Bond Enthalpies
Chemistry: AQA A Level, topic 1.4.6
Mean Bond Enthalpies
Mean bond enthalpies can be used in rough calculations to predict enthalpy changes.
Bond enthalpy
- Bond enthalpy is the enthalpy change when one mole of bonds is broken in the gas phase.
- This varies from molecule to molecule and from bond to bond.
Mean bond enthalpies
- Some molecules (e.g. methane - CH4) have multiple identical bonds.
- The energy to break the second CH bond in methane will be greater than for the first bond because the molecule will rearrange after the first bond is broken.
- The CH bond enthalpy value for methane is the average of all four bond enthalpies.
Example
- Looking at the previous example of methane:
- First C-H broken: 401 kJmol-1
- Second C-H broken: 413 kJmol-1
- Third C-H broken: 428 kJmol-1
- Fourth C-H broken: 439 kJmol-1
- So overall, the enthalpy change will be an average:
- (439 + 428 + 413 + 401) ÷ 4 = 420.25 kJmol-1
Inaccuracies
- There are two main reasons mean bond enthalpies are inaccurate:
- Mean bond enthalpies are only valid in the gas phase. You might not be doing your reaction in the gas phase.
- Bond enthalpies depend on the particular molecule, whereas mean bond enthalpies are quoted generally.
- So we can't expect a perfect answer - they are only good as a guide.
Test yourself on Bond Enthalpies
Which of the following best describes bond enthalpies?