Bond enthalpies

Definitions

Bond enthalpy is the enthalpy change to break one mole of a given bond in gaseous molecules. Breaking is always endothermic (positive). Mean bond enthalpy is the average value for that bond over a range of different compounds, since the exact value depends on the rest of the molecule.

Calculating ΔH

ΔH = sum of bond enthalpies of bonds broken - sum of bond enthalpies of bonds made.

Draw displayed formulae and count every bond. If more energy is released making bonds than is needed to break them, the reaction is exothermic.

Limitations

Values calculated from mean bond enthalpies differ from experimental values because:

• mean values are averages, not the actual values in these molecules
• they apply only to gases; if any substance is liquid (like water), energy changes of state are ignored

Values calculated from enthalpies of formation are more accurate.

Finding a mean bond enthalpy

Use a Hess cycle. For example, ΔatH for CH4(g) -> C(g) + 4H(g) breaks four C-H bonds, so mean C-H = ΔH / 4.

Worked example

Estimate ΔcH for CH4(g) + 2O2(g) -> CO2(g) + 2H2O(g). Bond enthalpies (kJ/mol): C-H 413, O=O 498, C=O 805, O-H 464.

1. Broken: 4(413) + 2(498) = 2648
2. Made: 2(805) + 4(464) = 3466
3. ΔH = 2648 - 3466

Answer: ΔH = -818 kJ/mol

Check yourself

1. Find ΔH for H2(g) + Cl2(g) -> 2HCl(g). (H-H 436, Cl-Cl 243, H-Cl 432 kJ/mol)
   Show answer

   -185 kJ/mol. Broken 679; made 864; ΔH = -185 kJ/mol.

2. Find ΔH for C2H4(g) + H2(g) -> C2H6(g). (C=C 612, C-C 347, C-H 413, H-H 436 kJ/mol)
   Show answer

   -125 kJ/mol. Broken: C=C and H-H = 1048. Made: C-C and two new C-H = 1173. ΔH = -125 kJ/mol.

3. ΔH for NH3(g) -> N(g) + 3H(g) is +1170 kJ/mol. Find the mean N-H bond enthalpy.
   Show answer

   390 kJ/mol. Three N-H bonds broken: 1170 / 3 = 390 kJ/mol.

4. Bond breaking is
   • exothermic
   • endothermic
   • neither
   • exothermic for double bonds only
   Show answer

   endothermic. Energy is needed to separate the atoms.

5. Why does a value from mean bond enthalpies differ from the experimental ΔcH of methane?
   • Bond enthalpies are measured at 0 K
   • Methane has no C-H bonds
   • Oxygen has a single bond
   • Mean values are averages and water is a liquid under standard conditions
   Show answer

   Mean values are averages and water is a liquid under standard conditions. Averages are not exact, and standard ΔcH forms liquid water, but bond enthalpies apply to gases.

6. Estimate ΔH for 2H2(g) + O2(g) -> 2H2O(g). (H-H 436, O=O 498, O-H 464 kJ/mol)
   Show answer

   -486 kJ/mol. Broken: 872 + 498 = 1370. Made: 4 x 464 = 1856. ΔH = -486 kJ/mol.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.