Question Details

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

Options

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 3 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.

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Correct Answer :

Option C

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

Option D

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

Solution :

To determine the correct statements about intermolecular forces, let us analyze each statement step-by-step:

1. Potential energy of point charges versus point charge-point dipole:
The potential energy between two point charges (ion-ion interaction) is inversely proportional to the distance r between them:
Vion-ion1r
The potential energy between a point charge and a point dipole (ion-dipole interaction) is inversely proportional to the square of the distance:
Vion-dipole1r2
As the distance r approaches infinity (r), the term 1r2 approaches zero faster than 1r. Therefore, the ion-dipole potential energy approaches zero more rapidly than the ion-ion potential energy, making the first statement incorrect.

2. Average potential energy of two rotating polar molecules:
For stationary polar molecules, the dipole-dipole potential energy depends on 1r3. However, when polar molecules are free to rotate, thermal motion averages the interaction. The resulting average potential energy (Keesom interaction) depends on the temperature T and has a 1r6 dependence:
Vrotating dipole-dipole1Tr6
Thus, the second statement is incorrect.

3. Temperature independence of dipole-induced dipole interaction:
A dipole-induced dipole interaction (Debye force) occurs when a polar molecule with a permanent dipole moment μ induces a temporary dipole in a neighboring nonpolar molecule with polarizability α. The average interaction potential energy is given by:
Vdipole-induced dipole=-2μ2α(4πε0)2r6
This equation does not contain temperature T. Therefore, the dipole-induced dipole average interaction energy is independent of temperature, making this statement correct.

4. Attraction between nonpolar molecules:
Even though nonpolar molecules lack permanent dipoles, continuous electron motion produces instantaneous dipoles at any given moment. These instantaneous dipoles induce temporary dipoles in adjacent molecules, creating an attractive force known as London dispersion force. Therefore, nonpolar molecules attract one another, making this statement correct.

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