Energetics (AQA A-Level Chemistry): Revision Notes

1.6.7 Bond Enthalpies

What are Bond Enthalpies?

Bond enthalpy (also known as bond dissociation enthalpy) is the amount of energy required to break one mole of a specific covalent bond in a molecule, in the gaseous state. This process is always endothermic because energy is required to break bonds, and therefore the enthalpy change ($ΔH$) is positive.

Mean Bond Enthalpy

The mean bond enthalpy is the average energy needed to break one mole of a specific type of bond across a range of compounds where that bond exists. Since the strength of a bond can vary slightly depending on its chemical environment, the "mean" value accounts for these variations.

• Breaking bonds: Always requires energy, so bond breaking is endothermic and $ΔH$ is positive.
• Forming bonds: Always releases energy, so bond formation is exothermic and $ΔH$ is negative.

General Trends

• Stronger bonds require more energy to break and release more energy when formed.
• Shorter bonds are typically stronger:
  • Triple bonds are generally shorter and stronger than double bonds.
  • Double bonds are shorter and stronger than single bonds.

Calculating Enthalpy Changes Using Mean Bond Enthalpies

The enthalpy change ($ΔH$) of a reaction can be calculated using mean bond enthalpies by comparing the energy needed to break the bonds in the reactants to the energy released when new bonds form in the products.

The formula is:

$$ΔH = \sum (\text{mean bond enthalpies of bonds broken}) - \sum (\text{mean bond enthalpies of bonds formed})$$

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Differences Between Mean Bond Enthalpy Calculations and Hess' Law

Calculations using mean bond enthalpies often differ from those done using Hess' Law for several reasons:

1. Mean bond enthalpy values are averages, and actual bond energies can vary depending on the chemical environment of the bond.
2. State of the substances: Mean bond enthalpy calculations assume all substances are in the gaseous state. However, real reactions may involve liquids or solids, where intermolecular forces also contribute to the enthalpy change.
3. Accuracy: Calculations based on enthalpies of formation or combustion are generally more accurate because they are experimentally determined for specific reactions under standard conditions, rather than using average bond energies.