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Chemical Bonding and Molecular Structure question

2025 · Shift 2 · Q5
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Chemical Bonding and Molecular Structure question

2025 · Shift 2 · Q5

JEE AdvancedChemistryChemical Bonding and Molecular StructureMultiple correct+4 / −2
The correct statement(s) about intermolecular forces is(are)
  1. A
    The potential energy between two point charges approaches zero more rapidly than the potential energy between a point dipole and a point charge as the distance between them approaches infinity.
  2. B
    The average potential energy of two rotating polar molecules that are separated by a distance r has 1/r31/r^31/r3 dependence.
  3. C
    The dipole-induced dipole average interaction energy is independent of temperature.
  4. D
    Nonpolar molecules attract one another even though neither has a permanent dipole moment.
View written solutionFree

Correct answer: C, D

  1. Recall distance dependence of common intermolecular interactions

    • Charge–charge interaction: Uq−q ∝ 1rU_{q-q} \,\propto\, \frac{1}{r}Uq−q​∝r1​
    • Charge–dipole interaction: Uq−μ ∝ 1r2U_{q-\mu} \,\propto\, \frac{1}{r^2}Uq−μ​∝r21​
    • Dipole–dipole interaction (fixed orientation): Uμ−μ ∝ 1r3U_{\mu-\mu} \,\propto\, \frac{1}{r^3}Uμ−μ​∝r31​
    • Average dipole–dipole interaction for freely rotating polar molecules (Keesom interaction): ⟨U⟩ ∝ −1Tr6\langle U \rangle \,\propto\, -\frac{1}{T r^6}⟨U⟩∝−Tr61​
    • Dipole-induced dipole interaction (Debye interaction): ⟨U⟩ ∝ −1r6\langle U \rangle \,\propto\, -\frac{1}{r^6}⟨U⟩∝−r61​ and is essentially independent of temperature.
    • London dispersion interaction: U ∝ −1r6U \,\propto\, -\frac{1}{r^6}U∝−r61​ This explains attraction between nonpolar molecules.
  2. Check option A

    Statement: The potential energy between two point charges approaches zero more rapidly than the potential energy between a point dipole and a point charge as the distance between them approaches infinity.

    Compare: Uq−q∝1r,Uq−μ∝1r2U_{q-q} \propto \frac{1}{r}, \qquad U_{q-\mu} \propto \frac{1}{r^2}Uq−q​∝r1​,Uq−μ​∝r21​

    Since 1/r21/r^21/r2 goes to zero faster than 1/r1/r1/r, the charge–dipole interaction approaches zero more rapidly, not charge–charge.

    Therefore, A is false.

  3. Check option B

    Statement: The average potential energy of two rotating polar molecules that are separated by a distance rrr has 1/r31/r^31/r3 dependence.

    For freely rotating polar molecules, the thermal average dipole–dipole interaction is: ⟨U⟩∝−1Tr6\langle U \rangle \propto -\frac{1}{T r^6}⟨U⟩∝−Tr61​

    So it does not vary as 1/r31/r^31/r3.

    Therefore, B is false.

  4. Check option C

    Statement: The dipole-induced dipole average interaction energy is independent of temperature.

    Debye interaction (dipole-induced dipole) has the form: ⟨U⟩∝−1r6\langle U \rangle \propto -\frac{1}{r^6}⟨U⟩∝−r61​

    It does not involve temperature explicitly.

    Therefore, C is true.

  5. Check option D

    Statement: Nonpolar molecules attract one another even though neither has a permanent dipole moment.

    This is due to London dispersion forces, arising from instantaneous dipole-induced dipole interactions.

    Therefore, D is true.

  6. Final conclusion

    Correct statements are: C, D\boxed{C,\ D}C, D​

  7. Comparison with stored correct answer

    Stored answer: C, D

    My derived answer matches the stored answer.

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