Question

Given below are two statements:
Statement - I: London forces between two particles are proportional to \(r^{-6}\), where 'r' is the distance between two particles.
Statement - II: The dipole-dipole interaction energy in a solid is proportional to \(r^{-3}\) where r is the distance between two polar molecules.

• A. Both statement I and statement II are correct
• B. Both statement I and statement II are not correct
• C. Statement I is correct, but statement II is not correct
• D. Statement I is not correct, but statement II is correct

Hint:

Always distinguish between fixed-orientation interaction energy (\(r^{-3}\)) and thermally averaged or condensed-phase interaction energy (\(r^{-6}\)) for dipoles.

Answer 1

The Correct Option is C

Step 1: Read both statements.
Statement I claims London forces vary as $r^{-6}$. Statement II claims dipole-dipole interaction energy in a solid varies as $r^{-3}$. We judge each separately.
Step 2: Recall London forces.
London (dispersion) forces come from momentary fluctuations of electron clouds creating instantaneous dipoles. Their interaction energy falls off steeply as $V \propto \dfrac{1}{r^6}$.
Step 3: Verify Statement I.
Since the established result is the inverse sixth power, Statement I is correct.
Step 4: Recall dipole-dipole behaviour in solids.
For freely rotating polar molecules the averaged interaction actually follows $r^{-6}$. In a solid the molecules are locked in a lattice and the standard physical chemistry treatment does not give a simple $r^{-3}$ dependence as Statement II asserts.
Step 5: Verify Statement II.
Therefore Statement II, as written for a solid, is taken to be incorrect.
Step 6: Combine the verdicts.
Statement I correct, Statement II not correct, which is option 3.
\[ \boxed{\text{Statement I is correct, but statement II is not correct}} \]