Bond Enthalpies
Bond breaking absorbs energy (Endothermic) and bond forming releases energy (Exothermic).
The Golden Rule of Bonds
Breaking Bonds
Takes energy to pull atoms apart.
Making Bonds
Releases energy when atoms snap together.
Calculating
The Formula
(Reactants - Products)
Examiner's Warning: Average Values
You will often see a discrepancy between values calculated using Bond Enthalpies and values from Hess's Law (using Formation data).
Why? The bond enthalpies in the data booklet are Average Bond Enthalpies.
- They are averaged over many different compounds (e.g., C-H in methane, ethane, benzene).
- The actual C-H bond strength in your specific molecule might differ slightly.
- Standard Enthalpies of Formation/Combustion are specific to the compound, so they are more accurate.
State Symbols Matter
Bond Enthalpies are defined for the GASEOUS STATE only.
If you use them for liquids (like Ethanol, ), your calculation will be wrong because you ignored the Enthalpy of Vaporization needed to turn the liquid into gas first.
Putting it into Practice
Bond Enthalpy Calculation
Paper 2 StyleCalculate the enthalpy change for the hydrogenation of ethene, using the bond enthalpy data below:
C-H: 412 kJ/mol
C-C: 348 kJ/mol
C=C: 612 kJ/mol
H-H: 436 kJ/mol
Reaction:
1. List Bonds Broken (Reactants):
- 1 × C=C (612)
- 4 × C-H (4 × 412 = 1648)
- 1 × H-H (436)
Total Energy IN =
2. List Bonds Formed (Products):
- 1 × C-C (348)
- 6 × C-H (6 × 412 = 2472)
Total Energy OUT =
3. Calculate Difference:
Practice: Bond Definitions
Define the term Average Bond Enthalpy.