Bond Enthalpies

Full Notes

Understanding Bond Enthalpy

Bond enthalpy (bond dissociation energy) is defined as: "The energy required to break one mole of a particular bond in the gaseous state."

• Breaking bonds requires energy → endothermic (+ΔH).
• Making bonds releases energy → exothermic (-ΔH).

Example:

• Breaking H–H bond: H₂(g) → 2H(g), ΔH = +436 kJ mol⁻¹
• Forming H–H bond: 2H(g) → H₂(g), ΔH = −436 kJ mol⁻¹

Mean Bond Enthalpy

The mean bond enthalpy is the average energy required to break a bond type, averaged over several molecules containing that bond.

Example: The mean bond enthalpy of a C–H bond is +412 kJ mol⁻¹, but in different molecules the exact value will vary slightly.

Limitation: Because bond enthalpies are averages, enthalpy calculations using them are approximate compared to experimental values from calorimetry.

Calculating Enthalpy Change Using Bond Enthalpies

The enthalpy change of a reaction can be estimated using:

• Bonds broken (reactants) → energy absorbed (+ΔH).
• Bonds formed (products) → energy released (-ΔH).

Matt’s exam tip

Bond enthalpies apply to gaseous substances. If the substance isn’t in the gas phase, you may need to include enthalpy of vaporisation in your calculations.

Ex

Worked Example: Combustion of Methane

Reaction: CH₄ + 2O₂ → CO₂ + 2H₂O

Bond enthalpies:

• C–H = +412 kJ·mol⁻¹
• O=O = +498 kJ·mol⁻¹
• C=O = +805 kJ·mol⁻¹
• O–H = +463 kJ·mol⁻¹

1. Bonds Broken (Reactants)
   • CH₄: 4 × C–H = 4 × 412 = 1648 kJ
   • O₂: 2 × O=O = 2 × 498 = 996 kJ
   • Total broken = 1648 + 996 = 2644 kJ
2. Bonds Formed (Products)
   • CO₂: 2 × C=O = 2 × 805 = 1610 kJ
   • H₂O: 4 × O–H = 4 × 463 = 1852 kJ
   • Total formed = 1610 + 1852 = 3462 kJ
3. Calculate ΔH
   ΔH = (bonds broken) − (bonds formed)
   ΔH = 2644 − 3462 = −818 kJ·mol⁻¹

ΔH = −818 kJ·mol⁻¹ (exothermic)

Accuracy of Bond Enthalpy Calculations

Experimental enthalpy values (e.g., from calorimetry) are usually more accurate than those calculated using average bond ethanlpies because:

• Average bond enthalpy values are approximations.
• We have to assume all substances are in gaseous state (not always true in real conditions).

Summary

Bond enthalpies provide a way to estimate reaction enthalpies by comparing energy required to break bonds in reactants with energy released when forming bonds in products:

ΔH_r = Σ(bonds broken) − Σ(bonds formed)

This method gives useful approximations and helps explain the energetic basis of chemical reactions.