Intermolecular Forces — Prelims Strategy

Prelims Strategy

To excel in NEET questions on Intermolecular Forces, a systematic approach is essential. First, thoroughly understand the definition and distinction between intramolecular and intermolecular forces. For each type of IMF (London Dispersion, Dipole-Dipole, Dipole-Induced Dipole, Hydrogen Bonding), memorize its origin, the conditions required for its existence, and the factors affecting its strength. Pay special attention to the relative strengths: Hydrogen bonding > Dipole-Dipole > LDFs.

When tackling questions:

1
Identify Polarity: — Determine if the molecule(s) are polar or nonpolar. This is the first step to narrowing down the types of IMFs. Remember, nonpolar molecules only have LDFs. Polar molecules have dipole-dipole forces and LDFs. If H is bonded to F, O, or N, then hydrogen bonding is also present.
2
Prioritize IMFs: — If multiple IMFs are present, identify the strongest one, as it will be dominant in determining properties. Hydrogen bonding is usually dominant if present. If only LDFs, consider molecular size and shape.
3
Relate to Properties: — Directly link the strength of IMFs to physical properties: Stronger IMFs mean higher boiling point, higher melting point, higher viscosity, higher surface tension, and lower vapor pressure.
4
Watch for Traps: — Be wary of distractors that confuse intramolecular with intermolecular forces. Also, remember that LDFs are universal; even molecules with hydrogen bonding still have LDFs, though LDFs might not be the dominant force. For hydrocarbons, remember that branching decreases surface area and thus LDFs, leading to lower boiling points.
5
Practice Anomalies: — Understand why water, ammonia, and HF exhibit unusually high boiling points due to hydrogen bonding. Practice ranking compounds with similar molar masses but different IMFs.