

  • Bond enthalpy The energy required to break one mole of a covalent bond in the gas phase.
  • Mean bond enthalpy The average energy to break a bond over a range of compounds.
  • Bond breaking An endothermic process that absorbs energy.
  • Bond making An exothermic process that releases energy.

Common Mistakes

  • Adding bonds formed and bonds broken in the wrong order, giving the wrong sign.
  • Forgetting to multiply the bond enthalpy by the number of such bonds.
  • Not including all bonds, such as the C–H bonds in an organic reactant.
  • Comparing a bond enthalpy result with a Hess's law answer without noticing the difference in state.
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