A-Level Chemistry October/November 2024 Q2(c)(i): Explain the trend in the boiling points of the Group 16 hydrides H2S to H2Te.

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  The boiling points of the Group 16 hydrides increase from H₂S to H₂Te.

  Going down the group from H₂S to H₂Te, the total number of electrons per molecule increases.

  This results in stronger instantaneous dipole-induced dipole forces (or London dispersion forces) between the molecules. Consequently, more energy is required to overcome these stronger intermolecular forces, leading to an increase in the boiling point.

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  How the marks are awarded

  • B1 — The first mark is for correctly identifying that as you go down the group from H₂S to H₂Te, the molecules have an increasing number of electrons.
  • B1 — The second mark is for linking the greater number of electrons to stronger instantaneous dipole-induced dipole / London dispersion forces, and stating that more energy is needed to overcome them.

  Common mistakes

  • Discussing H₂O and hydrogen bonding, which is not part of the specific trend from H₂S to H₂Te requested in the question.
  • Incorrectly attributing the trend primarily to changes in permanent dipole-dipole forces instead of the dominant effect of increasing London dispersion forces.
  • Confusing intermolecular forces with intramolecular bonds by stating that covalent bonds are broken during boiling.
  • Using vague terms like 'molecules get bigger' or 'stronger van der Waals' forces' without explicitly mentioning the number of electrons and specifying 'instantaneous dipole-induced dipole forces'.

  Examiner tip: For trends in boiling points of simple molecular substances, always connect the total number of electrons in the molecule to the strength of the resulting London dispersion forces.

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