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1.4.7Physical Chemistry

Bond Length and Bond Strength — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 5 parts Lesson 16 of 42

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Part 1 of 5Definitions

1.4.7 Bond Length and Bond Strength

Bond length and bond strength are closely related.

Bond length
  • The distance between the nuclei of two bonded atoms, at which attraction and repulsion are balanced.
Bond enthalpy
  • The energy needed to break one mole of a covalent bond in gaseous molecules, measured in kJ mol⁻¹.
Link
  • Shorter bonds generally have the nuclei closer to the shared electrons, so they are stronger and have higher bond enthalpy.
Direction
  • Breaking bonds takes in energy, so bond enthalpies are positive values.
Energy changes
  • Energy is released when a bond forms, with the same size as the bond enthalpy, but opposite in sign to the energy needed to break it.

Part 2 of 5Multiple Bonds

Multiple Bonds
More pairs
  • More shared pairs give a stronger overall attraction between the nuclei and the electrons.
Length
  • C–C is about 154 pm, C=C about 134 pm and C≡C about 120 pm, so more bonds means a shorter distance.
Strength
  • C–C is 347, C=C 612 and C≡C 838 kJ mol⁻¹, so the triple bond is strongest although not three times the single bond.
Nitrogen
  • N≡N has a bond enthalpy of 945 kJ mol⁻¹, which explains why nitrogen gas is so unreactive.
Halogens
  • Single bonds between larger halogen atoms are longer and weaker, apart from F–F,
    • which is unusually weak because its lone pairs repel strongly.
BondLength / pmEnthalpy / kJ mol⁻¹
C–C154347
C=C134612
C≡C120838

Part 3 of 5Trends in Hydrogen Halides

1
Down Group 17 the halogen atoms get larger, so the H–X bond gets longer.
2
A longer bond means the shared pair is further from the halogen nucleus, so the attraction is weaker.
3
Bond enthalpy decreases in the order H–F 568, H–Cl 432, H–Br 366 and H–I 299 kJ mol⁻¹.
4
This explains why HI is the most easily broken and hydrogen iodide is the least thermally stable hydrogen halide.

Part 4 of 5Worked Example

Question

  • The H–Cl bond enthalpy is 432 kJ mol⁻¹.
  • Calculate the energy needed to break one single H–Cl bond, in joules (Avogadro constant 6.02 × 10²³ mol⁻¹).

  1. Convert to joules per mole432 kJ mol⁻¹ × 1000 = 4.32 × 10⁵ J mol⁻¹.
  2. One mole contains 6.02 × 10²³ bonds, so divide: 4.32 × 10⁵ / 6.02 × 10²³.
  3. The result is 7.18 × 10⁻¹⁹ J for each bond.

Answer 7.18 × 10⁻¹⁹ J per H–Cl bond.

Part 5 of 5Exam Checklist & Tips

1.4.7 – Key Points for the Exam

  • Bond length Distance between the nuclei of two covalently bonded atoms.
  • Bond enthalpy Energy required to break one mole of a particular covalent bond in the gas phase.
  • Multiple bond A double or triple bond, made up of two or three shared pairs.
  • Bond strength How much energy is needed to break a bond, measured by bond enthalpy.

Common Mistakes

  • Saying that a triple bond is exactly three times as strong as a single bond.
  • Using bond enthalpy for liquid or solid states; the definition needs gaseous molecules.
  • Mixing up the trendlonger bonds are weaker, not stronger.
  • Giving bond enthalpy a negative sign; breaking bonds is endothermic.

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