Question

The correct statement(s) about intermolecular forces is (are)

• 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.
• B. The average potential energy of two rotating polar molecules that are separated by a distance \( r \) has \( 1/r^6 \) dependence.
• C. The dipole-induced dipole average interaction energy is independent of temperature.
• D. Nonpolar molecules attract one another even though neither has a permanent dipole moment.

Hint:

Always remember the distance dependence: \begin{itemize} \item Charge–Charge: \( \frac{1}{r} \) \item Charge–Dipole: \( \frac{1}{r^2} \) \item Dipole–Dipole (rotating): \( \frac{1}{r^6} \) \item Dipole–Induced Dipole: \( \frac{1}{r^6} \) \item Induced–Induced (London): \( \frac{1}{r^6} \) \end{itemize}

Answer 1

The Correct Option is C, D

Let’s examine each statement individually: (A) Incorrect - For two point charges, the potential energy falls off as \( 1/r \). - For a point dipole and point charge, the interaction energy falls off as \( 1/r^2 \). - Hence, the potential between two point charges decreases more slowly (not rapidly) compared to that between a point dipole and a charge. \[ \Rightarrow Incorrect statement \] (B) Incorrect - For two rotating polar molecules, the orientation averages out due to thermal motion. - The correct dependence for such interactions (Keesom interaction) is: \[ \text{Average potential energy} \propto -\frac{1}{r^6} \] - Statement says \( 1/r^3 \) which applies to aligned, non-rotating dipoles. \[ \Rightarrow Incorrect \] (C) Correct - Dipole–induced dipole interaction (Debye interaction) does not depend on thermal averaging. - It is due to a permanent dipole inducing a dipole in a non-polar molecule, and this interaction is independent of temperature. \[ \Rightarrow Correct \] (D) Correct - Even nonpolar molecules attract each other via London dispersion forces (instantaneous dipole-induced dipole). - These are weak Van der Waals forces but always present, especially significant in noble gases. \[ \Rightarrow Correct \]