JEE Advance - Chemistry (2025 - Paper 2 Online - No. 5)
Explanation
Option A
A charge–charge interaction falls off as
$$U_{qq}\propto\frac1r$$
while a charge–dipole interaction goes as
$$U_{q\mu}\propto\frac1{r^2}\,. $$
So the charge–dipole potential actually decays faster.
⇒ A is false.
Option B
A fixed dipole–dipole energy is
$$U_{\mu\mu}\propto\frac1{r^3}\,, $$
but once both molecules tumble freely, the thermal (Keesom) average gives
$$\langle U\rangle_{\rm Keesom}\propto -\frac{\mu^4}{kT\,(4\pi\varepsilon_0)^2\,r^6}\,. $$
That falls off as $1/r^6$, not $1/r^3$.
⇒ B is false.
Option C
The permanent–induced dipole (Debye) interaction is
$$U_{\rm Debye}=-\tfrac12\,\alpha\,E^2\propto -\frac{\alpha\,\mu^2}{(4\pi\varepsilon_0)^2\,r^6}\,, $$
and when you average over random orientations (assuming the interaction is weak compared with $kT$), no $T$ appears in the result.
⇒ C is true.
Option D
Even totally nonpolar species experience instantaneous‐induced (London) dispersion forces that attract as
$$U_{\rm disp}\propto -\frac1{r^6}\,. $$
⇒ D is true.
Answer: C and D.
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