The correct statement(s) about intermolecular forces is(are)
- 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.
- The average potential energy of two rotating polar molecules that are separated by a distance r has 1/r3 dependence.
- The dipole-induced dipole average interaction energy is independent of temperature.
- Nonpolar molecules attract one another even though neither has a permanent dipole moment.
Answer
(3) The dipole-induced dipole average interaction energy is independent of temperature., (4) Nonpolar molecules attract one another even though neither has a permanent dipole moment.
Solution
Distance dependence of potential energy: ion–ion 1/r; ion–dipole 1/r2; stationary dipole–dipole 1/r3; rotating dipole–dipole 1/r6 (and 1/T); dipole–induced dipole 1/r6; London 1/r6.
(A) False: 1/r falls more slowly than 1/r2.
(B) False: for rotating polar molecules the average energy varies as 1/r6.
(C) True: the induced dipole follows the permanent dipole instantly, so thermal motion does not average it out — no T dependence.
(D) True: London dispersion forces (instantaneous dipoles) act between nonpolar molecules.
Answer: (C), (D)